RELATION OF CORAL REEF FISH LARVAL DISTRIBUTIONS TO ...

BULLETIN OF MARINE SCIENCE, 54(1): 228-244,

1994

RELATION OF CORAL REEF FISH LARVAL DISTRIBUTIONS TO ISLAND SCALE CIRCULATION AROUND BARBADOS, WEST INDIES Robert K. Cowen and Leonardo R. Castro ABSTRACT An interdisciplinary study was undertaken to test whether larval reef fish are readily retained in the vicinity of an island in association with eddies embedded in the downstream flow. Theoretical and empirical data have suggested the existence of either stationary eddies or shedding eddy fields (i.e., von Karman vortex streets) in the vicinity of isolated islands, and reef fish ecologists have considered such eddies to be important in the retention and eventual return of reef fish larvae to the island reefs. Our physical data do not support the hypothesis that either a stationary or shedding eddy field exists downstream of the island of Barbados. Although eddies may exist, evidence suggests that the main oceanographic features and flow are topographically steered, being strongly influenced by subsurface (300 m) ridges running northwest and south of the island. The result is essentially longshore flow on the leeward side of the island, offshore convergences located at the southwest and northwest corners of the island, and possibly a downstream (~50 km) return flow which follows the northwest ridge. Our preliminary analysis of larval reef fish distribution suggests a strong concordance with certain features of the physical flow, notably the offshore flowing convergence and the return flow northeast of the island. Additionally, larval reef fish densities are extremely high very nearshore « 1.5 km) at mid-island suggesting either a region oflow flow, onshore flow, and/or active behavior on the part of the larvae resulting in the retention and/or return of larvae in this vicinity. Analysis of the vertical distribution of the larvae shows highest concentrations at 10-30 m and 30-60 m depths, with some family level differences. Onshore flow is also evident (especially nearshore) at depths of about 50 m corresponding to the depths of high larval concentration. These preliminary data are intriguing in the sense that they suggest a simpler, more general mechanism for larval retention and ultimate return to the reef habitat. With primarily longshore flow around the island due to topographic features and some nearshore, onshore flow at mid-depths, the larvae are essentially in a similar physical regime as shelf environments. Therefore, larval behaviors and adaptations may not need to be so complex as to be island specific, rather the same general adaptations and behaviors required of reef fish larvae from coastal areas may be successful for island populations as well.

Processes influencing the transport of fish eggs and larvae may operate on a variety of temporal and spatial scales, not all of which necessarily contribute to the maintenance oflocal populations (Cowen, 1985). For example, some species are capable of extended pelagic larval lives and, in association with oceanic current systems, may be dispersed over very long distances (Rosenblatt et a1., 1972). Though potentially important with respect to gene flow (Scheltema, 1972; Rosenblatt and Waples, 1986; Thresher et a1., 1989; Lacson, 1992), such long distance dispersal is unlikely to be the main source of replenishment to most populations. Instead it is probable that various attributes of the local currents may act to retain eggs and larvae, or to transport them to other nearby, suitable areas of adult habitat (e.g., along a coast or down an island chain). Coastal areas that possess shoreline and bottom irregularities may generate strong eddying motions and associated secondary circulations (Pingree et a1., 1978; Loder, 1980; Pingree and Maddock, 1985). Such eddies have been proposed as important physical mechanisms that may recirculate the eggs and larvae of coral reef fishes, thereby minimizing the loss due to transport away from suitable settlement sites (Sale, 1970). Though Sale was not the first to suggest such a mech228

COWEN AND CASTRO: LARVAL FISH DISTRIBUTION-BARBADOS

229

anism (Boden, 1952; Sette, 1955; Randall, 1961), his hypothesis has found favor among tropical fish ecologists. For example, Johannes (1978) hypothesized that by habitually spawning off headlands or underwater promontories, reef fish place their eggs and larvae into nearshore eddies which transport larvae back to the parent reef or island. Lobel (1978, 1989) suggested that the spawning season of Hawaiian reef fish corresponds to those periods in which ocean currents, i.e., mesoscale eddies, are most likely to retain larvae in the vicinity of reefs. Indeed, Lobel and Robinson (1986) clearly demonstrated a mesoscale eddy 55-85 km off the island of Hawaii; however, their ichthyoplankton data indicate that reef fish larvae within this feature were extremely rare. It is interesting that, in his 1970 paper, Sale clearly questioned the likelihood of an eddy located 25-50 km offshore being useful as a return transport mechanism for coral reef fish, yet his skepticism seems to have gone unnoticed. A series of propagating eddies consistent with the properties of a von Karman vortex street was proposed to occur in the wake of Barbados, West Indies (Emery, 1972). Subsequently, offshore maxima ofichthyoplankton were found in two areas corresponding to the likely position of recirculating eddies (Powles, 1975), and eddies have been invoked to explain observations of high, local self-recruitment in both invertebrate and vertebrate populations to Barbados (Hunte and Younglao, 1988; Hunte and Cote, 1989). None of these studies benefitted from extensive physical measurements, yet they, in association with the above studies in other regions, raise several interesting questions. Are eddies commonly associated with island circulation? Are larvae utilizing eddies in island wakes (if they exist) directly as retention areas, and if so, are eddies stationary long enough for larvae to complete their development to settlement and metamorphosis? Alternatively, are offshore eddies merely entrapping larvae which are already lost to the system? Without detailed knowledge of the circulation around an island and concurrent information on the spatial and vertical distribution of the larvae these basic questions remain unanswered. Our objective in this study was to determine whether eddies are a dominant feature of the leeward flow around an isolated oceanic island (Barbados, West Indies), by examining the relationship between the observed physical regime and

the distribution of reef fish larvae. In the case of no eddies, it was our intent to determine what features of the island scale flow may be responsible for retention oflarval coral reef fish. We chose an isolated oceanic island to limit the possibility of upstream sources of reef fish larvae to the island itself. Our approach was to concurrently sample the horizontal and vertical components of both the circulation and larval reef fish distribution with a focus on island-scale patterns. SITE DESCRIPTION The characterization of how larvae are retained in the proximity of shore and/or are transported back to shore requires the ability to separate possible larval sources, i.e., local from upstream sources. To this end, Barbados was selected due to its location as the most upstream of the Caribbean islands. Located at 13°10'N, 59°30'W, Barbados is 140 km east of the Lesser Antilles, lying within the predominantly northwest flowing North Equatorial Current. Although water characteristic of the Amazon outflow (some 1,600 km to the southeast) occasionally bathes Barbados, the minimum time required for the transit is 80 d (Steven and Brooks, 1972) and there is a general lack of suitable reef fish habitat along the intervening coastline of South America (Bohlke and Chaplin, 1968). Consequently, upstream sources of coral reef fish larvae arriving at Barbados may be considered neglible, if any at all. Barbados is a small island (15 by 25 km) located along the Barbados ridge, which runs 75 km northwest and 25 km south of the island. The ridge rises to a depth of approximately 250 m in the north and 75 m in the south from a surrounding depth of 2,000 m. Along the western and eastern coast of the island, the drop-off is very nearshore with depths in excess of 500 m only 4 km from shore. Though geographically isolated, the reef fish population around Barbados is speciose and

BULLETIN OF MARINESCIENCE, VOL.54, NO, I,

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Shipboard sampling was conducted from 25 April-19 May, 1990 on the R/V ENDEAVOR.We separated the cruise into three legs: the first (April 28-May 4) and third (May 13-18) legs sampled primarily within the leeward (west and north) side of the island and up to 45 km offshore, with the second (May 5-11) leg running around the entire island and up to 90 km from shore (Fig, I). During the first and third legs of the cruise, the ship's track started in the southwest portion of the island and ran a series of II transects perpendicular to shore from south to north. During the northward portion of these legs (2.5 d), physical sampling was conducted and during the return, southward portion (3.5 d), the biological sampling was conducted (see below). During the second leg, both physical and biological sampling were conducted concurrently due to the time required to transit the around-island cruise track. Due to the early stage of our analyses, discussion of results will be restricted to the first two legs only.

Physical Measurements. - Water column characteristics were sampled with a combination of XBT's and CTD's (Neil Brown MK III). The XBT measured temperature to a depth of 450 m and the CTD measured temperature and salinity to a depth of 500 m (or to within 25 m of the bottom when water depth was less than 500 m). Originally, we intended to utilize the more rapidly collected temperature data of the XBT casts, in association with temperature-salinity relationships derived from the CTD casts to characterize water masses, however, the T-S relationship was too complex for this purpose (Fig. 2). Currents were measured directly in two ways. First, a moored current meter array was deployed at the beginning of the cruise southeast of the island to measure "upstream" current velocity. This array was moored in 75 m of water and had two Aandera RCM-4 current meters, one at 5 m and the other at 60 m. The array was retrieved at the end of the cruise after a total of 25 d of sampling. Current information was also obtained with a ship-borne Acoustic Doppler Current Profiler (ADCP, RD Instruments, 150 kHz). A vertical resolution of 8 m was utilized, providing measurements of the vertical structure ofthe currents to a depth of (typically) 250 m. For analysis, we averaged the current velocities by depth strata in accordance with the depths sampled by our ichthyoplankton nets (see below). The ADCP was operated with 5 min averaging, giving a spatial resolution of approximately 1.5 km with the ship steaming at 10 knots. All ADCP currents are reported relative to a 250 m reference layer.

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Biological Sampling. - To sample the vertical and horizontal distribution of the ichthyoplankton, I m', 505 ILm mesh Multiple Opening-Closing Net and Environmental Sensing System (MOCNESS) was used. Leg I ichthyoplankton sampling locations were determined after preliminary analysis of the current patterns (obtained during the northward portion of the leg), thereby ensuring that the placement of transects coincided with specific features of the downstream flow field. In all, 42 stations were sampled along 6 transects (Fig. IA). These same stations were resampled during the third leg to assess temporal consistency of the observed distributions (not reported here). During the second leg, 22 additional MOCNESS stations were sampled along two transects downstream of the island and along five eastern and southern upstream transects (Fig. IB). Sampling depths of the nets were determined from the initial CTD survey to correspond with apparent layering of the upper 140 m of the water column. Based upon the strong salinity signal relative to the temperature signal, selected sampling depths were 140-90, 90-60, 60-30, 30-10, and 10-0 m. The MOCNESS was equipped with a flow meter and angle indicator that accurately calculated the volume filtered for each net, as well as temperature, conductivity and depth sensors. Tows were of 5 min duration for each depth interval sampled and were conducted in a stepped-oblique fashion. Sampling was conducted on a 24 h basis to maximize the synopticity of our sampling effort. On board, the samples were split in half with a Folsom plankton splitter. One half of the sample was preserved in 4% buffered formaldehyde for identification purposes and the other was preserved in 95% ETOH for otolith analysis. In the lab, the formalin preserved samples were sorted with the aid of a dissecting microscope and larvae identified to the lowest possible taxon. For this study, only family level determinations are utilized, and analyses are based on those families with predominantly nearshore, reef-associated adults. All counts are based on the total sample (i.e., doubling of the formalin counts) and are expressed as densities (number' 1,000 m-3). RESULTS

Physical Environment. - The upper water column was very complex, with evidence of multiple layers of differing salinity. Temperature did not vary significantly within the upper 100 m (26.9-27.6°C), whereas the salinity varied within that same depth range by nearly 4 psu (33.7-37.4 psu; Fig. 2). The upper 10 m was characterized by very low salinity water (e.g., less than 34 psu). Salinity increased with depth, often in distinct steps or layers, until reaching a salinity maximum (>37 psu) between 90-140 m. As depth increased beyond 140 m, salinity de-

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BULLETIN OF MARINE SCIENCE, VOL. 54, NO. I, 1994

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creased to approximately 35 psu. The above pattern was most evident during Leg 1 (Fig. 2A). During the second leg, and particularly far downstream (NW), the low salinity surface water was not as evident, and appeared to be mixed with water in the upper 30-40 m as evidenced by the depth of the mixed layer (Fig. 2B). There was a strong shift in the velocity ofthe "upstream" current that occurred between the first and second legs (Fig. 3). Early on, the prevailing current on the southeast side of the island averaged 13 cm· s- L predominantly toward the northwest. During the course of approximately 24 h, the current shifted toward the east, and the speed increased nearly ten-fold to 100 cm· S-I. Although the wind increased in association with the observed current shift, higher winds (17-21' ms-I) did not last more than several days, whereas the shift in the current remained for the duration of the study. Additionally, although the wind increased in intensity, the direction was still predominantly ENE. Therefore, we do not yet have a clear idea of what forcing was responsible for the observed current shift. The flow field, based on ADCP current vectors, was complex, though specific features were evident and persistent between legs. First, there was no evidence of von Karman type eddying motions in the lee of the island during either leg of the cruise (Figs. 4, 6). Rather, the flow was predominantly around the island ridge. During Leg 1, flow proceeded along the western shore until it reached the NW corner of the island, where it veered offshore towards the northwest (Fig. 4). This offshore flow was associated with a convergence zone at the northwestern tip of the island. There was also a zone of convergence in the waters off the SW comer of the island. Flow was generally higher (- 20 cm· s- I) offshore than nearshore (-10 cm· S-I), particularly within the middle portion of the western shore (Fig. 5). Nearshore, in the area of reduced flow, most vectors were either parallel to shore or slightly onshore in the near-surface waters (Fig. SA). With increasing depth, flow became strongly onshore (i.e., NE), especially at 50 m (Fig. SA-C).

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During Leg 2, the currents were stronger than during Leg 1, particularly south of the island (Fig. 6). Flow off the southwest corner of the island was from the south and then diverged as it approached the shore, proceeding northwest along (and slightly offshore) the western shore and east along the southern shore. A convergence occurred at the north tip of the island (as during Leg 1) which flowed

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BULLETIN OF MARINE SCIENCE, VOL. 54, NO.1, 1994

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off to the NW. This NW offshore flow was detected out to our most offshore transect. However, along the northeastern portion of each of the five offshore (northern) transects, flow occurred in the reverse (return) direction, with minimal flow in the center of the transects. The opposite nature of this flow was particularly evident in depth sections taken along each transect (Fig. 7). The NW flow was concentrated in ajet (>20 cm·s-I) 20-40 km along our transect and at a depth of 40-70 m, and the return (SE) flow (> 30 cm 'S-L) was concentrated 85-110 km along the same transect and at the same depth. The flow continued to the SE along the eastern shore of the island, except for a small portion which diverged at the northern tip of the island at the NW convergence (Fig. 6). In the southeastern portion of the island, the flow appeared to converge with the eastward flowing waters passing to the immediate south of the island. Our sampling did not extend far enough southeast to resolve the fate of this flow. Overall, the circulation seemed to be following the outline of the Barbados ridge, indicating topographically steered flow where bottom friction is important. There is no evidence for trapped eddies embedded in the flow past the island. Ichthyoplankton. - During the first two legs, a total of 13,850 larvae were collected comprising 62 taxa (57 families plus three suborders and two orders). Of these families, 36 were of predominantly reef-associated species, with 10 families occurring in high enough abundance to be considered in this analysis (Table I). Over all taxa, mean larval density was 278 larvae 1,000'm-3, which was dominated by myctophids (29.7% of the total), bregmacerotids (13.6%), nomeids (6.4%), and both ids (5.3%). The reef-associated taxa comprised a combined 27.9% of the total fish larvae collected and the 10 top families 25.4% of all fish. These families are commonly included in surveys of coral reef-associated fishes (see reviews by Leis, 1991a; Thresher, 1991). During Leg I, reef fish larvae (over all depths combined) were extremely abundant within 4 km of shore, especially along the middle two transects (densities greater than 400 larvae' 1,000 m -3; Fig. 8). The high concentrations of reef fish larvae nearshore were dominated by pomacentrids and gobiids. Further north, in


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Figure 6. Current vectors of flow around Barbados (8 m depth bin) as determined by the ADCP for Leg 2 (5-11 May). Topographic contours as in Figure I. MOCNESS stations (A-G) referred to in Figure 10 are noted along the two most offshore (NW) transects.

the area of the offshore flow, larval distributions extend offshore in relatively high numbers (primarily carangids, serranids, labrids and dactylopterids) as compared to the offshore stations of the middle transects. To the south, there was also a higher concentration of larvae offshore, compared to the middle transects. This southern area of larval concentration was primarily composed of four families: lutjanids, gerreids, carangids and sphyraenids. When larval distributions were examined for each depth sampled, finer scale patterns were evident (Fig. 9A-C). First, along the western side of the island, reef fish larvae were largely restricted to water no greater than 60 m. Second, highest larval densities (> 1,200 larvae' 1,000 m -3) occurred along the central transects and were most closely restricted to shore in the 30-60 m depth strata. Finally, the secondary highs in larval concentration which occurred along the southern and northern transects were restricted to the upper 30 m of water.

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Figure 7. Vertical cross-section of ADCP derived flow taken along the third transect NW of Barbados (see Fig. 6). Positive contours indicate current speed (cm·sec-I) into the page (i.e., NW), negative contours indicate currents out of the page' (i.e., SE).

Along the northwest two transects sampled in Leg 2 (Fig. 6), two distinct zones of larval concentration occurred. The peak concentrations were associated with the jets flowing in opposite directions along each side ofthe submerged ridge (Fig. 10). The first zone (> 140 larvae' 1,000 m-3) was at a depth of 10-30 m, centered 30 km along the transect, shallower but directly over the NW flow. The second zone was more dense (>380 larvae·I,OOO m-3) and distributed over a broader depth range (10-60 m) than the first, though highest concentrations were also at 10-30 m. This second zone was directly over the returning, SE flowing jet. Along the eastern side of the island, there was no onshore/offshore gradient in concentration as was present on the west side during Leg 1 (Fig. 11). Larval

Table I. List of the 10 most abundant larval coral reef fishes collected around Barbados during the first and second legs, 1990. Total number of stations sampled was 64 (maximum frequency). Larvae collected is the total number of larvae in all samples. Density is calculated as the total number of larvae by the total volume sampled in all stations. Maximum density is the highest larval density collected at a single depth stratum. All density estimates are standardized to 1,000 m3• Station Family

Bothidae Gobiidae Pomacentridae Carangidae Lutjanidae Scaridae Gerreidae Sphyraenidae Serranidae Labridae

frequency

61 54 32 48 48 34 18 25 29 23

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1,460 1,350 1,166 964 916 308 268 262 218 116

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Figure 8. Distribution of larval coral reef fish (N· 1,000 m -3) around the western side of Barbados during Leg I. Contours represent the average larval densities in the 140 m deep water column.

densities were moderately high and in the same range as those found well downstream of the island, particularly in the return, SE flowing water. There was a slight increase in concentration east of the eastern tip of the island in the same area as the east side convergence (Fig. 6). Overall, during Legs I and 2, the general pattern appears to be one of larvae being distributed in relation to the observed current flow field. Maximum larval concentrations occurred very nearshore on the western shore of the island, an area with reduced surface flow and deep (50 m) onshore flow. Other areas oflarval concentration were associated with near-surface convergence zones SW, NW and SE of the island, and with two opposite flowing jets NW of the island.

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Figure 9. Distribution by depth intervals oflarval coral reef fish (N'I,OOO m-3) around western side of Barbados during Leg 1. Depth intervals represented correspond to A) 0-10 m, B) 10-30 m, C) 3060 m. DISCUSSION

Caribbean coral reef fish occur over a broad geographic range. Within this range, the larvae of a given species may encounter a diversity of pelagic environments ranging from a broad, continental shelf to complex flows around and through island chains, to flow around isolated islands such as Barbados. The fact that these fish are successfully recruiting throughout their range suggests that some larvae are surviving their dispersive stage in all of these environments. This success must partially reflect larval adaptations that enhance their chance of being transported to a suitable settlement environment. Two alternate hypotheses involving o

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Figure 10. Vertical cross-section of reef fish larval concentrations (N' 1,000 m -3) along MOCNESS stations located along the two most offshore transects in Leg 2 (noted in Fig. 6). Stations are arranged in a linear fashion as if all were located along a single transect.

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Figure I] _ Distribution of larval coral reef fish (N - 1,000 m -J) around the eastern side of Barbados during Leg 2. Contours represent the average larval densities in the 140 m deep water column.

larval behavior may be envisioned. In one scenario, larvae would need to have the ability to sense their environment (e.g., both large and small scale flow features, vertical structure, etc.) and then have a suite of responses appropriate for each environmental parameter. An alternative explanation is that some characteristic features of the physical environment are present throughout most if not all of their range such that larval behaviors do not have to be specifically tailored to

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each individual island and headland. In the former case, the larvae must be very plastic in their ability to respond to sometimes very complex current regimes, and they also must have the ability to sense what kind of environment they are in. This scenario would call for extremely complex sensory and behavioral capabilities in the larvae. The alternative scenario would call for a single set of behaviors in the larvae which is successful in each environment due to some commonality in the physical regime. The latter option is clearly the most parsimomous. One oceanographic process that has often been invoked as a likely larval retention mechanism is eddying motion in the lee of an island or particular headlands (Sette, 1955; Randall, 1961; Sale, 1970; Emery, 1972; Powles, 1975; Johannes, 1978; Lobel and Robinson, 1986). The objective of the present study was to test the hypothesis that an eddy field (stationary or shedding) was embedded in the leeward flow of Barbados as previously predicted. Our preliminary analysis suggests that no such eddy field exists. Rather, the predominant flow appears to be topographically steered, resulting in circulation around the Barbados Ridge and parallel to shore. Additionally,

our findings show that the overall distribution

of

reef fish larvae corresponded to the general flow pattern. Where the flow veered offshore, the larvae were carried with the currents (e.g., NW and SE tips of Barbados). Where the flow returned around the northern end of the Barbados Ridge, the larval distribution reflected that return with high numbers around the edge of the flow. Another feature of the flow around Barbados was the somewhat slower current speed nearshore, possibly resulting from frictional forces. As in many other studies of larval distributions (see review by Leis, 1991a), we found very high larval abundances close to shore with highest concentrations at depth (30-60 m) where flow was predominantly onshore. The slower speeds and possibly the deeper, onshore flow of the coastal water may be a shared physical feature utilized by larvae in many areas. For example, in one of the more comprehensive studies to date of physical processes and distributions of ichthyoplankton, Boehlert et al. (1992) found an apparent zone of1ow flow (or possibly returning flow) nearshore of the leeward side of Johnston Atoll. Associated with this area were relatively high abundances of island-associated fish larvae. A close look at their data reveals that approximately 79% of the "island" fish larvae collected in this nearshore, low-flow area were composed of egg-brooding taxa whose larvae are typically found nearshore (i.e., gobiids, apogonids, blenniids, and pomacentrids; Leis and Miller, 1976; Leis, 1982; Smith et aI., 1987; Kobayashi, 1989). Similarly, the very high concentrations of larvae nearshore in Barbados were dominated by pomacentrids and gobiids. That these taxa are typically found nearshore in a variety of environments suggests that their larval behaviors are adapted to some "typical" nearshore current regime. Other studies of flow around oceanic islands have reported a range of circulation patterns, some that result from direct topographic interactions with the prevailing flow and some suggestive of hydrodynamic features that are derived elsewhere and which impinge on the island (Boden, 1952; Hogg, 1972; Chopra, 1973; Hogg et aI., 1978; see brief review by Farmer and Berg, 1990). In most cases, there is some nearshore zone of low flow and/or an area of entrainment which may be of substantial importance to reef fish larvae. However, missing from most of these studies is information about the temporal nature of these flows. The presence or absence of a particular flow pattern does not give any evidence about the typical duration or the frequency with which such a pattern is present. For instance, Boehlert et al. (1992) raised the question about the temporal nature of the flow


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affecting Johnston Atoll during their sampling. The flow they observed was potentially derived from a frontal feature to the north of the island which may change seasonally (Barkley, 1972) and/or episodically. Imposed upon the observed flow in our study were unexplained current shifts, such as those encountered south of the island. These current shifts may result from either inertial or wind forcing, and may have significant, "event-scale" impacts on larval distribution and eventual recruitment (Boekhoudt, 1992). The full scale of their importance to the prevailing flow around Barbados is not yet clear. The ultimate importance of a given circulation feature to the distribution of reef fish larvae is dependent on the likely duration of the flow feature and whether the occurrence of the flow is in phase with the production of the larvae. The results presented in this study are a synoptic view, and yet even within the short time period covered we observed a large current shift and intensification south of the island suggesting the possible dynamic nature of island-scale circulation. Nonetheless, some of the hydrodynamic features, such as the convergence at the northwest tip of the island, were present before and after the current shift. Similarly, it should be noted that we encountered a similar bimodal distribution oflarval reef fish west of Barbados as was found 17 years earlier by Powles (1975). Our physical data showed that the offshore highs in larval reef fish abundance were associated with near-surface, convergent flow which is onshore in the south and offshore in the north. While the larval distributions are consistent between studies, the flow direction is not consistent with the trapped eddy system proposed by Powles. Rather, the biological patterns are associated with the onshore and offshore components of the around-island ridge flow. The implications of our study are several. First, the apparently large scale (i.e., greater than island diameter) recirculation of water around Barbados may act to retain locally generated larvae in the vicinity of the island. This mechanism may thereby maintain a somewhat regular supply of larvae to the island, and provide the larvae with access to the island's reefs. Evidence supporting this contention is presented by Hunte and Younglao (1988) in their finding of a likely se1frecruiting population ofthe sea urchin, Diadema antillarum, in Barbados. Second, there are features of this flow that are similar to flow regimes which larvae may encounter in other areas of their range. In particular, when in the vicinity of the island, the flow is parallel to shore and vertically stratified. This type of flow is analogous to the flow along many mainland coastal regions. Simple vertical migratory behavior may enhance the onshore movement of larvae in this type of environment (Leis and Goldman, 1984; Leis, 1986; Boehlert and Mundy, 1988; Leis, 1991b; Cowen et ai., 1993). As seen in our study, when flow at the surface was parallel to shore in nearshore waters, at 50 m depth there was a distinct onshore component. Whether the larvae collected in our study were utilizing this mechanism to move onshore will require further analysis of our vertically stratified samples. Smaller scale features were not examined in this particular study. However, the reef structure around Barbados may result in very localized circulation features that are analogous to those described by Wolanski and Hamner (1988) and Kingsford (1990). While tidal excursions are small at Barbados, and probably have little influence on offshore circulation patterns, they may interact periodically with the very nearshore regime (S. Sponaugle, MSRC, unpubl. data). Coupling of these nearshore circulation features with behavioral responses may result in synchrony of spawning (Robertson et aI., 1988; Hunt von Herbing and Hunte, 1991) and/ or settlement to tidal or lunar stimuli (Victor, 1986; Robertson et aI., 1988; Sponaugle, unpubi. data). Other pulses in recruitment may be due to small scale,

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short duration changes in the flow (e,g" wind events) or to larger scale changes in the flow due to inertial forcing (Cowen, 1985; Choat et aL, 1988), Interruptions in the island-scale circulation around Barbados may result in the loss of a patch oflarvae by advecting them permanently away from the island or just temporarily away from the nearshore processes mentioned above. The observed flow regime would not necessarily be expected to retain all larvae in a permanent recirculation around Barbados. As seen in the ADCP mapping of the current, flow was stronger offshore than nearshore. Larvae which are carried further offshore (e.g., > 8 km), either by diffusive factors or highly localized flow features, may well be swept away from the island by not being caught in the recirculation at the northwestern perimeter of the Barbados ridge. Under such conditions, the larvae may be advected downstream where they may encounter other islands in the Lesser Antilles (e.g., 81. Lucia, Martinique). In addition, though we do not have a clear picture of the flow south of the island, the strong flow in that region could produce similar results. Other sources ofloss might include the retention of larvae in any ephemeral eddies that might occasionally spin off of the prevailing flow. In conclusion, no evidence was found to support the contention that an eddy field (stationary or shedding) is embedded in the leeward flow of Barbados. The observed flow appeared to follow the topography of the Barbados Ridge. The horizontal distribution of larval reef fish reflected the large scale circulation pattern. We suggest that larvae are being retained in the vicinity of the island via this around-island circulation pattern, however, it may not yet be possible to separate the scale at which the various flows and larvae are interacting. The larger scale, around-island flow may act to retain the larvae of some taxa in the general vicinity of the island until they are competent, or ontogenetically advanced enough to move into nearshore waters. Once there, these larvae may utilize smaller scale physical features (sensu Kingsford, 1990) for eventual transport to settlement sites. These smaller scale processes may also be utilized by other taxa which remain nearshore during their entire early life history. The various nearshore features may be entirely independent of the processes affecting the island wide circulation. Clearly, the transition between offshore dispersal to interactions with nearshore features needs further directed study. Whether the findings of this study are indicative of typical flow features around islands will require similarly scaled studies in other areas. However, the similarity in taxon-specific larval distributions suggests some commonality of vertically structured flows which enables larvae to remain near and/or return to their shallow water settlement sites. ACKNOWLEDGMENTS

Many persons have contributed to all phases of this study and we thank them all. For discussion of ideas and comments on the manuscript, we thank S. Sponaugle, E. Schultz, and J. Hare; all three materially improved the manuscript. T. Wilson provided technical and analytical assistance, including programming for near-real time utilization of the ADCP data. M. J. Bowman, K. Stansfield, S. Fauria, and K. Lwiza provided the physical oceanographic analyses. We would particularly like to thank W. Richards for his time and help with our larval id's, and his editorial help with this manuscript. B. Boekhoudt was also a great help with processing the larval samples. This work was funded by the National Science Foundation, Grant OCE-891ll20 to R.K.C. and M.J.B. This is MSRC Contribution Number: 920. LITERATURE CITED Barkley, R. A. 1972. Johnston Atoll's wake. J. Mar. Res. 30: 201-216. Boden, B. P. 1952. Natural conservation of insular plankton. Nature 169: 697-699.


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May 20, 1993.

ADDRESS: Marine Sciences Research Center, State University of New York at Stony Brook, Stony Brook, New York 11794-5000.