As part ofa Gulf-wide ichthyoplankton survey of the GulfofMexico in the spring of ... Ichthyoplankton surveys have been conducted in the Gulf of Mexico to define.
BULLETIN OF MARINE SCIENCE, 53(2): 475-537, 1993
LARV AL FISH ASSEMBLAGES AT THE LOOP CURRENT BOUNDARY IN THE GULF OF MEXICO W J. Richards, M. F. McGowan, T. Leming, J. T. Lamkin and S. Kelley ABSTRACT As part ofa Gulf-wide ichthyoplankton survey of the GulfofMexico in the spring of 1987, eight transects were made across the Loop Current boundary using bongo nets fished to 200 m depth and neuston nets fished at the surface. The boundary was determined by satellite images of the Loop Current and the ship was positioned to make a transect across the boundary whenever it approached a boundary during its normal survey operations. Eight transects were made and the composition and abundance of the fish larvae were determined for each tow. Eight to 10 ten tows were made at 2-to 4-km intervals, Taxonomic diversity of the ichthyoplankton was higher than previously reported for the Gulf of Mexico or Caribbean (100 families). Cluster analysis of families produced two major groups, oceanic and continental, but our hypothesized frontal assemblage is not coequal with the oceanic and shelf groups. Cluster analysis of stations also supported the hypothesis of contrasting oceanic and shelf assem blages, Principal-components analysis found more than one-half of the variance in the data to be summarized by three independent patterns. The high diversity of larval fishes is due to the mix offaunas from tropical and warm temperate oceanic, mesopelagic, and coastal demersal and pelagic species which is enhanced by the dynamics of the oceanographic system of the Loop Current.
Ichthyoplankton surveys have been conducted in the Gulf of Mexico to define spawning areas, times, and magnitude of larval populations since 1982 as part of the SEAMAP (Southeastern Area Monitoring and Assessment Program) program (Richards et a1., 1984; Kelley et a1., 1986, 1990). Particular emphasis has been placed on surveys in the spring of the year because more species spawn then and an important commercial and recreational species, the bluefin tuna, also spawns then. In recent years two surveys have been made between 15 April and 15 June in waters of the U.S. exclusive economic zone (EEZ) during the blue fin spawning season. Bluefin larval distributions indicated that the spawning was associated with the Loop Current boundary, the major hydrographic feature in the Gulf. The possible reasons for this association were not clear so in 1987 10 additional days were made available to define the blue fin distribution better for refinement of future surveys and to learn the processes which account for interannual differences in larval abundance and the association with the Loop Current boundary. The Loop Current boundary is clearly visible in satellite images prior to the Gulf surface becoming isothermal in May, thus enabling us to position the vesse1. At each feature plankton was collected across the boundary and the resulting fish larvae were analyzed. Part of the results from this study emphasizing the bluefin tuna has been completed (Richards et a1., 1989). This paper reports on the results of an examination of all fish larvae collected during a series of experiments where the vessel was directed to eight different hydrographic features determined from real-time satellite images of the Loop Current. The motivation for analyzing assemblages rather than single species stems from three different, although somewhat hierarchical objectives: description of apparent assemblages, hypothesis testing of their validity, and discerning their ecological dependencies. First, there must be a description of the specific composition of the assemblage or assemblages and their consistency, variability with time and space, or other pattern of occurrence. Second, the hypothesized and described 475
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BULLETIN OF MARINE SCIENCE, VOL. 53, NO.2,
1993
patterns should be confirmed objectively with statistical methods. This may in turn lead to subsequent hypotheses about the organization and function of the assemblages. Common lines ofinvestigation are determination of biogeographical provinces and boundaries, ecotone effects on species, and water mass or assemblage indicator species. For example, we hypothesized the existence of coastal, frontal, and oceanic assemblages across the front between the Gulf of Mexico coastal water and the tropically-derived Loop Current water. Third, the assemblages may be under environmental constraints such as limiting temperature or salinity ranges, advective convergence or retention, starvation/predation or competitive effects. Once objectively determined assemblages have been characterized then studies of their ecological control can be investigated just as in studies of single species. The assemblages can be studied as units whose individual taxa respond similarly to the environment without necessarily invoking emergent community properties. Background Broad scale ichthyoplankton studies were not carried out in the Gulf of Mexico until 1971 with the beginning of the interdisciplinary EGMEX and Western Continental Shelf Program from 1971 to 1972 (Rinkel, 1974). Ichthyoplankton surveys were continued by the University of Miami through 1974 for a total of 17 cruises in the eastern Gulf of Mexico (Houde et al., 1979). These resulted in a number of basic landmark publications on larval fish abundances and potential fisheries exploitation (see the series of papers by Edward D. Houde and his co-workers and students as listed in Houde et al., 1979). These studies focused on some of the principal clupeoid species and developed estimates of spawning stock size which agreed well with fishery-dependent estimates. Concurrent with the eastern Gulf surveys, a Caribbean-wide ichthyoplankton survey was made in 1972 and 1973 which covered both the cool and warm seasons (Richards, 1981, 1984). This study revealed a wide diversity, but no dominant group except for myctophids and gonostomatids. Only a few ichthyoplankton studies were carried out in the Gulf until the start of the SEAMAP Program in 1982. These included larval studies by Eldred and her colleagues (Eldred, 1966, 1967a, 1967b, 1969a, 1969b; Eldred and Lyons, 1966) which mainly concerned descriptions oflarval stages of those species with leptocephalous early life stages and focused on Florida waters. Surveys by Cuba in the southern Gulf emphasized scombrid distribution and abundance (Juarez, 1975); off Texas, Finucane et al. (1979) described distributions for all fish larvae; nearshore surveys off Louisiana concerned distribution and abundance of nearshore larvae, principally engraulids, clupeids and sciaenids (Ditty, 1986; Ditty and Truesdale, 1984); off the Florida Everglades, Collins and Finucane (1984) provided a list of species found and their abundance. Later studies of Florida Bay by Powell et al. (1989) described seasonal distributions and abundances. Off Mississippi, Stuck and Perry (1982) and Shultz and Richardson (1985) described the distribution of larvae in the nearshore environment, while several more recent studies have been conducted to describe the larval habitat associated with the Mississippi River plume (Govoni et al., 1989; Grimes and Finucane, in press) and distributions of king mackerel (Grimes et al., 1990). Nearshore at Campeche, Mexico, Flores Coto and Alvarez Cadena (1980) described the distribution and abundance oflarvae; and surveys to determine bluefin tuna spawning areas were described by Richards and Potthoff (1980a, 1980b). Species assemblages and interactions of predominant reef species were studied by McGowan (1985). His study focused on the Flower Garden Banks off Texas in the northern
RICHARDS ET AL.: ASSEMBLAGES IN THE GULF OF MEXICO
477
Gulf and the effects oil drilling could have on early life history stages of fish with an emphasis on comparing different ecological zones. The SEAMAP program has broad coverage of the U.S. EEZ of the Gulf with a Gulf-wide multivessel survey annually each spring plus multivessel surveys targeting specific areas and times each year. Cruises have targeted spawning times of mackerels and sciaenids in late summer and early fall along the northern Gulf coast. Summary reports have been produced for each year and the surveys are continuing (Richards et al., 1984; Kelley et al., 1986, 1990); detailed reports analyzing the data are forthcoming (Shaw and Drollinger, 1990a, 1990b). The main objective of the SEAMAP Program is to describe the spawning times and abundances of eggs and larvae for the Gulf of Mexico. Concurrent with SEAMAP and with the same objective, Mexico has initiated broad ichthyoplankton surveys of the Mexican EEZ. Results of that work are just appearing (Olvera et al., in press). Work has been done on other aspects of early life history of fishes in the Gulf of Mexico, but the amount of work is too voluminous to cover here. Much of that work is mentioned in the publications cited above. Taken as a whole our knowledge of the distribution and abundance of early life history of fishes in the Gulf is well advanced considering the high number of species and the large number of early life stage habitats. Both contribute to a very complex system. No one has combined all the historical information into a cohesive study, but this probably could yield a good picture ofGulfichthyoplankton. The main problem is that most of the studies identify only to the family level, thus limiting some of the possible interpretations. As work advances and more specimens are collected, filling in gaps in larval series, the taxonomic or identification problems should be solved, thus yielding greater analytical precision. A better understanding of the interactions of early life history stages can be achieved only through identification of all taxa in this species-rich environment. This must be coupled with discrete depth sampling within definable zones of habitat. There has been limited discrete depth sampling with the exception of some of the Mississippi River plume work and some coastal studies. Our work, reported here, is the first to study all taxa from the offshore Loop Current area. We have done additional work using discrete depth nets which is still being analyzed and will be reported on in the future. MATERIALS AND METHODS Plankton samples were collected from the R/V OREGON II during an ichthyoplankton survey of the U.S. EEZ of the Gulf of Mexico on cruise 166 from April 15 to May 22, 1987. At each station CTD casts to 200 m, expendable bathythermograph (XBT), neuston net collections (not reported here), and our experiment samples were collected using 60 cm paired bongo nets fitted with 0.333 mm mesh aperture netting. The nets were towed to 200 m or to within 5 m of the bottom in stations shallower than 200 m with the vessel holding a 45° wire angle (usually 1.5 kn vessel speed) and a winch retrieval rate of 20 m per minute. The samples were fixed immediately in 10% Formalin buffered with marble chips and transferred to 70% ethanol within 48 h. The samples were returned to the laboratory where plankton volumes were determined followed by sorting the entire sample for fish eggs, larvae, and juveniles, lobster phyllosomes, and cephalopod larvae. Only the right bongo sample was processed as the left sample was archived. For the experiment, eight transects were made across the Loop Current boundary (Table I, Fig. I). The locations of the eight transects were selected by examining real-time satellite images of frontal features, relaying this information to the vessel as it proceeded along the ichthyoplankton survey grid. At six of the transects (1-6) stations were at 2-km intervals and at two transects (7-8) they were at 4-km intervals. Since the normal cruise track of the survey followed the same course as Transect 6, additional stations at greater intervals were included in the transect (Table I). Transect 4 was a short transect with only seven stations. Although II stations were taken on Transect 5, the ichthyoplankton sample from station 45374 was lost and could not be included in the analysis. The samples from the ichthyoplankton survey were processed at the Plankton Sorting and Identification Center, Szezecin,
Figure I. Satellite images of the Loop Current system with locations of the eight transects shown by crosses (+) and roman numerals. The two images shown depict a period of sharp boundaries (upper) on 29 April 1987 and a period of weak boundaries (lower) on 13 May 1987.
479
RICHARDS ET AL.: ASSEMBLAGES IN THE GULF OF MEXICO
Table 1. Station data
Station
Latitude
fOl
the eight transects across the Loop Current Boundary
Time (GMT)
Tow depth (m)
Pl.nkton volume per 1,000 m'
Se. surface temper-
ature
Larv. abundance Number under 10 m2
Number of ta ••
Longitude
Month
D.y
Transect 1 45230 N26-00 45231 N26-00 45232 N26-00 45233 N26-00 45234 N26-00 N26-00 45235 45236 N26-00 45237 N26-00 45238 N26-00 45239 N26-00 45240 N26-00
W084-00 W083-58 W083-56 W083-54 W083-52 W083-50 W083-48 W083-46 W083-44 W083-42 W083-40
4 4 4 4 4 4 4 4 4 4 4
20 20 20 20 20 20 20 20 20 20 20
614 719 833 933 1036 1143 1251 1349 1459 1550 1644
125 125 120 110 105 100 95 85 80 65 65
156.9 332.2 398.4 441.8 416.8 433.5 419.8 564.6 537.6 658.3 541.1
24.5 24.4 24.3 24.1 23.6 22.9 21.7 21 21 21.4 21.4
813 1,418 1,481 2,780 3,149 2,066 2,265 2,246 1,190 1,486 1,380
36 31 43 44 41 40 34 23 22 24 25
Transect 2 45252 N24-30 N24-30 45253 45254 N24-30 45255 N24-30 N24-30 45256 45257 N24-30 N24-30 45258 N24-30 45259 N24-30 45260 45261 N24-30 45262 N24-30
W084-00 W083-58 W083-56 W083-54 W083-52 W083-50 W083-48 W083-46 W083-44 W083-42 W083-40
4 4 4 4 4 4 4 4 4 4 4
21 21 21 21 21 21 21 21 21 21 21
659 813 928 1051 1159 1311 1440 1546 1656 1802 1914
200 200 200 200 200 200 200 200 200 200 200
114.7 89.3 127.3 183.0 246.0 330.5 267.3 246.0 211.4 193.2 165.2
25.6 25.3 25.4 24.9 24.9 24.9 24 23 23 23.1 23.2
732 683 1,049 1,222 1,942 2,199 616 693 838 741 693
36 39 39 42 38 40 23 23 22 20 23
Transect 3 45287 N25-50 N25-52 45288 45289 N25-54 45290 N25-56 45291 N25-58 45292 N26-00 45293 N26-02 45294 N26-04 45295 N26-06 45296 N26-08 45297 N26-10
W086-00 W086-00 W086-00 W086-00 W086-00 W086-00 W086-00 W086-00 W086-00 W086-00 W086-00
4 4 4 4 4 4 4 4 4 4 4
24 24 24 24 24 24 24 24 24 24 24
37 212 421 543 652 801 910 1022 1126 1229 1326
200 200 200 200 200 200 200 200 200 200 200
115.7 185.9 181.0 160.3 160.1 201.2 122.2 72.8 90.7 64.0 55.2
27 27 26 25 26 24 24.1 23.5 23.4 24 23
757 1,301 931 921 974 1,012 713 772 578 448 559
41 42 40 36 37 30 34 20 26 19 20
Transect 4 45350 N26-48 45351 N26-50 45352 N26-52 45353 N26-54 45354 N26-56 45355 N26-58 45356 N27-00
W088-00 W088-00 W088-00 W088-00 W088-00 W088-00 W088-00
5 5 5 5 5 5 5
3 3 3 3 3 3 3
613 714 821 926 1031 1130 1229
170 217 213 200 217 217 200
205.2 88.8 250.0 89.4 169.5 73.3 48.5
26.2 26.1 25.9 25.9 25.8 25.2 24.4
2,352 1,983 2,401 814 819 651 642
51 44 44 35 29 23 21
Transect 5 45370 N26-00 45371 N26-00 45372 N26-00 45373 N26-00 45374 N26-00 45375 N26-00 45376 N26-00 45377 N25-59.7
W085-1O W085-08 W085-06 W085-03 W085-02 W085-00 W084-57.9 W084-55.8
5 5 5 5 5 5 5 5
9 9 9 9 9 9 9 9
13 126 239 346 507 637 807 922
190 190 200 189 200 213 200 200
123.5 55.9 69.4 42.3 18.4 202.9 71.3 170.0
27.1 26.7 25.8 25.5 25.1 25.1 25.1 27.4
447 630 965 800
18 24 30 29
907 818 1,189
25 21 27
('C)
480
BULLETIN OF MARINE SCIENCE, VOL. 53, NO.2,
1993
Table 1. Continued
Station
45378 45379 45380
Latitude
Longitude
Month
N25-59.5 N25-58.9 N25-59.8
W084-53.7 W084-51.8 W084-50.3
5 5 5
Transect 6 45388 N25-00 45391 N26-00 45393 N26-59.9 45395 N27-23.9 45396 N27-26 45397 N27-28 45398 N27-30 45399 N27-32 45400 N27-34 45401 N27-35.9 45402 N27-38 45403 N27-39.9 45404 N27-42.1 45405 N27-44.1 45406 N28-00
W086-00 W086-00 W085-59.9 W086-00.6 W086-00 W086-00 W086-00 W086-00 W086-00 W085-59.9 W085-59.9 W085-59.7 W085-59.9 W086-00 W086-00
5 5 5 5 5 5 5 5 5 5 5 5 5 5 5
Transect 7 45413 N27-55 45414 N27-51 45415 N27-47 45416 N27-43 45417 N27-39 45418 N27-35 45419 N27-31 45420 N27-27 45421 N27-23 45422 N27-19 45423 N27-15
W087-00 W087-00 W087-00 W087-00 W087-00 W087-00 W087-00 W087-00 W087-00 W087-00 W087-00
Transect 8 45470 N28-14 N28-IQ 45471 45472 N28-06 45473 N28-02 N27-58 45474 45475 N27-54 45476 N27-50 45477 N27-46 45478 N27-42 45479 N27-38 45480 N27-34
W085-48 W085-48 W085-48 W085-48 W085-48 W085-48 W085-48 W085-48 W085-48 W085-48 W085-48
Day
Time (GMn
Tow depth (m)
Plankton volume per I,OOOmJ
Sea surface temper-
ature ("C)
Larva abundance Number under 10 m'
Number of taxa
9 9 9
1038 1151 1311
200 200 200
99.7 94.6 89.5
27.5 27.4 27.5
1,159 535 707
30 16 19
10 12 12 12 12 12 12 12 12 12 12 12 12
2247 1032 1938 141 254 400 509 615 730 838 948 1056 1203 1307 1537
200 200 200 200 154 182 196 217 200 200 200 193 200 197 178
25.7 23.2 55.6 87.2 112.8 111.0 68.0 90.0 123.0 120.0 94.7 110.6 37.7 63.2 58.3
28.6 28.3 26.7 26.5 25.1 25.1 23.3 23.3 25.2 26.3 24.1 25 25.3 25.6 24.6
447 395 396 1,228 1,020 1,046 1,192 997 13,296 980 1,225 1,019 955 340 457
22 22 29 23 24 29 25 33 29 25 30 23 19 17 22
5 5 5 5 5 5 5 5 5 5 5
13 13 13 13 13 14 14 14 14 14 14
1812 1935 2056 2216 2336 48 205 325 450 620 801
200 203 200 187 196 200 200 182 197 217 182
47.1 64.0 46.4 87.4 136.5 90.2 119.4 92.4 48.6 127.7 153.8
25 25.7 27.3 27.6 27.9 27.7 27.7 25.5 27.7 27.7 27.9
585 561 484 545 504 575 1,125 894 494 1,734 1,419
21 20 17 17 16 28 38 31 25 58 53
5 5 5 5 5 5 5 5 5 5 5
21 22 22 22 22 22 22 22 22 22 22
2256 16 137 259 418 535 706 828 947 1114 1241
200 200 200 178 182 189 210 200 202 200 200
15.0 43.8 53.7 57.0 90.0 47.1 116.8 138.5 125.1 113.1 245.1
27.4 29 27.3 26.5 27.1 27.9 27.7 28.9 28.8 28.6 27.9
839 1,299 1,463 1,494 989 795 1,278 1,011 1,191 1,139 809
22 15 18 26 41 38 36 40 33 37 29
II II
Poland, but the 87 experiment samples were all processed at the Southeast Fisheries Science Center in Miami by the authors. Larval Fish identifications. - Identification problems of early life history stages are very pronounced in the Gulf of Mexico, but significant progress is being made. Richards (1990) summarized the available information for identification for the entire western central Atlantic, including the Gulf. There are close to 200 families of fish with over 1,700 species whose early life history stages may occur in the Gulf. Besides the resident fauna, many eggs, larvae and juveniles may be advected into the Gulf via the Loop Current from the Caribbean Sea. We encountered some very difficult identification problems
RICHARDS ET AL.: ASSEMBLAGES IN THE GULF OF MEXICO
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BULLETIN OF MARINE SCIENCE, VOL. 53, NO.2,
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Figure 2. Temperature COC) at 100 m depth for 15 April to 3 May 1987. Crosses (+) indicate CTD or XBT station locations. in this work despite many years of studying Gulf ichthyoplankton. We made no attempt to identify eggs from our collections because of the complexities. A large proportion of the larvae are very small «2 mm NL) and these are especially difficult to identify. In addition 3.5% were damaged (usually lost eyes), which greatly impeded identification. Table 2 summarizes much of the basic data from the study. The high percentage of unidentified specimens from Transect 6 is due to one station (45400) which had 12,341 unidentified larvae under 10 m2• If these very small specimens are discounted, the percentage of unidentified falls from 54.1 % to 4.7%. Likewise for Transect 8 the high percentage of unidentified and damaged larvae is due to station 45472 which had 717 damaged larvae. Discounting those specimens reduces the percentage of unidentified to 18.9% from 24.7%. Since our earlier report based on part of this experiment (Richards et aI., 1989), we have subsequently learned that we misidentified Myctophum affine larvae as M. nitidulum. Those specimen identifications have been corrected in this study. All our identifications follow the descriptions cited in Richards (1990). For our tables we used a three plus four letter code for species, the full generic name for genera, families, and higher groups. The identification of the code is given in Appendix Table l. Oceanographic Features. - The Loop Current is the major oceanographic feature of the eastern Gulf of Mexico. Its boundary is clearly visible in satellite images prior to the Gulf becoming isothermal in May (Fig. I). It is a warm, highly saline water mass which enters the Gulf from the Caribbean Sea through the Yucatan Straits. It is a strong geostrophic current, with speeds of up to 250 cm·s·'. It intrudes into the central Gulf, then turns 180 and flows south and exits the Gulf through the Florida Straits where it is known as the Florida Current, eventually becoming the primary source of the Gulf Stream (Maul, 1977; Leipper, 1970). The boundary of the Loop Current is a dynamic zone with meanders, eddies, strong convergences and divergences, that can concentrate planktonic organisms including fish eggs and larvae (Olson and Backus, 1985). At times the entire Loop breaks off as an anticyclonic eddy and moves into the western Gulf. These eddies are typically 300-400 krn in diameter and have a life span of I year (Elliot, 1982). Additional information on the Loop Current and the Gulf of Mexico is summarized in Richards and McGowan (1989). For this study we have plotted the temperature at 100 m depth for collections taken between April 15 to May 3 (Fig. 2) and for collections taken during May 7 to 22 (Fig. 3). Also shown are two satellite photographs of the area with the positions of the transects shown which compare a time of sharp 0
483
RICHARDS ET AL.: ASSEMBLAGES IN THE GULF OF MEXICO
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Figure 3. Temperature ("C) at 100 m depth for 7 to 22 May 1987. Crosses (+) indicate CTD or XBT station locations. boundaries and weak boundaries (Fig. I). Comparison of those figures with the sea surface temperatures for each station in Table I reveals that the vessel was accurately positioned across frontal boundaries. As the Gulfwas approaching isothermal conditions in late May, boundary features were not as evident as those found in Transect 8 on May 22. We did not plot bathymetry as only Transect I was over the shelf « 200 m) while the other transects were all at depths greater than 500 m. The Gulf has a wide continental shelf thus benthic and coastal faunal elements are in close proximity to deep waters. Larval Assemblage Analysis. - The high species diversity of tropical fish assemblages makes them difficult to summarize and this is further complicated by the dominant physical property of tropical waters: their warmth. This means that larvae will not survive long duration plankton tows in recognizable condition for reliable identification. The intrinsic diversity and short tows make it difficult to sample all taxa adequately. A further difficulty is that due to the high diversity and the relative neglect until recently of taxonomic work on tropical ichthyoplankton (e.g., Leis and Rennis, 1983; Leis and Tmski, 1989), many larvae cannot be identified to species level (Richards, 1985). In the present study there were 237 taxa identified in 87 samples on 8 transects. There were representatives of 100 families but when all data were aggregated by family only 53 families were present at a mean abundance of more than 1 individual per transect. [Higher taxa were either rare (Trachinoidei), represented by a few unidentifiable individuals (Myctophiformes), or merited more detailed treatment elsewhere (Anguilliformes).] Only 25 families were represented by >0.5 individual per sample. Our analysis of the assemblages at the Loop Current front was based on these 25 families in the interest of statistical validity and simplification for a generalized first look at these tropicalsubtropical assemblages. Standardized catches under 10 m2 were used in al\ analyses of these 25 taxa (Fig. 4) which occurred at a mean abundance of one (rounded up) or more per station for the 87 stations. Cluster analysis was used to describe both family and station groupings. Group average hierarchical clustering was used because it is known, from Monte Carlo simulation studies, to perform well under a variety of error conditions (Milligan, 1980). The similarity measure for the clustering was Pearson correlation which was converted to a distance measure by the transformation (I - R) where R stands for the Pearson product moment correlation coefficient. In station clustering, abundances of taxa were standardized to z-scores to prevent the numerically dominant myctophids from dominating the de-
484
BULLETIN OF MARINE SCIENCE, VOL. 53, NO.2,
1993
Myctophidae Bregmacerotidae Gonostomatidae Carangidae Synodonlidae Bothidae Scombridae Sternopwchidae Photictithy.idae Paralepiaidae Clupeidae Serranidae Gobiidae Ophidiidae Nomeidae Labridae Triglidae Scaridae Scopelarchidae Mullidae Tetraodontidae Gemp'ylidae Balistidae Scorpaenidae Acropomatidae
o
5
10
15
20
25
30
35
40
(Thousands) Figure 4. stations.
Catch of the 25 fish families with mean abundance of I or more per station for the 87
termination of similarity. In taxon clustering the use of correlation obviates this problem (Wilkinson, 1990). Principal-components analysis (PCA) was also applied to the 25 taxa: PCA is a method for decomposing the covariance or correlation structure of multivariate data (e.g., Tatsuoka, 1971). The results of PC A can summarize multivariate data in fewer composite variables and reveal patterns ofassociation among the original variables. We used PCA to examine relationships among the taxa, to explore the existence of assemblages, and to see if certain taxa were characteristic or indicators ofthese assemblages. These indicators could then be used as surrogates for the assemblages when investigating the ecological relationships of the assemblages. We plotted standardized abundance of selected indicator taxa by station along the transects to display how their abundance changed in different water masses across the Loop Current front. RESULTS
The list of taxa for each transect and the numbers oflarvae under 10 m2 of sea surface are given in Appendix Tables 2-9. A high percentage oftaxa (21.8%) were represented by single specimens. Of these 52 taxa, 27 were identified as singlespecies occurrences as follows: Acanthocybium solandri, Amblycirrhitus pinos, Antennariidae, Antigonia sp., Aulopus nanae, Cyclopsettaji.mbriata, Dactylopterus volitans, Eutaeniophorusjestivus, Fistularia tabacaria, Gobiesox sp., Hippocampus reidi, Liopropoma sp., Malacosteus niger, Nesiarchus nasutus, Physiculus sp., Pleuronectidae, Polymixia lowei, Polydactylus, Serrivomeridae, Sphyraenops bairdiana, Synagrops sp., Tetragonurus atlanticus, Tetrapterus pjluegeri, Tiluropsis, Trachinoidei, Xiphias gladius, and a Zeiformes. As an indicator of biological productivity we examined the plankton displacement volumes at each station. The highest volumes were found in water less than
485
RICHARDS ET AL.: ASSEMBLAGES IN THE GULF OF MEXICO
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Figure 5. Temperature ("C) section along a) Transect I on 20 April 1987; b) Transect 2 on 21 April 1987; c) Transect 3 on 24 April 1987; d) Transect 4 on 3 May 1987; e) Transect 5 on 5 May 1987; t) Transect 6 on 10 to 12 May 1987; g) Transect 7 on 13 May 1987; h) Transect 8 on 22 May 1987.
200 m during April (Table 1). As water temperatures increased and more stations were made in the warm Caribbean water, the volumes declined. Volumes also indicate the patchiness of plankton as evidenced especially in Transect 5. Surface temperature gradients across the Loop Current front were commonly 0.5°C· km-1 and as large as 1.0°C· km-1 (see sea-surface temperatures in Table I). Abundances of larvae and number of taxa from each transect are compared including displacement volume data and hydrographic data (Fig. 5a-h). Coefficients of determination (r2) are given in Table 3. These values are compared below. Transect 1 (Table 3. Appendix Table 2, Fig. 5a). - This was the only transect to occur in water shallower than 200 m. The transect went from west to east and moved up onto the continental shelffrom warm water> 22.9°C to cool shelf water :l
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