OCEANOGRAPHIC PROCESSES OF CORAL REEFS: Physical and Biological Links in the Great Barrier Reef - Chapter 13 pptx

The Effects of Water Flow around Coral Reefs on the Distribution of Pre-Settlement Fish (Great Barrier Reef, Australia) John H. Carleton, Richard Brinkman, and Peter J. Doherty CONTENTS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 Materials and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210 Helix Reef Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 Bowden Reef Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 Helix Reef Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 Bowden Reef Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 Hydrodynamics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 Fish Distribution and Abundance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 Dispersion Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 INTRODUCTION Coral reef fish, with very few exceptions, have planktonic egg, larvae, and pre-set- tlement juvenile stages that vary in duration from weeks to months. Most reef fish spawn buoyant eggs that have the potential to be transported many kilometers in wind-driven surface currents before hatching into larvae capable of influencing their dispersal. Others lay eggs in protected nests with the subsequent release to the water column of actively swimming larvae or juveniles, thus minimizing the time their off- spring are exposed to the vagaries of ocean or shelf currents and enhancing the 13 209 © 2001 by CRC Press LLC chances of recruitment back to their natal reef (Jones et al., 1999). In either case, the problem facing propagules expatriated from coral reefs is one of population closure—of finding shallow coral reef habitat suitable for the juvenile/adult phases in their life cycle. We now know that the physical, chemical, and biological composition of the water mass in the immediate vicinity of coral reefs is affected by fine-scale current patterns generated through the interaction of reef topography with prevailing, far- field currents (see Hamner & Wolanski, 1988 for review). Coral reefs, growing to within a few meters of the sea surface, act as barriers to the flow of oceanic or shelf currents. As currents approach they must diverge to flow around the reef edges, cre- ating a zone of relatively stationary water immediately upstream which becomes enriched with nutrients and plankton. At the reef face, topographical entrapment of tidal currents by coral buttresses results in the advection of deep water up the reef slope toward the crest. On the surface, wave turbulence mixes the chemical and par- ticulate matter from deep layers with shallow wind-driven material just prior to push- ing the mixture across the reef crest and onto the reef flat (Hamner et al., 1988). As diverging currents accelerate around the reef, strong longshore currents are generated close to and parallel with the reef sides. If longshore currents are strong enough, flow separation occurs adjacent to sharp projections or indentations in the reef margin with the resultant formation of particulate-rich eddies. In the lee of the reef, gyres and eddies, depending on their size, location, strength, and duration, vary in their ability to retain both neutrally buoyant material such as echinoderm larvae (Black, 1988) and positively buoyant material such as coral eggs (Willis & Oliver, 1990). A number of these reef-associated hydrodynamic processes must also affect the distribution and abundance of pelagic, pre-settlement fish in the near-reef environment, and therefore impact on the eventual success/failure of their recruitment back to suitable, coral reef habitats. To illustrate the role of flow dynamics on the retention of pre-settlement fish, we present the findings from two independent studies at two physically distinct platform reefs in the central section of the Great Barrier Reef (GBR). The first of these stud- ies was of short duration and occurred at Helix Reef, a small, topographically simple, oval-shaped reef; and the second, completed over a 3-month period, was at Bowden Reef, a considerably larger, topographically more complex, elongated reef. Synchronized light traps moored in close proximity to these reefs produced synoptic views of fish distribution and abundance patterns at various times of the night and states of the tide. By combining information on fish distribution patterns with physi- cal oceanographic data, we gain an insight into which hydrodynamic processes con- tribute most to the retention of juvenile fish near reefs. MATERIALS AND METHODS The light traps (Figure 1) were three-chambered devices similar in design to those described by Doherty (1987). These traps have no moving parts and depend upon the behaviour of photopositive organisms to effect their capture. Fish are attracted into 210 Oceanographic Processes of Coral Reefs © 2001 by CRC Press LLC the upper chamber through a number of tapered slits, then by a vertical array of oscil- lating lights moved down through the middle chamber and into the lower chamber where most of the fish remain alive until collected. Upon trap recovery, the catch is washed from the lower chamber with filtered seawater, concentrated into a smaller volume and fixed in 100% methylated ethanol. The lights were activated for three, 1-h periods each night (21:00 to 22:00, 24:00 to 01:00, and 03:00 to 04:00 GMT ϩ 10:00) around the new moon between November and January when seasonal and lunar spawning patterns produce the largest catches (Doherty, 1991). During the summer, the prevailing longshore currents in the central section of the GBR are driven by the East Australia Current (EAC) and flow from the northwest to the southeast, parallel to the major isobaths along the continental shelf (Andrews & Furnas, 1986). Tidal currents which flow across the shelf isobaths, flooding to the southwest and ebbing to the northeast (King & Wolanski, 1996), modulate the per- sistent southward flow pushing the resultant current more to the east during falling tide and more to the west during rising tide (Gay & Andrews, 1994). The interaction of these far-field currents with the variety of reef shapes and sizes found in the cen- tral GBR results in a complexity of flow pattern through the reef matrix. HELIX REEF STUDY Helix Reef (147° 18Ј E, 18° 38Ј S) is a small (Ͻ800 m diameter), relatively isolated (Ͼ10 km to the nearest neighbouring reef), platform reef which rises to the surface from a depth of 55 m (Figure 2). The surrounding seafloor is flat, composed of mud/sand sediment and devoid of any outcrops. These topographical features result in a relatively simple flow regime. The persistent, southerly set current splits around the northern margin of Helix Reef, accelerates along the reef flanks, and sets up a counterclockwise-rotating eddy in the lee (Figure 3, modified from Sammarco & Andrews, 1988). Although tidal modulation of the shelf currents causes the lee eddy to intensify or relax and to change its actual position, only during moderate to strong southeasterly winds does the eddy degenerate (Sammarco & Andrews, 1989). Pre-settlement fish were collected from the surface at 16 stations on the south- ern, downstream side of Helix Reef. Stations A to C were located around the south- ern reef margin within 50 m of the crest, while the remaining 13 traps were moored in a regular grid pattern at a spacing of 350 m across a northwest to southeast axis (see Figure 2). From the reef edge to the downstream side of the grid was 800 m and from the northeast side to the southwest side was 1.4 km. Samples were collected over three consecutive nights covering the new moon period in January 1992. On the first night, an attempt was made to clear all traps after each sampling period. This proved to be logistically very difficult and on the remaining two nights only the 11 traps closest to the reef (A to C, 1 to 8) were cleared after each period. To discern pattern in fish associations, the log transformed abundance data (indi- viduals h Ϫ1 of trapping) from stations closest to the reef (A to C, 1 to 8) during the three individual time periods of each night (21:00 to 22:00, 24:00 to 01:00, and 03:00 to 04:00) were subjected to agglomerative, hierarchical clustering techniques (n ϭ 80 samples). Bray-Curtis dissimilarity coefficients (Bray & Curtis, 1957) were The Effects of Water Flow around Coral Reefs 211 © 2001 by CRC Press LLC calculated for every possible pair of samples, the resulting association matrix sub- jected to the Ward’s incremental sum of squares fusion strategy (Belbin, 1987) and the results summarized by a dendrogram. Diagnostic routines developed for use with the Bray-Curtis metric (Cramer values) were applied to the results from the cluster analysis to determine the level of fidelity of the various fish species to sample group- ings (Abel et al., 1985). Catch rate, number of species, Shannon-Wiener diversity index (HЈ), and Pielou’s evenness index (JЈ) (Pielou, 1969 and 1975) were deter- mined for each of the sample groupings. BOWDEN REEF STUDY Bowden Reef (147° 56Ј E, 19° 02Ј S) is a much larger platform reef than Helix Reef (6.0 km long ϫ 3.0 km wide), is crescent shaped with a continuous reef flat along the northern, eastern, and southern sides, and has a semi-enclosed, shallow, sandy lagoon (Figure 2). The seafloor surrounding Bowden Reef is flat, smooth, and has a depth of 40 to 50 m. Light-traps were moored at the surface at 13 stations around the circumference of Bowden Reef and at 4 stations within the lagoon (Figure 2). On the northern, east- ern, and southern sides, two traps were anchored directly across from each other on either side of the shallow reef flat with an additional two or three traps moored far- ther out in deeper water, within 100 m of the reef crest. The outside near-crest traps were placed just in front of the breaker zone and the inside traps located just behind the reef flat. Traps were deployed for periods up to seven consecutive nights around the new moon during the months of November to January 1992/1993. The traps were activated each night for the same three 1-h periods as in the Helix Reef study, but cleared only once per day and not after each sampling period. Concurrently with trap sampling the strength and direction of far-field currents were measured at half-hourly intervals by two Aanderaa RCM4-S current meters moored at mid-depth to the west and south of Bowden Reef (Figure 2). Tidal height data were collected by Aanderaa tide gauges placed on the seafloor at the base of each current meter mooring, on the lagoon floor at Bowden Reef, and on two adjacent reefs (Figure 2). In addition, data on the strength of the poleward flowing EAC were obtained from a current meter moored at a depth of 35 m on the shelf slope seaward of Myrmidon Reef, approximately 100 km to the northwest. Wind data were obtained from a weather station at nearby Davies Reef. The longest continuous sets of far-field current measurements were obtained in November and January. These data, along with sea level and wind data, were used to force a two-dimensional, depth-averaged, hydrodynamic numerical model (Wolanski et al., 1989). This model was considered the most appropriate to simulate the flow field at the time of biological sampling for a couple of reasons. First, the model has successfully reproduced observed current fields at Bowden Reef in previous studies (Wolanski et al., 1989, Wolanski & King, 1990); and second, this relatively simple, two-dimensional model predicts very similar flow patterns at the sea surface, where most nocturnal, pre-settlement reef fish occur (Doherty & Carleton, 1996), to com- putationally more complex, three-dimensional models (Wolanski et al., 1997). The 212 Oceanographic Processes of Coral Reefs © 2001 by CRC Press LLC computational grid for the model was a square horizontal mesh of 386 m with the X-axis aligned parallel to the margin of the continental shelf. To provide an insight into how the reef-associated flow field may affect the dis- tribution of pre-settlement fish around Bowden Reef, the computed current fields from the two-dimensional hydrodynamic model were applied to a second-order advection-diffusion model (Oliver et al., 1992). As in Wolanski et al. (1997), the “simulated fish” were assumed competent and, in addition to passive advection by local currents, were given a behavioral repertoire of swimming speeds typical of pre- settlement coral reef fish (0.05, 0.1, and 0.2 ms Ϫ1 : Leis & Carson-Ewart, 1997). Fish, regardless of their size or swimming ability, within 3 km of the northern, eastern, and southern sides swam directly toward the reef in response to low frequency sounds generated by breaking waves (Leis et al., 1996) and only stopped swimming when they were within 800 m of the crest. If fish were washed out of this 800-m-wide enve- lope by local currents, they again swam directly toward the crest, stopping when they reached the seaward margin of the envelope. Fish on the western, open-lagoon, “quiet” side were not allowed a behavioral response to the reef. Pre-settlement fish are known to appear in the vicinity of coral reefs immediately following sunset in readiness for settlement (McIlwain, 1996). To incorporate this behavior into the model, a plume of pre-settlement fish with a density of 100 fish per cell and extend- ing across the entire width of the upstream model domain was released at 18:30 on 21 November and at 18:45 on 21 January. To calculate the time-integrated abundance around the reef, propagules were counted every 15 minutes during simulation runs along 13, evenly spaced, 1-km-long transects which projected seaward at right angles from the reef margin. These counts were added to running totals for each transect. Replicate abundance data were tested by three-way, fixed-factor analysis of vari- ance for differences between November and January (those months in which flow patterns were modeled), among stations around the reef circumference (stations 1 to 11, 16, and 17, Figure 2) and among three size classes (small Ͻ10 mm, medium Ն10 and Յ15 mm, large Ͼ15 mm). Prior to analysis, all data were log transformed to sta- bilise variances (Sokal & Rohlf, 1981) and tested for heteroscedasticity by Cochran’s procedures (Winer, 1971). RESULTS HELIX REEF STUDY Over the three nights of sampling more than 15,000 individual fish were caught, rep- resenting more than 160 species belonging to 31 families. However, a large number of the species occurred only once or twice. The dataset was dominated by Clupeids that accounted for 68% of all individuals, followed by Pomacentrids (10%), Nomeids (7%), Apogonids (4%), Carangids (3%), and Gobies (2%). Classification of all 80 samples identified four sample groupings highlighting both spatial and temporal distribution patterns (Figure 4). The fish community struc- ture at stations close to Helix Reef was consistent through time. All samples from sta- tion A, regardless of time of night or night of sampling, clustered together in a distinct The Effects of Water Flow around Coral Reefs 213 © 2001 by CRC Press LLC group, while all the samples from station B and approximately 50% of those from sta- tions C, 1, and 2 formed a second near-reef group. Fish community structure at sta- tions farther from the reef (stations 3 to 8 and the remaining 50% of samples from stations C, 1, and 2) varied with time of night. Approximately 70% of all late evening samples (21:00 to 22:00) formed a cluster distinct from the remaining far-grid sam- ples. Diagnostic routines indicated Spratelloides larvae, Apogonids, and Gobies con- tributed most in the characterization of near-reef communities and that Psene arafuensis was instrumental in distinguishing the late evening, far-grid community. The relevance of these key taxa in defining fish associations is evident from the composition of the four sample groupings (Table 1). Spratelloides larvae dominated the two near-reef groups, Apogonids and Gobies occurred in relatively large numbers only at station A, and P. arafuensis dominated the early evening, far-grid group. The catch rate at station A was almost an order of magnitude higher than the other near- reef group, which in turn was five to eight times higher than the far-grid groups. Although species richness was highest at station A, the species diversity index was 214 Oceanographic Processes of Coral Reefs TABLE 1 Composition of the Four Species Groupings from the Cluster Analysis (H؅؍Shannon-Wiener diversity index and J؅؍Pielou’s evenness index) Site A Abundance (%) # h Ϫ1 970.82 Spratelloides larvae 87 # Species 52 Apogonids 2.5 HЈ 0.7489 Gobies 2.5 JЈ 0.1895 Tripterygiids 2 Near Grid Group # h Ϫ1 175.54 Spratelloides larvae 83 # Species 24 Pomacentrus bankanensis 3.5 HЈ 0.8754 S. delicatulus 2.5 JЈ 0.2755 P. coelestis 2 Pomacentrids (unmetamorphosed) 1 Far Grid Group (22:00 to 23:00) # h Ϫ1 32.65 Psenes arafuensis 45 # Species 32 Pomcentrus bankanensis 9 HЈ 2.3036 Abudefduf vagiensis 8 JЈ 0.6647 Spratelloides delicatulus 6 Atherinids 3 Far Grid Group (01:00 to 05:00) # h Ϫ1 22.30 Pomacentrus bankanensis 28 # Species 26 Spratelloides larvae 22 HЈ 2.2518 S. delicatulus 10 JЈ 0.6911 S. gracilis 9.5 Pomacentrus unpigmented 7 P. coelestis 6 © 2001 by CRC Press LLC the lowest (HЈ ϭ 0.75) due to the dominance of Spratelloides larvae (JЈ ϭ 0.19). Species diversity indexes were highest at the far-grid groups (HЈ ϭ 2.30 and 2.25) due to an even proportioning of abundance among species (JЈ ϭ 0.66 and 0.69). Detailed scrutiny of catch rate data for each of the key taxa illustrates the con- sistency of spatial and temporal patterns. The distribution of Spratelloides larvae in the lee of Helix Reef becomes evident when the catch data from all three sampling periods is integrated over the night of 5 to 6 January (Figure 5). In this figure the columns and discs represent different information. The height and colour of the columns are proportional to the number of individuals collected at each station at a particular time (the numbers over each column are the actual catch rates). The diam- eter of the discs represents the proportion of the total catch, from all stations over the 3 days of sampling, taken at each station, whereas the colour of the disc represents the percentage of the station catch which was captured at that particular time. For example, the relatively small disc at station 10 indicates that few Spratelloides larvae were taken at this station—in fact, only one fish—and the bright red colour indicates that the single individual captured on the night of 5 to 6 January represents 100% of the station catch. A quick scan of disc diameters indicates that Spratelloides larvae were most abundant at stations A, B, and 1. Although there was some tempo- ral variation in catch rates, the spatial distribution pattern remained fairly con- sistent (Animation 1). Apogonids and Gobies had similar distribution patterns to Spratelloides larvae occurring primarily at stations A and B (Animations 2 and 3). Nomeids and Pomacentrids were distributed quite differently. Psene arafuensis avoided Helix Reef and was always most abundant at far-grid stations (Figure 6). Although this spatial pattern was evident at all times, specific catch rates differed consistently among sampling periods. The late evening samples (21:00 to 22:00) always contained considerably more of this species (Animation 4). Pomacentrids were distributed across the entire sampling grid but were most abundant at stations A, B, and C. Catch rates were highest during the late evening and declined steadily over the subsequent sampling periods (Animation 5). BOWDEN REEF STUDY Hydrodynamics Tides during the sampling periods in November and January were similar in both their amplitude (~2.5 m) and semi-diurnal nature. During November, winds were from the north to northwest at 5.5 ms Ϫ1 and a persistent, southeast flow at 0.47 to 0.59 ms Ϫ1 was recorded on the shelf slope seaward of Myrmidon Reef. During January, easterly winds prevailed (2.8 to 12.0 m Ϫ1 ) and the persistent, southeast cur- rent was slightly weaker (0.26 to 0.49 ms Ϫ1 ). For both November and January, computed current fields around Bowden Reef were dominated by tidal forcing (Animation 6) and displayed similar characteristics to previous observations and numerical studies (Wolanski et al., 1989). Current mag- nitudes of 0.3 to 0.4 ms Ϫ1 occurred in the far field during both maximum ebb and flood tides producing zones of strong lateral velocity shear (Figure 7). During flood The Effects of Water Flow around Coral Reefs 215 © 2001 by CRC Press LLC tide, the net southerly current bifurcated at the northern end of the reef, accelerated along the eastern and western flanks, and recombined to the south, resulting in a relatively narrow region of reduced velocity immediately adjacent to the reef (Figures 7a and d). During ebb tide, the net northeasterly current bifurcated to the west of the reef, accelerated around the northern and southern ends, and recombined some distance to the east, producing a wide region of still water along the reef face (Figures 7b and e). The size and strength of tidally generated hydrodynamic features were modu- lated by prevailing winds and the low-frequency, southeasterly shelf flow, both of which varied between November and January. The combination of northerly winds and stronger southward shelf flow in November resulted in an enhanced net south- ward flow which, during ebb tide, produced a number of re-circulation features including the formation of a closed eddy to the southeast of the reef (Figure 7b). Features of this strength were not evident in the computed circulation for January. Removal of tidal effects by temporal averaging of the time-varying currents over a tidal cycle revealed a stronger southward flow during November, with a more clearly defined convergence zone and associated region of relatively reduced velocity to the southeast of the reef (Figures 7c and f). Fish Distribution and Abundance Almost 50,000 individual fish, representing 45 families and over 300 species, were captured at Bowden Reef during the new moon sampling periods in November and January. As in the Helix Reef study, a large number of the species occurred only once or twice. However, unlike the Helix Reef study, Pomacentrids were the most abun- dant family comprising approximately 40% of the catch, followed by Clupeids (9%), Apogonids (8%), Blennies (6%), and Gobies (3%). Clupeids and Nomeids, families instrumental in defining near-reef and far-grid communities at Helix Reef, were of less significance at Bowden Reef. Nomeids were poorly represented, occurring only in extremely low numbers at the more exposed stations, while Clupeids, comprised primarily of Spratelloides gracilis and S. deli- catulus, were ubiquitous (Animations 7 and 8). As the effect of local currents on pre- settlement fish was of primary interest, the Clupeidae were removed from all subsequent analyses. A shift in size frequency distribution toward larger fish in January (p Ͻ0.001, ␹ 2 ϭ 1059; Figure 8) resulted in a highly significant interaction between months and sizes (p Ͻ0.001, Table 2). Pre-settlement Pomacentrids and Blennies were slightly larger in January but juveniles of the more open water families, Scombrids, Carangids and Monocanthids, were substantially larger. The larger size in Pomacentrids appears to be due primarily to a shift in species composition, from a greater number of smaller species in November (e.g., Chrysiptera rollandi and Dischistodus spp.) toward more medium-sized species in January (e.g., Pomacentrus bankanensis), although some Pomacentrids, such as P. coelestis, were larger in January. The other two highly significant interactions (months X stations, and sta- tions X sizes, p Ͻ0.001, Table 2) reveal more as to the possible role of hydrodynamic processes in the distribution of pre-settlement fish. 216 Oceanographic Processes of Coral Reefs © 2001 by CRC Press LLC The interaction between months and stations indicates a significantly different distribution pattern around Bowden Reef between November and January. In November, fish were more abundant at stations 1, 5, 6, 7, and 11 and less abundant at station 3 (Figure 9). The November flow pattern averaged over a tidal cycle (Figure 7c) clearly shows an area of divergence and reduced flow near station 1; a large region of reduced flow along the reef face surrounding stations 5, 6 and 7; and a narrow band of still water immediately to the south near station 11. Station 3, to the north of the reef, lies in an area of relatively strong currents. The tidally averaged flow regime in The interaction between stations and sizes denotes a size-dependent distribution pattern around the reef circumference. Small fish (Ͻ10 mm) had the greatest variabil- ity in abundance among stations, large fish (Ͼ15 mm) had the most uniform distribu- tion, and medium-sized fish (Ն10 to Յ15 mm) had a distribution similar in pattern to small fish but less variable (Figure 10). Small fish were most abundant at stations 7 to 11 and at station 1. During flood tide, stations 8 to 11, which lie along the southern mar- gin, are in a clearly defined convergence zone with an associated reduction in flow velocity (Figures 7a and d). During ebb tide, these southern stations are in zone of flow separation and subsequent eddy formation, and station 7, moored on the eastern reef face, is surrounded by still water (Figures 7a and d). As noted previously, the tidally averaged flow regime places station 1 in an area of divergence. The distribution of medium-sized fish closely parallels that of small fish (Figure 10), but differences in abundance among stations are much smaller. Large fish, with the exception of station 3, are uniformly distributed. Catch data for Chrysiptera rollandi and Pomacentrus coelestis in November illustrate the distribution of a small/medium (mean ϭ 9.98 mm) and large (mean ϭ 15.09 mm) fish (Animations 9 and 10). A plot of mean size at each station for November and January data combined (Figure 11) shows that most fish were captured at station 11 and that their mean size was significantly smaller than anywhere else (p Ͻ0.05, Games and Howell multiple range procedure; Sokal & Rohlf, 1981). The Effects of Water Flow around Coral Reefs 217 TABLE 2 Analysis of Variance Table Examining the Catch of Pre-Settlement Fish as a Function of Month, Station, and Size Source of Variation df MS F P Month 1 0.071 0.52 ns Station 12 0.905 6.66 *** Size 2 20.591 151.60 *** Month ϫ station 12 0.371 2.74 ** Month ϫ size 2 1.776 13.08 *** Station ϫ size 24 0.364 2.68 *** Month ϫ station ϫ size 24 0.159 1.169 ns Note: ns ϭ not significant. ** ϭ p Ͻ0.01. *** ϭ p Ͻ0.001. © 2001 by CRC Press LLC January, although similar in pattern, was substantially weaker (Figure 7f). Dispersion Model The theoretical distribution of pre-settlement fish around Bowden Reef, as predicted by the second-order, advection-diffusion model, varied between November and January (Figure 12). With the exception of passive, non-swimming particles in November, fish were retained along the reef face from the northeast sector (location 0.3, Figure 12) around to the southern sector (location 0.8). However, the exact loca- tion of maximum retention, the most abundant size class of fish, and the total num- ber of propagules retained differed between months. In November, the primary peak in retention for all swimming speeds occurred in the northeast sector; a secondary, weaker peak was located on the eastern face near stations 5, 6, and 7; and abundance declined steadily around the southeast reef margin toward the southern end (Figure 12). The difference in height between the primary and secondary peaks was most pro- nounced for large, faster-swimming fish, indicating a fairly variable distribution, and was least pronounced for small, slower fish, indicating a more even distribution. The model also predicted a greater abundance of large rather than medium-sized or small fish, and a total lack of retention in either the northwest or southwest sectors. In January, a single peak was located to the southeast and a greater number of medium-sized and small fish were retained (Figure 12). Within the sector of maxi- mum retention, medium-sized fish (swimming speed of 0.1 ms Ϫ1 ) were most abun- dant, while at other locations, such as in the northeast sector, predicted abundance was proportional to swimming ability. Generally, the overall level of retention was higher—the total number of fish was higher, retention occurred in the northwest and southwest sectors, and a greater number of passive particles were trapped, especially along the southern margin. Detailed examination of the interaction between local currents and various swimming abilities may help explain the resultant distribution pattern generated by the diffusion model. If the “simulated fish” were treated as passive particles, they were either swept past the reef or retained temporarily in regions of reduced velocity, but without an increase in concentration (Animations 11a and 12a). If the fish were allowed to swim to and remain near the reef, then “hot spots” of increased concen- tration appeared at size-specific locations around the reef. Larger, strong-swimming fish reached the reef first, quickly aggregating at a point on the upstream end in con- centrations exceeding an order of magnitude above that of the incident plume. Smaller, slow-swimming fish spent more time in the prevailing currents and were advected farther downstream before making their first contact with the reef. These aggregations developed more slowly, but were still at concentrations above that of the incident plume. Once near the reef, all fish, regardless of size, were subjected to the same suite of tidally dominated local currents. Multiple, size-specific aggregations which had formed as the incident plume was wrapped around the reef, were washed back and forth along the reef face by the ebb and flood of the tide. Variability in current strength pulled at and deformed these aggregations, but still they maintained their cohesion. After a period of time, similar distribution patterns emerged for the “hot spots” in each month, although concentrations within size-specific aggregations 218 Oceanographic Processes of Coral Reefs © 2001 by CRC Press LLC [...]... persisted during the mass coral spawning event in November 1983, the combination of constant northerly winds, prevailing southerly flow, and falling tide at the time of our sampling would most likely generate a very similar flow pattern Wind stress on the sea surface has the greatest influence on surface flow and the conϪ1 stant northerly winds during this study (2.2 to 8.3 m s ) would have accelerated the. .. stronger-swimming fish were able to maintain their position on the more exposed western side Inter-month hydrodynamic variability resulted in distinctive dispersal patterns in each month During the November simulations, passive particles were not retained in the vicinity of the reef for more than 2 days, zones of aggregation remained on the eastern face, and there was very little trapping on the western... fish, and water currents on a windward reef face: Great Barrier Reef, Australia Bulletin of Marine Science 42, 459–479 Hamner, W.M & Wolanski, E 1988 Hydrodynamic forcing functions and biological processes on coral reefs: a status review In Proceedings of the 6th International Coral Reef Symposium, Townsville, Australia, August 8 to 12, 1988 Vol 1: Plenary Addresses and Status Reviews, 1988, The 6th International... Sustained swimming abilities of the late pelagic stages of coral reef fishes Marine Ecology Progress Series 149, 35–41 Stobutzki, I.C & Bellwood, D.R 1998 Nocturnal orientation to reefs by late pelagic stage coral reef fish Coral Reefs 17, 103 –110 Thorrold, S.R 1992 Evaluating the performance of light traps for sampling small fish and squid in open waters of the central Great Barrier Reef lagoon Marine... LLC The Effects of Water Flow around Coral Reefs 225 FIGURE 1 Diver with light trap FIGURE 2 Map showing the location of Helix Reef and Bowden Reef and the location of oceanographic instruments (red diamonds are current meters, green diamonds are tide gauges) within the central GBR Insets show three-dimensional detail of reef topography and location of sampling stations FIGURE 3 Postulated, tidal-averaged... around coral reefs At Helix Reef, station A, located near the indentation in the southern margin, consistently had the highest catch rates and greatest number of species Overlaying our station grid with the flow pattern generated by the time-averaged hydrodynamic model of Sammarco and Andrews (1988) places station A at the centre of an eddy (Figure 13) Although this flow regime was developed with the. .. What the pelagic stages of coral reef fishes are doing out in blue water: daytime field observations of larval behavioural capabilities Marine and Freshwater Research 47, 401 –411 Leis, J.M & Carson-Ewart, B.M 1977 In situ swimming speeds of the late pelagic larvae of some Indo-Pacific coral- reef fishes Marine Ecology Progress Series 159, 165–174 McIlwain, J.L 1996 Hydrodynamic flows and flux of larval... were in the south and southeast sectors where aggregations of fish generally remained at or above the concentration of the incident plume DISCUSSION The findings of these two independent studies suggest that fine-scale hydrodynamic features generated through the interaction of reef topography with prevailing, farfield currents have a considerable impact on the distribution and abundance of presettlement... Wolanski, E & King, B.A 1990 Flushing of Bowden Reef lagoon, Great Barrier Reef Estuarine, Coastal and Shelf Science 31, 789 –804 Wolanski, E., Doherty, P., & Carleton, J 1997 Directional swimming of fish larvae determines connectivity of fish populations on the Great Barrier Reef Naturwissenschaften 84, 262 –268 Wolanski, E & Spagnol, S 2000 Sticky waters in the Great Barrier Reef Estuarine, Coastal and Shelf... Press LLC 220 Oceanographic Processes of Coral Reefs Willis & Oliver, 1990), headlands (Alldredge & Hamner, 1980; Murdoch, 1989), and islands (Hernandez-Leon, 1991) Also, Sammarco and Andrews (1988 and 1989) found the highest concentration of coral spat (up to 90% of all settlement) on settlement plates moored to the south of Helix Reef in areas of high water residence time, again suggesting that lee . on the retention of pre-settlement fish, we present the findings from two independent studies at two physically distinct platform reefs in the central section of the Great Barrier Reef (GBR). The. moored far- ther out in deeper water, within 100 m of the reef crest. The outside near-crest traps were placed just in front of the breaker zone and the inside traps located just behind the reef flat per- sistent southward flow pushing the resultant current more to the east during falling tide and more to the west during rising tide (Gay & Andrews, 1994). The interaction of these far-field