A COMPARISON OF LARVAL FISH ASSEMBLAGES ...

BULLETIN OF MARINE SCIENCE, 53(2): 588-644,1993

A COMPARISON OF LARVAL FISH ASSEMBLAGES IN THE TEMPERA TE ZONE OF THE NORTHEAST PACIFIC AND NORTHWEST ATLANTIC OCEANS Miriam J. Doyle, Wallace W Morse and Arthur W Kendall, Jr. ABSTRACT Numerical classification is used to examine broad scale spatial patterns in the larval component of the ichthyoplankton off the west and east coasts of the United States, based on data collected during extensive surveys over several years. The multispecies spatial patterns that emerged imply the existence of persistent and geographically distinct larval fish assemblages off both coasts. Four assemblages were identified off the west coast. They include a coastal assemblage that was restricted to coastal and continental shelf waters mainly off Washington and Oregon; a slope/transitional assemblage that occurred largely along the shelf edge and slope; a Columbia River plume assemblage that was associated with the Columbia River plume during summer; and an oceanic assemblage that prevailed in deep water beyond the shelf edge and for which northern and southern components were apparent during winter and spring. The east coast assemblages include a Gulf of Maine and Georges Bank assemblage; an oceanic assemblage that was associated with the continental shelf edge and slope; and a Middle Atlantic Bight assemblage that occurred along the shelffrom Cape Cod, Massachusetts, to Cape Hatteras, North Carolina. Northern, southern, inshore, and offshore components of the Middle Atlantic Bight assemblage were apparent at certain times of the year. In general, the boundaries to the assemblages are fluid, and seasonal variation in occurrence and abundance of species within assemblages is strong. The distribution of the larval fish assemblages reflects spatial structure in the oceanographic environment and, in some instances, can be related to specific hydrographic features. Among the fish taxa in both regions, adaptation of the spawning patterns to the prevailing oceanographic conditions is apparent. Co-evolution among the fishes' spawning strategies within the complex and variable marine ecosystems may have given rise to the high degree of structure observed in the ichthyoplankton spatial patterns and to the larval fish assemblages themselves. It is not possible to conclude from this limited study that the multispecies larval fish assemblages are independent ecological entities that enhance survival ofthe constituent species. Further investigations of finer scale spatial patterns within the larval fish assemblages and among different ontogenetic categories, as well as consideration of the zooplankton, of which fish larvae form only a small part, are necessary to understand fully the multispecies spatial patterns that prevail.

Spatial patterns among planktonic animals in pelagic ecosystems are assumed to be functional, enhancing survival of the organisms. More specifically, this functional interpretation of spatial pattern has been put forward to explain patchiness in the distribution of fish eggs and larvae, and it has been proposed that larval fish distribution patterns may be more important to survival oflarvae than absolute abundance (Hewitt, 1981; Smith, 1981). Extending this idea to a multispecies level, Frank and Leggett (1983) suggested that the synchronous emergence and resulting co-occurrence of capelin (Mallotus villotus) larvae and larvae of other demersal spawning species in Newfoundland coastal waters enhances survival and contributes to species richness and community stability. They propose that multispecies larval fish associations are adaptive and are the result of similar responses among species to the pelagic environment. By focusing on spatial patterns in the ichthyoplankton, it is possible to gain some understanding of interrelationships among fish species during their early life histories, as well as at the adult spawning phase. Examining these patterns in relation to oceanographic conditions provides insight into the adaptation of spawning strategies to the prevailing physical and biological processes. Infor588

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DOYLE ET AL.: TEMPERATE PACIAC AND ATLANTIC OCEAN ASSEMBLAGES

SPATIAL SCALE

ENVIRONMENTAL FACTORS

EvoIuUonary

1 000-1

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Annual

BATHYMETRY TEMPERATURE

REGIMES

SPAWNING

STRATEGIES

• Tlmlng/duraUon CIRCULATION

PATTERNS

• LocaUon

Evolutionary Annual

01 spawning

1OOD-l OOkm 2

Seasonal

01 spawning

• Pelagic/demersal WATER MASS STRUCTURE PLANKTON

PRODUCTION

ESTUARINE

OUTFLOW

• Fecundity

LARVAL ABUNDANCE • Larval drill • Larval behavior

I DISTRIBUTION

(e.g., vertical

mlgreUon)

Annual Seesonal

< lDOkm 2

Dlel

• Food availability • Predator

abundance

• Larval growth/development/survival

Figure I. Hierarchy of factors contributing at different temporal and spatial scales to the occurrence, distribution, and abundance of larval fish species.

mation concerning optimal environmental conditions for larval fish survival may also be gained. Such knowledge is important to understand resource utilization and niche occupation among fish species in marine ecosystems. Integrating these multi species interactions and ecosystem processes into fisheries investigations is a much more realistic approach to understanding natural fluctuations in fish populations than the traditional single species approach (Sharp, 1980). The interpretation of spatial patterns in the ichthyoplankton involves consideration of many factors pertaining to the biology and early life history of fish species and the environment in which they live and reproduce. A complex suite of environmental factors interacts with the biology of fish populations at different temporal and spatial scales to influence the occurrence, distribution, and abundance of the larvae (Fig. 1). On a spatial scale of 1,000 to 100 km2, the distributional range of fish populations and the spawning strategies they employ have evolved under the influence of the dominant oceanographic features ofthe region in which they occur (Parrish et al., 1981; Hewitt, 1981; Loeb et aI., 1983a; Norcross and Shaw, 1984). At this broad temporal and spatial scale, the patterns of distribution of the spawning products are initially set. Variation in the oceanographic environment on an annual scale may cause interannual changes in both the distributional range of the adult fish and features of their spawning strategies such as timing, duration, and location of spawning. These changes will be reflected in the patterns of distribution and abundance of their larvae. Over shorter time periods, a combination of physical and biological factors act directly on larvae to influence their distribution, abundance, growth, and survival. These direct effects act at meso-and fine scale levels « 100 km2) and are independent of the distribution and spawning strategies of the adult fish. Such environmental factors include water movement and temperature, the occurrence of physical discontinuities, primary and secondary production in the plankton, and the associated abundance and distribution of larval fish prey and predator organisms. These factors can affect larval behavior such as vertical migration on a diel basis, swimming ability, and foraging behavior. A recent approach to investigating ichthyoplankton spatial patterns has been to identify larval fish assemblages and relate their occurrence and variability to the biology of the component fish species and to the pelagic ecosystem in which they exist (Richardson and Pearcy, 1977; Richardson et aI., 1980; Loeb et aI.,

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1983b; Frank and Leggett, 1983; Young et aI., 1986; Smith et aI., 1987; Moser et aI., 1987; Olivar, 1987; Sabates, 1990; Suthers and Frank, 1991). Given the fluid nature of the pelagic environment, gradual spatial variation in abundance oflarval fish is the norm rather than concentrations within sharply defined boundaries. Considerable overlap in the distribution of different species does occur. Nevertheless, all the above studies have identified assemblages of larval fish species that correspond to geographic or oceanic zones and which tend to reflect the habitat and behavior of the spawning adult fish. Many questions remain, however, concerning the origin ofichthyoplankton assemblages. For instance, are the observed multispecies distributional patterns merely the result of the random co-occurrence of the spawning products of many species? Alternatively, has the synchronous evolution among the spawning strategies of these species resulted in the occurrence of "functional" assemblages of larval fish, the existence of which enhances survival of the constituent members? Relationships between the occurrence and distribution ofichthyoplankton assemblages and features of the oceanographic environment also need to be investigated and clarified. The extensive ichthyoplankton data sets for waters off the V.S. northwest and northeast coasts (Morse et aI., 1987; Sibunka and Silverman, 1989; Dunn and Rugen, 1989) afford an opportunity to examine broad scale spatial structure in the larval fish populations of both regions. Vsing these data sets, the present study was undertaken to investigate and compare spatial patterns among species offish larvae in the temperate northeast Pacific and northwest Atlantic oceans and to relate these patterns to the oceanography of each area. The primary purpose of this study is to identify and delineate species assemblages of larval fish in both areas. Although the investigation is essentially exploratory, another objective is to gain some insight into the origin and maintenance of these assemblages with reference to the spawning strategies of the fish species and various features of their oceanographic environment. STUDY AREAS

West Coast.-The survey area extends from 48°N off northwest Washington to 40oN, south of Cape Mendocino off northern California, and from 3 to 200 mi (4.8 to 322 km) offshore (Fig. 2). The oceanography of this region is characterized by the California Current system, a typical eastern boundary current regime (Hickey, 1979, 1989). The bathymetry of the region is characterized by a narrow continental shelf. Off Washington and northern Oregon, the shelf width is less than 70 km, whereas off southern Oregon and northern California it narrows to less than 30 km, reaching a minimum of about 10 km off Cape Mendocino. A series of submarine canyons transect the shelf and slope off Washington and California. These canyons are absent off Oregon where rocky submarine banks are found along the shelf. The easterly flowing West Wind Drift, which traverses the subarctic portion of the northern Pacific Ocean, diverges into a northerly flowing component, the Alaskan Current, and a southerly flowing component, the California Current, as it approaches the North American land mass (Hickey, 1979). The main California Current is slow, meandering, broad, and indistinct (Loeb et aL, 1983b) and, as it proceeds southwards along the U.S. west coast, its subarctic characteristics are modified by heating, evaporation, and intrusion of water from the west and south. Subcomponents of the California Current system include the northerly flowing California Undercurrent and Davidson Current (Hickey, 1989). The California Undercurrent consists of a jet-like poleward flow with a subsurface maximum, whose core appears to be confined to the continental slope. The northerly flowing Davidson Current is a seasonal surface current which prevails on the coastal side of the California Current during winter. A schematic illustration of the deep ocean boundary currents and their seasonal variation in the surface zone off the U.S. west coast is given in Figure 3 (after Hickey, 1989). Coastal surface currents in this region are primarily wind driven and display strong seasonal variability (Huyeret aI., 1975; Hickey, 1979; Strubet aI., 1987). Spring and autumn transitions in prevailing winds and associated coastal currents are driven by large scale changes in atmospheric pressure systems over the north Pacific Ocean. In winter, southerly winds result in the northward flowing Davidson Current, onshore Ekman transport of surface water, and downwelling close to the coast. A transition

DOYLE ET AL.: TEMPERATE

PACIFIC AND ATLANTIC

127°

129°W

OCEAN ASSEMBLAGES

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41°

40°

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Figure 2. Survey area, sampling stations, and bathymetry off the U.S. west coast. Encircled stations are those for which temperature profiles are plotted. Horizontal lines divide the area into three zones for which mean surface temperatures are calculated: Washington, northern Oregon, and southern Oregon/northern California (see Fig. 7).

from southerly to northerly winds occurs during spring, and by summer, prevailing conditions include a southward flowing coastal current, offshore Ekman transport, and upwelling of cold, oceanic water close to the coast. The autumn transition from northerly to southerly winds leads back to the winter conditions. The intensity of Ekman transport and associated upwelling is variable along the northwest coast. Mean monthly upwelling indices (derived from geostrophic wind stress; see Bakun, 1973) for four locations along the l250W meridian, from northern Washington to northern California, are plotted

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50°

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N

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50°

SCC

= CALIFORNIA CURRENT = DAVIDSON CURRENT = SOUTHERN CALIFORNIA

SCE

=

CC DC

40°

COUNTERCURRENT SOUTHERN

CALIFORNIA EDDY 30°

30°

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Figure 3. Seasonal variation in deep ocean boundary currents off the U.S. west coast and Baja California, Mexico: Col. R. = Columbia River; CB = Cape Blanco; CM = Cape Mendocino; PC = Point Conception. in Figure 4 and show that the extent and intensity of upwelling increases from north to south in the study area. Off Washington and northern Oregon, the upwelling season is confined largely to summer while winter is characterized by vigorous downwelling. Along the northern California coast, winter downwelling is weaker and less extensive, and summer upwelling indices are considerably higher than off Washington and northern Oregon. The region of maximum upwelling along the U.S. west coast is between Cape Blanco, off southern Oregon, and Point Conception, southern California, with a local maximum at Cape Mendocino (Parrish et aI., 1981). The oceanography of waters off the U.S. northwest coast is modified significantly by the Columbia River, which divides the Washington and Oregon coastal regions (Hickey and Landry, 1989). The Columbia River is the largest point source offreshwater flow into the eastern Pacific Ocean, and water from it forms a low salinity plume which extends outwards from the river mouth above a shallow « 20 m) halocline (Fielder and Laurs, 1990). The extent and orientation of the plume is variable and subject to seasonal changes in runoff and longshore flow of coastal waters. The period of highest runoff is late spring and early summer with a peak occurring in June (Landry et aI., 1989). During winter, the Columbia River plume lies over the Washington shelf and slope, as a result of onshore Ekman transport and northward geostrophic flow. After the spring transition to upwelling, favorable winds (northerly), and offshore Ekman transport, the summer orientation of the plume is generally farther


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PACIFIC AND ATLANTIC OCEAN ASSEMBLAGES

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Figure 4. Mean upwelling indices (monthly means 1946 to 1989) for four locations along the west coast: northern Washington (0), northern Oregon (*), Oregon/California border (+), and northern California (.). offshore and southerly, extending over much of the deepwater zone off southern Washington and Oregon. In a biogeographical context, the California Current lies within the Transitional Zone between the Pacific Subarctic and Eastern Tropical Pacific oceanic regions (McGowan, 1974). This oceanic zone extends from Vancouver Island to Baja California. The coastal area of this transitional zone is included in the Oregonian province of the Eastern Boreal region described by Briggs (1974). Species diversity is high and many are endemic within the Transitional Zone. Parrish et al. (1981) describe the ichthyofauna of the California Current region as being composed principally of elements of a cold temperate fauna centered off the U.S. northwest coast (area north of Cape Blanco) and a subtropical fauna centered off the southern California-Baja peninsula region. They consider the region between Cape Blanco and Point Conception (region of maximum upwelling) to be transitional in nature, containing elements of both faunas. The region off California is dominated by migratory, pelagic stocks of fish including E ngraulis mordax, Sardinops sagax, M erluccius product us, and Trachurus symmetricus. Sebastes jordani, a semi-pelagic rockfish species, comprises the largest resident stock in this region. Other resident coastal populations include various Sebastes species, Gadus macrocephalus, Ophiodon elongatus, cottids, hexagrammids, and flatfish species of the families Pleuronectidae and Paralichthyidae. Two very important species, both in terms of biomass and exploitability, are the epipelagic Cololabis saira and the semidemersal Anoplopomafimbria. Both these species are associated mainly with the slope region off the North American west coast, but they also extend into deeper water. Apart from a subpopulation of Engraulis mordax, spawning of the migratory pelagic species does not occur or is negligible off Washington and Oregon. In addition, the family Osmeridae is prominent off Washington and Oregon but not off California. Otherwise the ichthyofauna of the northern region is similar to that off California in that the coastal zone is dominated by a diverse assemblage of Sebastes species, cottids, hexagrammids, and various pleuronectid and paralichthyid species. Species composition, however, does vary between the two areas, and northerly and southerly complexes have been identified among the ichthyoplankton (Loeb et aI., 1983a; Moser et aI., 1987). The oceanic zone of the California Current region is characterized by the occurrence of mesopelagic species, mainly of the families Myctophidae and Bathylagidae. Again, a distinct north-south trend in species occurrence and abundance among the meso pelagic group is apparent (Willis, 1984; Moser et aI., 1987). East Coast. - The east coast survey area, comprising the continental shelf and slope between Nova Scotia (44°N) and Cape Hatteras, North Carolina (35°30'N), can be clearly divided into four major

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BULLETIN OF MARINE SCIENCE, VOL. 53, NO.2, 1993

Figure 5. MARMAP survey area and sampling stations, and bathymetry off the east coast. Encircled stations are those for which temperature profiles are plotted. Depths are in meters,

subareas, based on bathymetry and oceanographic patterns (Fig. 5). The first area is the deep, stratified basin of the Gulf of Maine to the north. The second area is the shallow offshore bank, Georges Bank, on the southern flank of the Gulf of Maine. The third is the smooth, gently sloping shelf between Cape Cod, Massachusetts, and Cape Hatteras, North Carolina. The fourth area is the slope waterGulf Stream area, which extends offshore from about the edge of the continental shelf (Ingham et aI., 1982). The Gulf of Maine is surrounded on three sides by land and by Georges Bank on its southern flank. The Gulfwaters are anomalously cold because ofits location in the lee ofthe North American continent and its isolation from the warmer Atlantic waters. It receives oceanic water through a deep, narrow channel on its eastern extreme, the Northeast Channel. Tidal and wind-driven currents in the Gulf of Maine produce a sluggish, cyclonic gyre (Fig, 6) that is affected by a complex bottom topography (Brooks, 1985). The Gulf remains stratified for much of the year, with relatively warm and dense oceanic waters on the bottom and with cooler, fresher waters in the surface layer. Summer warming produces a relatively shallow thermocline, thus producing three water masses throughout much ofthe Gulf (Flagg, 1987). The two near surface layers reflect the seasonal cycles of warming and cooling, while the seasonal range in temperature for the deep layer is relatively slight. Tidal and wind mixing keep the shallow waters around the rim of the Gulf of Maine well mixed all year.


Figure 6.


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Residual surface circulation patterns off the U.S. east coast.

The first description of currents on Georges Bank by Bigelow (1927) was refined by Ingham et al. (1982) and Butman et al. (1987). In general, currents on the bank follow the isobaths in an anticyclonic gyre (Fig. 6) with the center of the bank «60 m) remaining well mixed all year. This keeps the water in the center relatively cool. The edges of the bank stratify during spring and summer with warm water overlying cooler, denser waters. The southern edge of Georges Bank at depths greater than 100 m interacts with the warm, oceanic slope waters. Along this edge, Gulf Stream warm core rings often impinge upon the bank and produce complex patterns of water temperature and currents. The central area ( < 100 m) is characterized by the anticyclonic gyre which intensifies during spring and summer. During autumn and winter, the gyre breaks down and a drift to the west and south occurs. The northern edge of Georges Bank often contains strong frontal zones where the Gulf of Maine gyre abuts the Georges Bank gyre. The continental shelfin the Middle Atlantic Bight area is narrow to the south and widens to about ISO km in the north. General current patterns are southerly alongshore and northerly offshore in the area of the Gulf Stream. Spring and summer warming, as well as freshwater runoff from the major estuaries, produces a warm water layer over a pool of cold water isolated from the influence of surface warming by a thermocline (Ingham et aI., 1982). The cold pool extends over much of this subarea at depths of 40 to 100 m (Ingham et aI., 1982). Autumn turnover occurs in October or November and the shelf waters become isothermal until spring. The shelf-slope front, where warm oceanic water meets the shelf water, moves inshore during the summer to depths of75 to 100 m and offshore during

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Table la.

Year

Sampling coverage on the west coast

Total Cruises stations (No.) (No.)

1980 1981 1982 1983 1984 1985 1987

2 2

I I I

216 248 49 237 124 124 88

Total

10

1,086

I

2

Months sampled Ian

Feb

Mar

Apr

May

lui

100

Aug

Sep

Oct

Nov

Dec

------

7

winter. The location of this front is quite variable and responds to atmospheric forcing and Gulf Stream meanders. Beyond the shelf-slope front is the slope water-Gulf Stream area, where warmer, higher salinity oceanic water mixes with cooler, lower salinity shelf water. The inner slope water moves southwest along the isobaths (Ingham et aI., 1982) and it is here that Gulf Stream warm core rings are often entrained. Offshore of this southwest flow is the northeasterly flowing Gulf Stream, which contains warm, high salinity water originating in the Gulf of Mexico. The major zoogeographic boundaries in the east coast sampling area include Cape Hatteras to the south and Cape Cod to the north. Cape Hatteras marks the boundary between the warm temperate fauna to the south and cold temperate fauna to the north (Briggs, 1974; Grosslein and Azarovitz, 1982; Azarovitz and Grosslein, 1987). The shelf waters from Cape Hatteras to Cape Cod have unusually large seasonal temperature fluctuations that cause many species to migrate in order to stay within their preferred temperature range (Parr, 1933). Approximately three fourths of the 250 species of shorefishes that occur in the southern part ofthe study area in spring and summer move south during autumn and winter (Briggs, 1974). Grosslein and Azarovitz (1982) found that of the 180 species of trawl-caught fishes, only 10 species were endemic to the Middle Atlantic Bight. They found that in the southern part of the bight, over 77% of all species were migratory, warm temperate taxa. As waters warm in the spring, cold temperate species move north and warm temperate species migrate into the area from the south. North ofthe Middle Atlantic Bight, seasonal shifts in species composition decrease dramatically. On Georges Bank, a 10% change occurs from autumn to spring while only a I% change occurs off Cape Sable, Nova Scotia (Azarovitz and Grosslein, 1987). Three families account for most of the adult fish biomass in the study area: Gadidae (cod fishes), Paralichthyidae (lefteye flounders) and Pleuronectidae (righteye flounders). These are largely demersal fishes. Gadidae and Pleuronectidae are cold temperate (boreal) fishes, while Paralichthyidae are warm temperate types. Some gadids, such as Gadus morhua, Merluccius bilinearis. and Urophycis spp. migrate south in winter, but most remain in the northern part of the study area, primarily in the vicinity of Georges Bank and in coastal waters of the Gulf of Maine. Most pleuronectids migrate very little and occur to the north, while some paralichthyids are highly migratory and are most abundant in the Middle Atlantic Bight. The semi-pelagic sand lances (Ammodytes spp.) are abundant throughout the study area and play an important role in the food web as they are major food items in the diets of other fish (Grosslein and Azarovitz, 1982). A population explosion of Ammodytes spp. began in the mid-1970s, and since then sand lance larvae have been outstandingly abundant in east coast ichthyoplankton collections (Smith and Morse, 1985a). A number of seasonally abundant taxa, mainly those that are highly migratory, also characterize the ichthyofauna of the study area. These include the pelagics Clupea harengus and Brevoortia tyrannus (Clupeidae), Scomber scombrus (Scombridae), Pomatomus saltatrix (pomatomidae), and Peprilus triacanthus (Stromateidae), as well as the demersal Centropristis striata (Serranidae), Micropogonias undulatus (Sciaenidae), Prionotus spp. (Triglidae), and members of the sculpin family (Cottidae). METHODS AND MATERIALS Sampling Programs and Protocol. - The National Marine Fisheries Service conducted plankton and oceanographic surveys off the U.S. northwest coast from 1980 to 1987 and off the U.S. northeast coast from 1977 to 1987. Off the west coast, the surveys were carried out by the Northwest and Alaska Fisheries Center (now the Alaska Fisheries Science Center) in Seattle (Dunn and Rugen, 1989). A total of 10 cruises was conducted over a period of 7 years (Table la), of which eight were aboard

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Table I b. Sampling coverage on the east coast

Year

Total Cruises stations (No.) (No.)

1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987

7 6 7 6 8 6 7 9 8 8 6

1,158 867 910 958 971 870 963 1,131 1,151 1,161 850

Total

78

10,990

Months sampled Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

12

Soviet research vessels, as part of a cooperative research program between the U.S. and U.S.S.R. During these cruises, an area of 249,000 km' was sampled (Fig. 2). The sampling grid contained a maximum of 125 stations that were more closely spaced in the shelf and slope zone than in deep water west of the I,OOO-m isobath. Plankton sampling for fish eggs and larvae and hydrographic casts for determination of temperature, salinity, dissolved oxygen, and, in some instances, nutrient concentrations were carried out at all stations. Bongo-net samplers (61-cm diameter frames) were used for plankton sampling. Seasonal coverage was limited in the west coast sampling program (Table la). Six of the 10 cruises were carried out during spring months (March to early June), whereas sampling was restricted to one cruise each in summer (August 1980) and winter (January 1987) and two in autumn (October and November 1981, 1983). The Northeast Fisheries Center of the National Marine Fisheries Service was responsible for the 1977 to 1987 surveys in the northwest Atlantic off the U.S. east coast. The surveys were part of a comprehensive fishery ecosystem study known as MARMAP (Marine Resource Monitoring, Assessment, and Prediction; Sherman, 1980, 1988). MARMAP surveys off the northeastern United States covered continental shelf waters from Cape Hatteras to Cape Sable, an area of 260,000 km2 (Morse et aI., 1987; Sibunka and Silverman, 1989). A total of 78 surveys was carried out over the II-year period, and 10,990 stations were sampled (Table Ib). Approximately 150 to 180 stations, spaced 25 to 35 km apart, were occupied during each survey (Fig. 5). At each station, plankton samples for invertebrate zooplankton and ichthyoplankton were collected using bongo-net samplers. Water samples were taken for measurements of temperature, salinity, dissolved oxygen, nutrients, plant pigments, and primary productivity. Good seasonal coverage was achieved for the east coast program and cruises were relatively evenly distributed throughout spring, summer, autumn, and winter months (Table 1b). The plankton and hydrographic sampling procedure was the same for the west and east coast surveys. Following standard MARMAP procedures (Smith and Richardson, 1977), oblique tows to approximately 5 m from the bottom or a maximum of 200 m depth were carried out at each station using 61-cm frame bongo-net samplers (Posgay and Marak, 1980) to which 0.505-mm mesh plankton nets were attached. The sampler was deployed at 50 m' s -1 and retrieved at 20 m· S-1 at a towing speed of 1.5 to 2 kn to maintain a 45° wire angle. Tow profiles and maximum depth were determined with a bathykymograph. Plankton samples were preserved in a 5% buffered formalin solution. Plankton samples from one of the bongo nets were processed by the Polish Plankton Sorting Center in Szczecin, Poland. Fish larvae were sorted, identified to the lowest taxon possible, enumerated, and measured to the nearest 0.1 mm. Total water filtered by the nets was monitored with flowmeters and catches were standardized to the number oflarvae under 10 m' sea surface. In both survey areas, water samples were collected with Niskin bottles fitted with reversing thermometers. Nominal sampling depths were 0,5, 10, 15,20,25,30,35,50,75, 100,200,250 and 300 m, as water depth permitted. Temperatures integrated over the water column were calculated at each station and weighted according to sampling depths. Data Analysis. -Prior to analysis, the data for each coast were combined into four sets representing spring, summer, autumn, and winter to characterize spatial patterns for the different seasons. For each data set, rare taxa that occurred at less than 3% of all stations or did not contribute significantly to total larval fish abundance were eliminated from the analysis. For the west coast, data from the six spring cruises, March to June 1980 through 1985 (Table 1a),

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were combined, and mean levels of abundance of each larval fish taxon were calculated for station locations within the sampling grid, Summer, autumn, and winter data sets for the west coast are represented by single cruises, August 1980, OctoberlNovember 1981, and January 1987, respectively (Table la). Data from a second autumn cruise (November 1983) were not combined with the October/ November 1981 data sets, as anomalous conditions prevailed along the west coast in the summer and autumn of 1983 due to the strong EI Nino event (meteorological and oceanographic anomaly) that occurred that year (Miller et aI., 1985; Brodeur et aI., 1985). The October/November 1981 cruise results were therefore considered to be more representative of "normal" conditions in the ichthyoplankton during autumn. For the east coast, surveys were grouped into spring, summer, autumn, and winter categories based on water temperatures. The months from January to April are characterized by the coldest average water temperatures « 1O°C) and were assigned to the winter season. Dramatic increases in temperature occurred during May and June, which were considered spring months, and maximum temperatures prevailed from July to September (summer) with a peak in August. October, November, and December, when temperatures decline steadily to winter values, were assigned to the autumn season. According to this scheme, mean abundance levels of the dominant taxa throughout the east coast station grid (Fig. 5) were calculated for spring, summer, autumn, and winter. A multivariate technique of numerical classification was used to examine spatial patterns in the larval fish data. Numerical classification involves grouping similar entities together based on numerical data: in this instance, species abundance among stations (Clifford and Stephenson, 1975; Boesch, 1977). Normal and inverse classifications were carried out on the data sets; that is, both the species and the stations were classified into groups. To reduce the weighting of the most abundant species, the four data sets for each coast that comprised mean levels of abundance of dominant larval fish taxa among the stations were log-transformed. The first step in the numerical classification procedure calculated correlation coefficients for each pair of species or stations in a data set. The Bray-Curtis dissimilarity coefficient was chosen here (Bray and Curtis, 1957). An agglomerative, hierarchical sorting strategy was then used to produce dendrograms depicting clusters of stations and species. The "flexible sorting" strategy was used and a recommended value of -0.25 was chosen as the clustering intensity coefficient (Lance and Williams, 1967; Boesch, 1977). Caution in the interpretation of dendrograms is necessary because the identification of distinct groups in dendrograms is somewhat subjective. Dendrograms tend to overemphasize discontinuities and may force a graded series into discrete classes (Field et aI., 1982). To aid in the identification of groups, the original data sets (species abundance x stations) were rearranged into two-way tables according to the order that species and stations appeared in the dendrograms. In this manner it was possible to see how a chosen group of stations from a dendrogram was characterized by the occurrence or definitive range of abundance of a particular species or group of species. After the final species and station groups were chosen, the two-way tables (species x station group) were reduced by calculating the mean abundance of each species, within the different species groups, for each station group. The station groups were then plotted on maps of the sampling area in order to identify geographically distinct ichthyoplankton assemblages. Indices of constancy and fidelity (Boesch, 1977) were calculated for species in station groups. These indices give a measure of the "distinctness" ofa station group based on species occurrences. Constancy is the ratio of the actual number of occurrences of a species in a station group to the total number of occurrences possible within that group. It has a value of I when the species occurs at all the stations in the group and a value of 0 when the species occurs at none of the stations, Fidelity measures the degree to which a species is limited to or associated with a station group. The index is the ratio of the constancy of the species in the particular station group to the constancy over all stations. It is equal to I when the constancy ofthe species in the station group is equivalent to its overall constancy, greater than I when its constancy in that station group is greater than that overall, and less than I when its within-group constancy is less than its overall constancy. Values greater than 2 suggest a strong association of a species with a station group, whereas values much less than I indicate negative fidelity or "avoidance" by the species of the area represented by the station group. Two-way tables of constancy and fidelity indices for species among station groups are presented along with the twoway coincidence tables of mean abundance of species among station groups for each data set. RESULTS

Temperature and Salinity Data. - WEST COAST. Mean annual surface temperature curves for the continental shelf-slope zone (inshore of the I,OOO-m isobath) and the oceanic zone (> 1,000 m depth) are plotted in Figure 7a and b. Figure 2 shows the three areas for which mean surface temperatures were calculated. Mean annual surface temperatures did not vary significantly with latitude apart from a slight

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o JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Figure 7. Mean annual surface temperature in different zones along the U.S. east and west coasts: a) west coast shelf-slope zone, b) west coast oceanic zone, c) east coast shelf-slope zone. WA = Washington, N-OR = northern Oregon, S-ORIN-CA = southern Oregon/northern California, GOM = Gulf of Maine, GB = Georges Bank, MAB = Middle Atlantic Bight. See Figures 2, 5 for location of zones.

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Figure 8. Surface temperature contours for summer (August 1980; a) off the west coast and winter (January 1987; b) off the west coast.

divergence in the three curves during winter and spring months in the oceanic zone (Fig, 7b), reflecting a small increase in temperature from north to south. Seasonal variation was also slight with surface temperature means restricted largely to the range 10 to 16°C. In the oceanic zone, peak temperatures of approximately 15 to 16°C prevail during August (Fig. 7b), due to summer warming of the surface layers, In contrast, surface temperatures in the shelf-slope zone during summer are moderated by the upwelling of cold oceanic water along the coast. August values do not differ significantly from those recorded during spring and winter months, remaining less than 13°C. Off Oregon and northern California, mean surface temperatures in coastal waters peak during October and November when upwelling has diminished. In contrast, the peak for the Washington coastal region does occur in summer, reflecting the reduced intensity of upwelling in this region. Horizontal patterns of surface temperature during the summer and winter off the west coast are illustrated in Figure 8a and b. These represent the extreme

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conditions in the annual temperature cycle, with spring and autumn being the transitional seasons. In summer, temperature increases from inshore to offshore when cold, upwelled water prevails along the shelf (Fig. 8a). The increasing intensity of upwelling in a southerly direction is shown by the extent of cold water (8 to 12°C) south of Cape Blanco. During winter, a narrow range of surface temperatures (9 to 12.5°C) was recorded throughout the sampling area. A slight increase in temperature is apparent in both an offshore and southerly direction (Fig. 8b). Surface salinity contours for summer and winter off the west coast are plotted in Figure 9a and b. These plots represent seasonal extremes in the extent and orientation ofthe Columbia River plume. In summer (August 1980), the offshore Ekman transport and southerly longshore flow deflects the plume offshore and to the south (Fig. 9a). Low salinity water « 320/00)occurs throughout an extensive area of the slope and oceanic zones off Washington and northern Oregon during August, though the peak period of Columbia River runoff occurs from June to July (Fiedler and Laurs, 1990). Salinities are highest off southern Oregon and

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Figure 10. West coast temperature profiles: a) Washington/northern Oregon shelf zone, b) Washington/northern Oregon oceanic zone, c) southern Oregon/northern California shelf zone, and d) southern Oregon/northern California oceanic zone. See Figure 2 for location of stations.

northern California where coastal upwelling is strongest. During winter and early spring, the low salinity water of the Columbia River plume is restricted mainly to the shelf and slope area off Washington due to northerly longshore flow and onshore Ekman transport (Fig. 9b). Vertical profiles of temperature at four locations (Fig. 2) are plotted for summer (August 1980) and winter (January 1987) in Figure 10. During winter, vertical variation in temperature is similar for both the northern and southern sectors of the sampling area. In the shelf zone, temperature is uniform throughout the water column both off Washington/northern Oregon and southern Oregon/northern California (Fig. lOa, c). In the oceanic zone, temperature remains at about lOoC in the upper 75 m and then decreases gradually to 7 to goC at 200 m, both at northern and southern locations (Fig. lOb, d). During summer, surface warming

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induces a shallow thermocline in the oceanic zone at around 25 m, with water above this being approximately 15 to 16°C. A similar vertical temperature profile is observed over the Washington/northern Oregon shelf during summer (Fig. IDa). However, the temperature decreases steadily from the surface and is about 1DoC at 30 m depth. In contrast, the shelf zone off southern Oregon and northern California is characterized by a lack of vertical temperature structure during the summer (Fig. lOc). Cold oceanic water, less than 10°C, extends throughout the water column reflecting the persistent and intense upwelling that occurs in this region during summer months. EASTCOAST.Mean annual surface temperature curves for the three subareas shown in Figure 5 are plotted in Figure 7c. In contrast to the west coast survey area, seasonal variation is strong and mean monthly temperatures range from 4 to 22°C. In the three areas, minimum temperatures (4 to goC) occur from January to April. During spring and early summer, surface temperatures increase steadily and by August have reached a peak, after which they decline steadily throughout autumn and the early winter months. In contrast to the west coast, latitudinal differences in surface temperature are significant. The Middle Atlantic Bight region is considerably warmer than the Gulf of Maine and Georges Bank, particularly during the summer when its maximum values reach 22°C compared to 16°C for both of the other areas. On average, the difference between monthly mean values in the Gulf of Maine and over Georges Bank is slight, with the former being marginally colder (by 1 to 2°C) than the latter. Horizontal patterns of temperature during summer and winter off the east coast are illustrated in Figure 11a and b. Mean water column temperatures for the months of August and March were used to represent summer and winter, respectively. Integrated water column temperature is more representative of general conditions off the east coast than surface temperature because of strong water column stratification, mainly in summer. During summer, the most obvious trend is an increase in temperature from north to south throughout the sampling area (Fig. lla). The entire Gulf of Maine remains relatively cold « 10°C). Farther south on Georges Bank, the water is warmer and in the center of the Bank the temperature reaches 14 to 16°C. Similar temperatures are recorded along most of

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Figure 12. Surface salinity contours for summer (a) and (b) off the east coast.

the Middle Atlantic Bight shelf except in the southern sector. Here, at depths of less than 50 m, the water is considerably warmer (18 to 26°C) than farther north or offshore. This warm water can be related to solar warming and outflow from the Hudson, Delaware, and Chesapeake estuaries. In winter, integrated temperature values range from 0 to 12°C throughout most of the sampling area and the general trend is for increasing temperature in an offshore direction. The water is warmest beyond the shelf edge where the shelf/slope front separates the warmer oceanic water from the colder shelf waters. Freshwater influence off the east coast includes runoff from the Hudson, Delaware, and Chesapeake estuaries. This is reflected in the surface salinity distribution with lenses of low salinity water (26 to 300/00)occurring in the vicinity of these estuaries (Fig. 12). The volume offreshwater runoff and associated extension of low salinity water is greatest off the Chesapeake estuary. In the rest of the sampling area, salinity increases gradually from 310/00in the coastal zone to 320/00 along the outer shelf and to 33 to 350/00along the shelf edge and over the slope. Freshwater runoff is usually at a peak during spring. Vertical profiles of temperature are plotted for the three station locations marked on Figure 5, representing the Gulf of Maine, Georges Bank, and the Middle Atlantic Bight (Fig. 13). Temperature remains fairly uniform throughout the water column in the three areas during winter with a slight increase, over a range of not more than 2°C, from the surface to the bottom. In summer, strong stratification is prevalent, particularly in the Middle Atlantic Bight, where surface warming results in temperatures as high as 26°C (Fig. 13c). From approximately 35 m depth to the bottom, winter temperatures «8°C) prevail. Marked thermoclines also occur in the Gulf of Maine and on Georges Bank (Fig. 13b), but they are less dramatic because mean surface temperatures generally do not go above 17°C (Fig. 7c). In the shallowest region of Georges Bank «60 m depth), the water column remains well mixed all year and no summer thermocline develops. Larval Fish Composition and Abundance. - WESTCOAST.A list of the dominant taxa of fish larvae, including levels of relative abundance, is given in Table 2. These are the taxa that were included in the numerical classification analysis. The

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Table 2.

Relative abundance of west coast dominant larval fish taxa

Family

Engraulidae Osmeridae Argentinidae Bathylagidae

Chauliodontidae Melanostomidae Myctophidae

Paralepididae Melamphaeidae Scorpaenidae Hexagrammidae Conidae Cyc10pteridae Centrolophidae Paralichthyidae

Pleuronectidae

Species

Common name

Engraulis mordax Unidentified Nansenia candida Bathylagus ochotensis Bathylagus pacificus Bathylagus milleri Chauliodus macouni TaCIOSlOmamacropus Unidentified Diaphus theta Larnpanyclus spp. Lampanyctus ritteri Lampanyctus regalis ProtomyclOphum crockeri Protomyctophum thompsoni Tarletonbeania crenularis Stenobrachius leucopsarus Lestidiops ringens Unidentified Sebastes spp. Sebastolobus spp. Hexagrammos decagrammus Artedius harringtoni Hemilepidotus spinosus Unidentified lcichthys lockingtoni Citharichthys spp. Citharichthys sordidus Citharichthys stigmaeus Errex zachirus Pleuronectes isolepis Eopsetta exilis Pleuronectes vetulus

northern anchovy smelts bluethroat argentine eared blacksmelt slender blacksmelt stout blacksmelt Pacific viperfish longfin dragon fish lanternfishes California headlightfish lampfish broadfin lampfish pinpoint lamp fish California flashlightfish northern flashlightfish blue lanternfish northern lampfish slender barracudina bigscales rockfishes thornyheads kelp greenling scaly head sculpin brown Irish lord lumpsuckers medusafish sanddabs Pacific sanddab speckled sanddab rex sole butter sole slender sole English sole

T01al abundance

7.15 3.04 0.41 7.31 0.84 0.07 0.63 0.22 0.96 10.64 0.36

0.46 0.19 1.68 0.69 4.32 38.90 0.45 0.22 10.29 0.62 0.12 0.14 0.09 0.47 0.14 1.00 0.35 0.27 0.46 0.35 1.70 1.10

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Figure 14. Annual variation in total larval fish abundance for the west and east coast survey areas. Data points for the west coast represent individual cruises except for late spring; April-May = mean abundance over three cruises (1980, 1983, and 1985); May = mean abundance for two cruises (1981 and 1982). East coast data points are mean monthly levels of abundance.

most abundant taxa are Engraulis mordax, the mesopelagic families Myctophidae (lanternfishes) and Bathylagidae (deepsea smelts), the rockfish complex Sebastes spp., and the family Osmeridae. The myctophid Stenobrachius leucopsarus. accounting for 38.9% of all fish larvae caught, is the most numerous species. Other prominent myctophids present are Diaphus theta (10.6%) and Tarletonbeania crenularis (4.3%). Among the bathylagids, Bathylagus ochotensis was much more numerous than any of the other species and accounted for 7.3% of all fish larvae in the bongo-net samples. Engraulis mordax and Osmeridae comprised approximately 7% and 3%, respectively, of total larval fish abundance; Sebastes spp. accounted for 10%. Most of the remaining taxa among the dominants accounted for less than 1%, each, of total numbers. The annual patterns of variation in total larval fish abundance off the west and east coasts are depicted in Figure 14. Because data points are missing for the months of February, June, July, September, and December off the west coast, the plotted line may not accurately represent the prevailing annual pattern. The August data point on this plot consists almost entirely of E. mordax, whose peak spawning period off the northwest coast is essentially from June to August (Doyle, unpubl. observations). Otherwise, the peak spawning period for most species off the west coast is spring, and this is reflected in the late spring peak in total larval fish abundance (Fig. 14). Seasonal variation in abundance among the dominant taxa of west coast fish larvae, most of which are combined into families, is illustrated in Figure 15a and b. The most abundant of these are myctophids and E. mordax (Fig. 15a). Although various myctophid larvae occur in the plankton throughout the year, their densities are at a peak during spring months. In contrast, the spawning season of E. mordax is short and larval fish abundance is at a peak during late summer (Fig. 15a). The remaining less abundant dominant taxa have peak periods of abundance



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15°C) on the Middle Atlantic Bight shelf. This warmer water and associated larval fish assemblages may also be related to the reduced salinities in the vicinity of the Hudson, Delaware, and Chesapeake estuaries (Fig. 22c). The distribution of the oceanic assemblage is primarily related to bathymetry and reflects the occurrence of high salinity water along the shelf edge and slope. The Gulf of Maine assemblage during autumn, defined by the occurrence of high densities of Clupea harengus larvae (station groups 8-10), is again associated with the relatively cold water prevailing in this area (Tables 11, 13; Fig. 23a, b). The Georges Bank assemblage during autumn is species poor and characterized by low to moderate levels of abundance of taxa that are most abundant in the Middle Atlantic Bight area. Its distribution reflects the occurrence of water tem-



635

peratures significantly cooler than those prevailing farther south in the Middle Atlantic Bight (Fig. 23b). The inner (station group 1) and outer shelf (station groups 3,4) components of the Middle Atlantic Bight assemblage observed during autumn appear to be related to spatial variation in surface salinity (Fig. 23c). For instance, the distribution of the inner component dominated by mainly shallow water or estuarine associated species such as Brevoortia tyrannus, Paralichthys dentatus, Scophthalmus aquosus, and Tautogolabrus adspersus (Table 11) reflects the occurrence of low salinity water along the coast from north of Long Island to Cape Hatteras. Assemblage structure in the ichthyoplankton is poor during winter when the east coast catches are dominated by Ammodytes spp. larvae (Tables 12, 13; Fig. 24a). Nevertheless, there appears to be some association between distribution and abundance oflarval fish species and spatial variation in hydrography. The scarcity of Ammodytes spp. larvae in the northern sector of the Gulf of Maine and along the New England coast (station groups 5, 8, 9) coincides with the occurrence of the coldest water «4°C; Fig. 24b). In addition, the occurrence of the myctophid Benthosema glaciale, which defines the oceanic assemblage, is associated with the relatively warm oceanic water occurring along the shelf edge and slope. DISCUSSION

The multispecies spatial patterns which emerged from analysis of the west and east coast ichthyoplankton data sets suggest the existence of persistent and geographically distinct larval fish assemblages off both coasts. Consistent interspecies relationships indicate that the observed distribution patterns result from the synchrony and spatial cohesion among the spawning patterns of various groups of fish species rather than the mere random co-occurrence of their spawning products. Furthermore, the distribution of larval fish assemblages reflects spatial structure in the oceanographic environment and, in some instances, can be related to specific oceanographic features. Such observations concur with those of previous investigations in other geographic areas in which larval fish assemblages or complexes have been identified (Richardson et al., 1980; Loeb et al., 1983b; Moser et a1., 1987; Smith et a1., 1987; Olivar, 1987; Sabates, 1990; Suthers and Frank, 1991). In general, the boundaries of the assemblages identified in both sampling areas change due to seasonal variation in species composition and abundance within the assemblages. Some assemblages, however, remain distinct while others are transient and are manifest only for one or two seasons in the year. The considerable overlap in occurrence of species among the larval fish assemblages, which confers plasticity to the assemblage boundaries, can be attributed to the fluidity of the pelagic environment. Planktonic organisms, including fish larvae, are in a constant state of motion and may migrate vertically in the water column and be transported far beyond the range of their swimming ability by water currents. However, a common feature of most assemblages is their geographical distinctness. Constituent species are consistently associated with clearly defined zones within the two sampling areas. Species belonging to a particular assemblage are usually similar in terms of distributional range, habitat and temporal and spatial spawning patterns of the adults. In their investigation of demersal fish assemblages on the Scotian shelf, Mahon and Smith (1989) stated that assemblage definition is particularly problematic in marine systems which are largely "open." Nevertheless, despite the vagaries of the pelagic environment, assemblage structure is observed among plankton organisms in general, and ichthyoplankton in particular. Moser et al. (1987) re-

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marked that the distinctions among the larval fish species complexes they observed off California are exceedingly clear, considering that the system is embedded in a current that is moving several thousand kilometers each year and mixing with coastal temperate, subarctic, and subtropical waters. Off the west coast, the coastal and oceanic assemblages identified are most distinct and during all seasons contain an exclusive set of definitive species. Osmerids, hexagrammids, cottids, cyclopterids, paralichthyids, and pleuronectids characterize the coastal assemblage, whereas bathylagids, myctophids, and several other mesopelagic species define the oceanic assemblage (Tables 4-8). The coastal assemblage is associated with the narrow con tinental shelf off Washington, Oregon, and northern California where cold temperatures prevail throughout the year and seasonal variation in circulation patterns is strong. The predominant spawning season is from winter to spring, when downwelling and onshore Ekman transport prevail. It seems that many ofthe coastal assemblage species have evolved spawning patterns that result in retention of their larvae in the shelf zone. Some members of the coastal assemblage of larvae, such as the cottids and hexagrammids, are most abundant in the neuston (Doyle, unpubl.). Patterns of surface circulation are most important in terms of regulating the transport of these larvae, and onshore Ekman currents favor retention of their larvae close to the coast. Demersal spawning is also a characteristic prevalent among the coastal assemblage species that aids retention of larvae in the coastal zone. Osmerids, cottids, hexagrammids, and cyclopterids deposit their eggs in the substratum or attach them to rocks or seaweed, a strategy which reduces the duration of planktonic drift in their early life history. The demersal spawning mode has been identified as a major factor contributing to the retention of larvae in shallow water environments and the formation of coastal assemblages of species in various parts of the world, including the Oregon shelf (Richardson et aI., 1980), the shelf off northwest Australia (Young et aI., 1986), coral reefs in the Caribbean Sea (Smith et aI., 1987), the Catalan shelf in the northwestern Mediterranean (Sabates, 1990) and the shelf off southwestern Nova Scotia (Suthers and Frank, 1991). Identification of three major larval fish assemblages off the Oregon coast by Richardson et al. (1980) is particularly relevant to the west coast section of this study. They documented the occurrence of a coastal, a transitional, and an offshore assemblage in this region during March and April from 1972 to 1975. These correspond to the spring slope/transitional and oceanic assemblages offish larvae identified in this paper, both in terms of species composition and distribution. Richardson and Pearcy (1977) and Richardson et aI. (1980) attributed the consistent occurrence of different larval fish assemblages off the Oregon coast to the spawning location of the adults and to the predominate longshore circulation patterns. Further evidence that spawning patterns among coastal assemblage taxa are adapted to prevailing circulation patterns off the west coast is the apparent northsouth trend in larval fish abundance. This trend is most pronounced during spring when the coastal assemblage of larvae is confined largely to the shelf area off Washington and northern Oregon. Farther south, the scarcity of larvae is associated with the region of upwelling and offshore Ekman transport that commences there earlier in the year. It appears that the coastal species avoid spawning in areas of upwelling, thus avoiding offshore transport of their larvae. Parrish et al. (1981) reviewed the reproductive strategies among the coastal fishery species in the entire California Current system and concluded that they show a pattern of correspondence to the major features of surface transport. Off Washington and Oregon, inshore and shelf-dwelling fish species tend to spawn during late winter


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and early spring when surface drift is generally directed toward the coast rather than during the more productive upwelling season. They include the osmerids, hexagrammids, cottids, and pleuronectids, which belong to the coastal assemblage offish larvae that is manifest primarily off Washington and Oregon during winter and spring. The southern portion of the west coast survey area, particularly south of Cape Blanco, corresponds to the northern sector of the maximum upwelling region identified by Parrish et al. (1981). In this region of vigorous upwelling and strong offshore surface transport, there is a paucity of species with pelagic eggs (Parrish et al., 1981), such as Engraulis mordax. paralichthyids, and pleuronectids. Larvae of the demersal spawning osmerids, cyclopterids, hexagrammids, and cottids are also less abundant along this section of the coast. Thus, upwelling and offshore transport help explain the scarcity of members of the coastal larval fish assemblage observed here during the present study. Year-round spawning is predominant among the oceanic assemblage, resulting in the occurrence of their larvae in the plankton during all seasons. This pattern of spawning reflects the relative stability that characterizes the oceanic zone. In contrast with the coastal zone, seasonal variation in oceanographic conditions, including production in the plankton (Perry et al., 1989), is moderate in this essentially oligotrophic realm. Pearcy (1976) found that over a 5-year period, standing stocks of planktonic herbivores, including copepods and euphausiids, were much higher in shelf waters off Oregon than in the oceanic zone. This is due to the high production associated with the summer upwelling along the coast. Here, seasonal variation in herbivore biomass is much greater than in the deep water beyond the shelf. The near steady state type of production offood organisms in the oceanic zone supports the persistence of larvae throughout the year. Some of the dominant myctophids and bathylagids display a strong peak in abundance of larvae during spring. This peak may be related to a spring peak in primary production observed in the oceanic zone off the U.S. northwest coast (Perry et al., 1989). The amplitude ofthis peak, however, is considerably weaker than seasonal variation in primary production in the coastal zone. The majority of the mesopelagic species are most abundant in the southern sector of the oceanic zone during winter and spring and their spatial distributions

suggest that spawning is associated with the warmest water (> 9°C) off the U.S. northwest coast during these seasons. Although the distributional range of most of these species is from Baja California to the Bering Sea (Hart, 1973), and spawning is widespread, the preferred spawning area along the U.S. west coast appears to be off southern Oregon and northern California (Doyle, unpubl.). Among these species, Bathylagus ochotensis. Stenobrachius leucopsarus, and Tarletonbeania crenularis belong to a northern complex of larval fish species, associated with central California, and Protomyctophum crocked belongs to a southern complex, associated with southern California and Baja California (Moser et al., 1987). Their spawning centers are south of our study area, which helps explain the predominance of their larvae off southern Oregon and northern California. In contrast, Protomyctophum thompsoni has a more northerly distribution, its southernmost extreme being off central California (Matarese et al., 1989). Its preferred spawning area is probably off Washington and Oregon (Doyle, unpubl.) and associated with the colder water prevalent in the northern sector of the study area during winter and spring. The distribution of the slope/transitional larval fish assemblage is primarily related to the bathymetric range of the adults. These species, including Sebastes spp., Errex zachirus and Eopsetta exilis. live and spawn on the outer shelf and continental slope off the U.S. west coast (Hart, 1973; Matarese et al., 1989). The

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more widespread distribution and higher levels of abundance of their larvae to the north may be related to the longshore variation in coastal upwelling. Onshore Ekman transport associated with downwelling at the coast is more persistent and stronger off Washington and northern Oregon than farther south. Larvae occurring in this area therefore havlt a better chance of being returned to the nursery areas and adult habitat in slope waters. The Columbia River plume assemblage is characterized by high densities of Engraulis mordax larvae during summer and is associated with the offshore and southerly extension oflow salinity plume water. Spawning of E. mordax off Oregon is associated with warm, near surface waters of the Columbia River plume (Richardson, 1973; Richardson et aI., 1980). Richardson (1973) proposed that a separate spawning stock of the northern anchovy (E. mordax) occurs off Oregon, and that the spawning period is correlated with the time when warm plume water is a dominant oceanographic feature in the area. High levels of macrozooplankton biomass also have been observed in association with the Columbia River plume during summer and may reflect high concentrations of phytoplankton and microzooplankton (Brodeur, 1990). The spawning strategy of E. mordax off the U.S. northwest coast, therefore, seems to take advantage of high levels of plankton production that prevail in the plume during the summer. The association of highest densities of larvae of the myctophids Stenobrachius leucopsarus and Tarletonbeania crenularis with the plume in summer suggests a similar relationship with warm surface temperatures or spatial variation in plankton production. There is no evidence to suggest, however, that myctophid spawning is associated with the surface layer or that their larvae are concentrated in this zone (Richardson, 1973; Doyle, unpub1.). Strong thermohaline and color (from Coastal Zone Color Scanner data) fronts have been observed in association with the edges of the plume (Fiedler and Laurs, 1990) and may represent a physical boundary that enhances the retention of larvae within the plume. Larval fish assemblages off the east coast were strongly related to water temperature and reflect, to a large extent, the zoogeographic structure among the adult fish populations of this area. During all seasons, the Gulf of Maine and Georges Bank assemblages consisted almost exclusively of boreal taxa, and their distribution was associated with the relatively cold water prevalent in these areas. Many of these boreal taxa were also part of the Middle Atlantic Bight assemblage but they formed a northern subcomponent which was associated mainly with the shelf between Long Island and Cape Cod where water was cooler than in the southern sector. During summer, and to a lesser extent in spring and autumn, the diversity of species in the Middle Atlantic Bight assemblage increased considerably with the occurrence of many warm temperate taxa. These taxa formed a southerly subcomponent associated with the warmest water from Cape Hatteras to Long Island. The predominant spawning season among the Gulf of Maine and Georges Bank assemblages is winter and spring, reflecting the affinity of these species for cold water. Clupea harengus is an exception, being an autumn spawner. Its location of spawning, however, ensures that its larvae are retained essentially in the cold waters of the Gulf of Maine and Scotian shelf. All the warm temperate taxa that occur in the Middle Atlantic Bight assemblage have their peak spawning periods in summer or autumn, reflecting their affinity for warm water. In addition, some of the boreal taxa, such as the hakes (Urophycis spp. and Merluccius spp.) and some pleuronectids, that are most prominent in the northern sector of the Middle Atlantic Bight, have a peak abundance oflarvae during summer. They commence


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spawning in spring, however, and are also part of the Gulf of Maine and Georges Bank assemblages during spring and summer. The high degree of seasonal variation in occurrence, distribution, and abundance of larval fish taxa in the Middle Atlantic Bight is due to the high proportion of migrants. Among the most abundant taxa of larvae occurring here, only two are considered endemic as adults: Scophthalmus aquosus and Paralichthys dentatus (Grosslein and Azarovitz, 1982). The rest are seasonal migrants whose population centers of abundance lie either to the north or south of the bight. Grosslein and Azarovitz (1982) concluded that the large temperature fluctuations in shelf waters here serve as strong ecological stimuli and force the migration of fishes. For instance, the large influx and spawning of the warm temperate forms (e.g., Pomatomus saltatrix, Prionotus spp., Peprilus triacanthus, Centropristus striatus, and Scomber scombrus) from the south is associated with rising water temperature in spring and summer. Conversely, some cold temperate forms (e.g., Clupea harengus, Gadus morhua, Melanogrammus aeglefinus, Hippoglossoides platessoides, and Glyptocephalus cynoglossus) move south into the Middle Atlantic Bight during winter, spawn in the northern sector during the cold months, and migrate out of the area when seasonal warming commences. Water temperature, therefore, seems to be a critical environmental factor influencing the migration and spawning patterns of the east coast species and, subsequently, the larval fish assemblages. Water circulation patterns along the east coast may also serve to enhance and maintain the observed structure in larval fish assemblages of this region. The integrity and persistence of the Georges Bank assemblage, particularly during spring and summer, is likely to be related to the anticyclonic gyre that prevails there. The gyre is most intense during spring and summer; therefore larvae of the winter and spring spawners, such as G. morhua, M. aeglefinus, H. platessoides, and Pleuronectes americanus, and the summer spawners S. aquosus and P. ferrugineus, may be retained there by this circulation feature. It is known that the principal spawning grounds for G. morhua and M. aeglefinus are over the eastern sector of Georges Bank and that their larvae are advected by the anticyclonic gyre along the southern flank and subsequently retained in the central shoal part of the bank (Smith and Morse, 1985a; Lough and Bolz, 1989).

Smith and Morse (1985a) also concluded that M. aeglefinus larvae originating on the bank are largely retained there and that the Georges Bank larvae do not intermix with larvae spawned in other locations such as the Gulf of Maine and the Scotian shelf. The cyclonic Gulf of Maine gyre may also contribute to the occurrence and maintenance of the larval fish assemblage identified here. This applies mainly to Sebastes spp. and Clupea harengus larvae, the definitive species in the Gulf of Maine during spring/summer and autumn, respectively. Sherman et al. (1984) identified Sebastes spp. as a gyre-associated (Gulf of Maine gyre) spawner in their review of the spawning strategies of fish species off the northeastern United States. The distinction between the Gulf of Maine and Georges Bank assemblages may be enhanced by the frontal zone associated with the northern edge of Georges Bank between the two gyres. The strong fronts that often occur here may act as a boundary to the movement of larvae. The station groupings and plots of temperature and salinity, particularly for spring and summer, suggest the occurrence of a biological and physical discontinuity between Georges Bank and Gulf of Maine waters (Figs. 21, 22). The other persistent frontal feature off the east coast is the shelf-slope front which separates oceanic water from shelf water. Although the location of this front is seasonally variable and is influenced by atmospheric forcing and Gulf Stream meanders, its persistence probably contributes to the

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retention of larval fish on the shelf, particularly along the Middle Atlantic Bight. The clear distinction between the species-rich shelf assemblages and the speciespoor, marginal oceanic assemblages observed here may be enhanced by this structure. Again, the station groupings and plots of temperature and salinity reflect the persistence of a biological and physical discontinuity along the shelf edge/slope region (Figs. 21-24). Although the concept of frontal retention cannot be generalized to all pelagic fish larvae (Taggart et aI., 1989), coastal fronts have been identified as barriers to advection oflarvae in various parts of the world (Iwatsuki et ai., 1989; Nakata, 1989; Stephenson and Power, 1989). Colvocoresses and Musick (1984) used numerical classification to investigate community structure in the demersal fish fauna ofthe Middle Atlantic Bight shelf, based on 9 years of survey data collected between winter/spring and autumn. The consistent species associations and faunal zones which they identified tended to follow isotherms and isobaths and were comparable to the Middle Atlantic Bight assemblages described here, despite the restriction to bottom-dwelling species. They concluded that continental shelf communities in the Middle Atlantic Bight are largely structured by temperature on the inner and mid-shelf and by depth on the outer shelf and shelf break. Ichthyoplankton assemblages off the east coast show a measure of temporal correspondence with prevailing patterns in zooplankton standing stocks (Sherman et aI., 1984). In the Gulf of Maine area, there is a prolonged spring-throughautumn peak copepod standing stock with corresponding peak abundance of Sebastes spp. larvae in spring/summer and Clupea harengus larvae in autumn. The spring Georges Bank assemblage of larvae coincides with a spring peak in zooplankton there. The northern subcomponent of the Middle Atlantic Bight larval fish assemblage, dominated by various Urophycis and Merluccius species, may be exploiting a spring and late summer peak in cope pod abundance in the New England section of the shelf. Farther south, larvae belonging to the southern subcomponent of this assemblage are most abundant during summer and autumn, in synchrony with the later peak in production there. It is interesting to note that Ammodytes larvae, which cannot be assigned to any assemblage in particular, are most abundant during winter when production in the plankton is at a minimum everywhere. Sherman et aI. (1984) concluded that the ubiquitous nature oftheir distribution, plus their relatively large size and dominance in the ichthyoplankton, compensates for the low densities of zooplankton prey organisms prevalent at this time of year. The larval fish assemblages identified off the northwest and northeast U.S. coasts in this study suggest that adaptive commonality exists among the spawning patterns offish taxa in both areas. Co-evolution among the fishes' spawning strategies within the complex and variable marine ecosystems may have given rise to the high degree of structure observed in the ichthyoplankton spatial patterns. In support of this claim is Colvocoresses and Musick's (1984) observation regarding demersal fish communities on the Middle Atlantic Bight shelf; for the most part, these communities are structured by species associations that behave as a group in response to environmental variation. A complex of environmental variables exerts selective pressure on the spawning strategies of fish species but the degree of influence of individual factors probably varies among areas. Likely factors operative in both survey areas include current patterns, water temperature, estuarine influences, and plankton production cycles. Off the west coast, circulation patterns (particularly those associated with the upwelling regime) seem to be the most influential factor, especially among the



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coastal species, whereas seasonal variation in water temperature and zooplankton production seem most influential off the east coast. Retention of larvae in fixed geographical locations may be more important as a survival strategy than widespread dispersal of larvae in both survey areas. Retention mechanisms may contribute to the development and maintenance of multispecies assemblages of larvae. This is true for the coastal and, to a lesser degree, the slope/transitional assemblages off the west coast, as well as the Gulf of Maine and Georges Bank assemblages off the east coast. The converse, widespread placement and advection of spawning products, seems to hold true mainly for the mesopelagic species, as observed in the oceanic zone off the west coast, but also to a certain extent for taxa such as Ammodytes spp. and Merluccius and Urophycis species off the east coast. Though patterns of distribution and abundance of larval fish species are obviously functional and contribute to survival, it is difficult to conclude from this study that the multi species associations, and thus the assemblages themselves, are of adaptive significance. Further investigations are necessary to determine the actual degree of species association, particularly on a small scale, within an assemblage and the ecological interactions that may prevail. Sampling at a finer scale than that achieved in our surveys would be required. The vertical distribution patterns oflarvae are particularly important. Larvae occurring together in a bongonet sample may inhabit different depths in the water column and therefore not interact. Ontogenetic variation in ichthyoplankton assemblages is also worthy of investigation. For instance, investigating multispecies spatial patterns among eggs and different size or age groups oflarvae, independently, is likely to provide additional information on advection/retention processes and the contribution of behavior to the occurrence and maintenance of larval fish assemblages. Another important consideration in the investigation and interpretation of larval fish assemblages is the relative scarcity of fish larvae in the plankton. Larval fish are part of a much larger community of zooplankton and tend to be rare compared to other taxa (McGowan and Miller, 1980). Many species of invertebrate macrozooplankton dominate their biotic environment and may prey on larval fishes or compete with them for the same microzooplankton prey organisms. Levels of abundance and patterns in distribution of fish larvae may be affected significantly by the patterns in distribution and abundance of other zooplankton organisms. The implication is that it may not be valid to consider a larval fish assemblage as a distinct and independent ecological entity. It may be necessary to focus on the zooplankton assemblage in its entirety, of which the ichthyoplankton is only a small part, to understand fully the multi species spatial patterns that prevail among larval fishes. ACKNOWLEDGMENTS We would like to thank S. Picquelle of the Alaska Fisheries Science Center, National Marine Fisheries Service, Seattle, for assistance with computer programming and data analysis. In addition, T. Brown of the Alaska Fisheries Science Center and T. Vance of the Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Seattle, helped with the plotting and presentation of the west coast data. LITERATURE CITED Azarovitz, T. R. and M. D. Grosslein. 1987. Fishes and squids. Pages 316-346 in R. H. Backus and P. W. Bourne, eds. Georges Bank. MIT Press, Cambridge, Massachusetts.

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Bakun, A. 1973. Coastal upwelling indices, west coast of North America, 1946-71. NOAA Tech. Rep, NMFS SSRF-671. 103 pp. Bigelow, H. B. 1927. Physical oceanography of the Gulf of Maine. Fish. Bull. U.S. 40: 511-1027, Boesch, D. F. 1977. Application of numerical classification in ecological investigations of water pollution. Environ. Prot. Agen. Ecol. Res. Ser. EPA-600/3-77-033. 115 pp. Bray, J. R. and J, T, Curtis. 1957, An ordination of the upland forest communities of southern Wisconsin. Ecol. Mono. 27: 325-349. Briggs, J. C. 1974. Marine zoogeography. McGraw-Hili Series in Population Biology. McGraw-Hili, New York. 450 pp. Brodeur, R, D. 1990, Macrozooplankton abundance and distribution along the Oregon and southern Washington coasts: May-August, 1981. University of Washington Fish. Res. Inst. Tech, Rep, 9003.33 pp. ---, D. M. Gadomski, W. G. Pearcy, H. P. Batchelder and C. B. Miller. 1985. Abundance and distribution of ichthyoplankton in the upwelling zone off Oregon during anomalous EI Nino conditions. Estuarine Coastal Shelf Sci. 21: 365-378. Brooks, D. A. 1985. Vernal circulation of the Gulf of Marine. J. Geophy. Res, 90C: 4687-4705. Butman, B., J. W. Loder and R. C. Beardsley. 1987. The seasonal mean circulation: observation and theory. Pages 125-138 in R. H. Backus and P. W. Bourne, eds. Georges Bank. MIT Press, Cambridge, Massachusetts. Clifford, H. T. and W. Stephenson. 1975. An introduction to numerical classification. Academic Press, New York. 229 pp. Colvocoresses, J. A. and J. A. Musick. 1984. Species associations and community composition of Middle Atlantic Bight continental shelf demersal fishes. Fish. Bull., U.S. 82: 295-313. Dunn, J. R. and W. C. Rugen. 1989. A catalog of Northwest and Alaska Fisheries Center ichthyoplankton cruises, 1965-1988. NOAA Proc. Rep. NMFS-F/NWAFC-89-04. 87 pp. Fiedler, P. C. and R. M. Laurs. 1990. Variability of the Columbia River plume observed in visible and infrared satellite imagery. Int. J. Remote Sens. 11: 999-1010. Field, J. G., K. R. Clarke and R. M. Warwick. 1982. A practical strategy for analysing multispecies distribution patterns. Mar. Ecol. Prog. Ser. 8: 37-52. Flagg, C. N. 1987. Hydrographic structure and variability. Pages 108-124 in R. H. Backus and P. W. Bourne, eds. Georges Bank. MIT Press, Cambridge, Massachusetts. Frank, K. T. and W. C. Leggett. 1983. Multispecies larval fish associations: accident or adaptation. Can. J. Fish. Aquat. Sci. 40: 754-762. Grosslein, M. D. and T. R. Azarovitz. 1982. Fish distribution. MESA New York Bight Atlas Monograph 15. New York Sea Grant Institute, Albany, New York. 182 pp. Hart, J. L. 1973. Pacific fishes of Canada. Bull. Fish. Res. Board Can. 180. 740 pp. Hewitt, R. 1981. The value of pattern in the distribution of young fish. Rapp. Reun., Cons. Int. Explor. Mer 178: 229-236. Hickey, B. M. 1979. The California Current system-hypothesis and facts. Prog. Oceanog. 8: 191279. ---. 1989. Patterns and processes of circulation over the shelf and slope. Pages 41-116 in M. R. Landry and B. M. Hickey, eds. Coastal oceanography of Washington and Oregon. Elsevier, Amsterdam. -and M. R. Landry. 1989. Coastal oceanography of Washington and Oregon: a summary and a prospectus for future studies. Pages 595-599 in M. R. Landry and B. M. Hickey, eds. Coastal oceanography of Washington and Oregon. Elsevier, Amsterdam. Huyer, A., R. D. Pilsbury and R. L. Smith. 1975. Seasonal variation of the alongshore velocity field over the continental shelf off Oregon. Limnol. Oceanog. 20: 90-95. Ingham, M. C. ed., J. L. Chamberlin, S. K. Cooke, D. G. Mountain, R. J. Schlitz, J. P. Thomas, J. J. Bisangi, J. F. Paul and C. E. Warsh. 1982. Summary of the physical oceanographic processes and features pertinent to pollution distribution in the coastal and offshore waters of the Northeastern United States, Virginia to Maine. NOAA Tech. Memo. NMFS-F/Nec-17, 166 pp. Iwatsuki, Y., H. Nakata and R. Hirano. 1989. The thermohaline front in relation to fish larvae. Rapp. Reun., Cons. Int. Explor. Mer 191: 119-126. Lance, G. N. and W. T. Williams. 1967. A general theory of classificatory sorting strategies. 1. Hierarchical systems. Compo J. 9: 373-380. Landry, M. R., J. R. Postel, W. K. Peterson and J. Newman. 1989. Broad-scale distributional patterns of hydrographic variables on the Washington/Oregon shelf. Pages 1-40 in M. R. Landry and B. M. Hickey, eds. Coastal oceanography of Washington and Oregon. Elsevier, Amsterdam. Loeb, V, J., P. E. Smith and H. G. Moser. 1983a. Ichthyoplankton and zooplankton abundance patterns in the California Current area, 1975. Calif. Coop. Oceanic Fish. Invest. Rep. 24: 109131.

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