MARINE ECOLOGY PROGRESS SERIES Published June 8 Mar. Ecol. Prog. Ser.

Patterns of settlement, survival and growth of across the Great Barrier Reef

Paul K. ay ton', J. H. carleton2, Andrew G. ~ackley~,Paul W. Sammarco2

' Scripps Institution of Oceanography, A-001, La Jolla, California 92093, USA Australian Institute of Marine Sciences, PMB 3, Townsville, Queensland 4810, 15 Ardyne St., Murrumbeena, Victoria 3163, Australia

ABSTRACT: Settlement plates of coral rock were moored on the Central Great Barrier Reef in order to evaluate the distribution of competent bivalve larvae and post-settlement growth and survivorship of oysters in different regions. Oysters were chosen because they dominate plates. The regions and sites were outer reef (Myrmidon), central reef (John Brewer), inner reef (Pandora) and an inshore non-reef habitat (Bowling Green Bay). Cross-shelf differences were studied by transferring plates from John Brewer Reef to the other sites with the return of some plates to John Brewer to control transfer effects. Cross-reef and fish predation effects were studied by transferring caged and uncaged plates to the fore reef, reef flat and back reef at John Brewer. Transfer to the back reef also evaluated the effect of Acropora colonies as relative refugia from fish predation and sand flats as an area relatively free of the invertebrate predators associated with coral rubble. There were clear regional settlement differences. Myrmidon Reef had lowest densities (6 2 ind 500cn1-'), high species richness (l? of 24 species identified in the project), and low equitabllity dominated by foliurn. John Brewer (l6 species) had high densities (> 100 ind. 500cm-~)and hlgh specles equitability. Pandora Reef had the most species (20)and the highest densities because of a massive settlement of maxima, most of which fell off the plates after recovery. Bowling Green Bay had low density (- 16 ind. 500cn1-~)and only 13 species, Ostrea angasi being a strong dominant. Average bivalve sizes increased from the outer to the inner reef. The cross-shelf transplant experiments showed strong trends in the growth rate with Myrmidon having the slowest and John Brewer and Pandora having the fastest growth. Growth rates of Plnctada and Ostrea spp. and commercialis differed significantly across the shelf. Ostrea spp. were largest at John Brewer, while Pinctada spp. and most Crassostrea spp. were largest at Pandora. At Bowling Green Bay oysters were smaller than at Pandora. Survivorship of Crassostrea spp. was about the same in all areas, but Pinctada and Ostrea spp. had significantly higher mortality at Bowling Green Bay and Pandora. In the cross-reef experiment immediate fish predation was highest on plates set in sand and lowest on plates under an Acropora thicket and there was much less fish predation at the reef flat and back reef sites. Oysters suffered very large mortalities from predation by invertebrates, especially gastropods, at all sites, but this was reduced for plates over sand substrata.

INTRODUCTION exist for hard corals (Done 1982, Sammarco 1986), soft corals (Dinesen 1983), sponges (Wilkinson 1986) cal- Most ecologists recognize that recruitment processes cified green algae (Drew 1983), and fishes (Williams are the most important yet least understood component 1982, Williams & Hatcher 1983, Doherty & Williams of community ecology. This is especially true for coral 1988). While these patterns appear general, it is not reef communities which have extremely high pro- clear whether the zones result from larval availability, portions of planktonic larvae. In general, recruitment habitat selectivity or post-settlement growth and sur- patterns are influenced by availability of competent vivorship. Recent fish and coral larval research (Wil- larvae, habitat selectivity and post-settlement survivor- Liams et al. 1984, Sammarco & Andrews 1988, Wolanski ship and growth. & Hamner 1988, Willis & Oliver in press) had found A great deal of recent research has demonstrated a important differences in larval transport over several gradation in physical parameters and community struc- time scales. This emphasizes the importance of inter- ture across the continental shelf in the Central Great reef and cross-shelf patterns in larval dispersal. This Barrier Reef region. Similar broadscale zonal patterns study attempts to evaluate larval availability and

O Inter-Research/Printed in F. R. Germany 76 Mar. Ecol. Prog. Ser. 54: 75-90, 1989

growth/survivorship of oysters over the same cross- prevailing current patterns so as to eliminate the effects shelf gradient. of the reefs or reef-associated planktivores on water The primary objective of this study was to evaluate movement or entrained plankters (Hamner & Hauri settlement, growth, and survivorship patterns of oysters 1981, Hamner et al. 1988). Post-settlement growth and across the shelf, independent of possible reef-associ- survivorship was evaluated by transfemng plates with ated effects. Oysters were chosen because previous attached fauna from the central reef to other regions. At work (Sammarco 1983, Sammarco & Andrews 1988) Myrmidon and John Brewer Reefs they were placed found that bivalves, especially oysters, had massive - 1 km northeast of the reefs to minimize reef effects on settlements on coral plates in open water in the central local hydrography and to have the plates 'upstream' of Great Barrier Reef region. With few exceptions oysters the reefs such that they sampled the outer and mid- almost completely covered the plates. The only other shelf water that impinges on the reef. At each site 2 to occur on many of our plates were gastropods replicate moorings were set. Inshore, at Pandora Reef, and flatworms, both predators on the young oysters. In moorings were about 2 km from the reef, about 0.5 km addition to being available, the planktotrophic larvae apart, and well away from the plume of the reef. The are probably in the water about 2wk, thus offering a Bowling Green Bay site was chosen to be near the good compromise between very long-lived (e.g. mouth of the Haughton Rver which we thought might echinoderms) and short-lived (e.g. tunicates) larvae enhance the local productivity, thus maximizing any which should be studied at different spatial scales. differential growth rates. The moorings were about Specifically we tested several general hypotheses 300 m apart. At Myrmidon and John Brewer Reefs they regarding: (1) the availability of larvae as estimated by were anchored in 60 and 50m water depth respec- recruitment onto coral plates in 4 distinct cross-shelf tively, and 150 to 200m apart. The plates were 15m regions of the Central Great Barrier Reef; (2) differ- below the surface. Water depth at Pandora Reef was ential post-settlement growth of each species (assum- only 15 m; thus the settling plates there were placet at ing equivalent settling times); (3) differential growth 12m. At Bowling Green Bay, water depth was 12m, and su~vorshipof species on coral plates transplanted and the settling plates were placed at 8m. from a central shelf to each of the 4 reglons; and (4) Plates were cut from natural uncleaned coral heads, differential growth and survivorship of oysters trans- mostly from Poritesspp. The corals were cut on a rock planted across a reef within the Great Barrier Reef. saw into discs ca 2 cm thick. Their average surface area was 615cm2. They were drilled and attached to gal- vanized steel racks with stainless steel pins supporting MATERIALS AND METHODS the plates at a 45" angle (Carleton & Sammarco 1987). A previous study (Sammarco & Andrews 1988) con- Study areas and mooring designs. Mooring locations firmed that this design attracts dense settlements of are illustrated in Fig. 1; they were placed upstream of oysters. The mooring design ensured the plates faced

CORAL SEA

AUSTRALIA

Fig. 1 Location of mooring sites at Bowling Green Bay. Pandora, Brewer and Myrmidon Reefs, Great Barrier Reef Dayton et al settlement on the GBR 77

directly into the prevailing current (Sammarco & 10m deep, but the reef flat site and two of the adjacent Andrews 1988). back reef sites on a small patch reef were at depths of AI1 the moorings were deployed between 1 and 4 5 m. One pair of racks at the back reef was covered with October 1985. The moorings at John Brewer were a layer of branching Acropora coral to reduce the checked by diving on 20 November and 6 December predation pressure of large fishes. The third pair was 1985. No oysters were visible on either date, but very placed on a large nearby sand flat at 9m depth at least small ones may have been hidden in the algal turf. All 20m from the closest hard substratum to see whether plates were retrieved between 14 and 17 February 1986. non-reef associated fishes are effective predators of In most instances the only other conspicuous sessile small oysters; by comparing caged plates on the sand animals on the plates were bryozoans and rare hydroids and on the reef we could evaluate the effects of inverte- which had settled on the oysters. The only cases where brate predators associated with the coral rubble of the other recruits may have affected oyster recruitment reef. The patch array offered a comparison to the fore- were 3 plates at Pandora Reef (not analyzed) which had reef and reef flat sites as well as comparisons among large bryozoa and compound tunicate colonies and most backreef sites (Table 1). of the plates at Bowling Green Bay which initially had All transplant experiments were retrieved between barnacles and later various species of compound 14 and 17 April 1986. Upon recovery, plates were tunicates. Plates used to evaluate settlement on each individually bagged and frozen. In the laboratory the mooring at every location were immediately bagged plates were thawed, cleaned of sediment and dried at and frozen. The remaining plates at John Brewer Reef 50°C. Oyster identification followed Hynd (1955) for were randomly placed in shaded tanks with running Pictada spp. and Thompson (1954) and Dinamani seawater on board the RV 'Lady Basten' for transport to (1971) for species of Ostrea, Crassostrea and Pyc- other study sites (which required, at most, 2 d). At each nodonte. Voucher specimens were retained and are location a minimum of 8 plates from John Brewer Reef housed at the Australian Institute of Marine Science. were redeployed on the moorings. Selection of plates Sizes of the oysters were measured with calipers. was randomized. Thus, plates from Brewer Reef were Ostrea and Crassostrea spp. were measured on longest well mixed to prevent the transfer of any possible and narrowest axis and size presented as the area of an mooring effects to the translocation experiment. A elhpse. This approach was necessary because the indi- Brewer-Brewer transfer was included and served as a viduals are extremely plastic. For Pinctada spp., hinge control for possible handling effects. In addition, a total length was the most consistent measurement for small of 50 plates from the John Brewer moorings were specimens (though for larger specimens the dorsovent- attached to racks, 5 to each rack, 35 cm apart, held at a ral measurement may be the most satisfactory; 45"angle and placed in pairs on John Brewer Reef on the Alagarswami & Chellain 1977).The condition of oysters fore-reef slope, mid-reef flat and 3 back reef lagoon (dead or alive) on the reef transplant plates was noted. sites. One rack from each pair was caged. The cage was On dead specimens it was also noted whether gas- a broad metal frame covered with a 1 cm2 nylon mesh, tropod drill holes were present. changed at monthly intervals. The fore reef slte was Data analysis. After the initial settlement and survi-

Table 1. Summary of c?xperimental design

Hypothesis I: Bivalve larvae equally available across the continental shelf Treatment: Settlement plates placed at 4 sites: Myrmidon, John Brewer and Pandora Reefs and Bowling Green Bay. Established 1 to 4 Oct 1985; retrieved 14 to 17 Feb 1986 Hypothesis 11: Equal growth and survival rates of initial recruits at each site Treatment: Assumes equivalent settling time; evaluate bivalves collected 14 to 17 Feb Hypothesis 111: Equal growth and survival of oysters across the continental shelf Treatment: Standardize initial collection by using oysters from 1 site (John Brewer Reef) and translocating them to other sites and back to John Brewer Reef to control handling effects Hypothesis IV: Equal growth and survivorship of oysters across John Brewer Reef Treatment: Oysters placed at 3 sites across John Brewer Reef Hypothesis V: Fish and benthic invertebrates have no effects on oyster survivorship Treatment A: One rack of 5 plates each was caged at each site to prevent fish predation Treatment B: At inner reef site caged and uncaged racks placed under Acropora and adjacent to Acropora patch because Acropora restricts access of bigger fishes Treatment C: At inner reef site caged and uncaged plates placed on sand substratun~whch had none of the natural benthic predators (mostly gastropods and flatworms) 7 8 Mar Ecol. Prog. Ser. 54: 75-90, 1989

val period, mooring effects on total bivalve density (no. Ostrea spp., 300 mm2 for Crassostrea spp., and 20 mm per 500 cm2) and the densities of the 10 most abundant for Pinctada spp. These sizes were derived from max- bivalve species were tested by l-way nested analysis of imum sizes on initial plates. For each of the 4 sampling variance in which moorings were nested within loca- locations and 2 data sets (initial settlement and trans- tion. Only data from Myrmidon, Pandora and Bowling plant data), size frequency distributions were con- Green Bay were used in these analysis as no replicate structed for all species of Ostrea, Crassostrea and Pinc- data were available from the second mooring at tada. For both data sets, size measurements for all Brewer. With the exception of one Bowling Green Bay species within each of the 3 major genera were then mooring which had several Patro australis found combined and distributions calculated for each genus nowhere else, there were no mooring effects on settle- by sampling location. ment density and survival of original recruits (p> 0.05). All individual replicate data from the 4 sampling Density data from both moorings at each location were locations and 2 treatments (initial and transplant combined and the differences among the 4 locations plates) - a total of 77 settling plates - were subjected to was examined by single fixed-factor ANOVA's or by a multivariate classification analysis to discriminate Kruskal-Wallis test in those cases where the variance associations among the 18 bivalve species encountered remained heteroscedastic after transformation (Sokal & during the study. Bray-Curtis similarity coefficients Rohlf 1981, Underwood 1981). (Bray & Curtis 1957) were calculated between all sam- Density of surviving bivalves on transplant control ples and the 2 most similar samples fused to form a (Brewer-Brewer) and plates transplanted from Brewer cluster. This process was repeated using Burr's incre- Reef moorings to other moorings were compared using mental sum of squares strategy (Burr 1970). The sample l-way fixed-factor ANOVA's. Separate analyses were groupings were then subjected to diagnostic techni- performed on total density data for each of the 3 most ques which used the algorithms of Abel et al. (1985) to important genera as well as each of the individual determine which bivalve species contributed most in species which comprised them. In order to eliminate discriminating among the associations. Brillouin's the effects of post-transplant settlement, the analysis diversity index (H,,'), being the most appropriate mea- included only individuals larger than 250mm2 for sure for a fully censused community or collection

Table 2. Recruitment patterns of all bivalves observed on plates acr.oss the continental shelf. Settlement plates were placed I to 4 Oct 1985 and recovered 14 to 17 Feb 1986. Species order is to simplify cross-shelf comparison. Data are mean densities per 500 cm2 (f standard deviation). Total mean density for Pandora Reef excludes the single recruitment event of several thousand Pictada maxima (see text). +: Spec~esobserved but not counted: -: Species not observed

I species Myrmidon Brewer Pandora Bowling Green Bay 1

Pa tro a ustraLis Ostrea angasi Pododesmus ? Pinctada rnargn tifera Modjolus 7 Ostrea trapezina Crassostrea commercialis Ostrea folium Unknown mytilld Crassostrea tuberculata Pinctada albina carchariarum Pinctada albina sugillata Pinctada maxima Pinctada maculata h-nctada furcata Crassostrea sedea Ostrea nomades Pinna bicolor Pateria sp. Crassostrea echina ta Pycnodonte h yotis Crassostrea amasa Streptopinna sp. Pinctada chemnitzi Total density per plate Dayton et a1 : Oyster settlement on the GBR 79

(Pielo~?1975), along with a measure of eveness (Jbl), ALL OPEN-WATER SAMPLES (Orlglnal and Transplant) were calculated for each group. For the transplant experiments across John Brewer Reef, all individual replicate data from the 5 reef loca- tions (reef front, reef flat, back reef, Acropora, and sand) and the 2 treatments (caged and uncaged) - a total of 50 plates - were also subjected to multivanate classification analysis. As replication was even throughout the experiment the significance of the groupings produced by the hierarchical classification could be tested by the Sandland & Young (19?9a,b) method. However, the fact that the plates on the rack are not independent of each other violates an assump- tion of this method. Because the method is robust we use it to provide preliminary indications of relation- ships or patterns but the plates are not true replicates.

RESULTS

Cluster analysis Fig. 2. Dendrogram from classification analysis of all replicate initial and transplant settlement plates from Bowling Green The cluster analysis on all the mooring data pro- Bay, Pandora, Brewer and Myrmidon Reefs (N= 77) for the 18 duced ? distinct groups (Fig. 2). Three of these groups bivalve species encountered during the study. G1: initial John (G2 to G4) contained replicate samples from only one Brewer Reef settlement clustered with majority of trans- planted plates. G2: 9 of 14 plates from Bowling Green Bay initial recruitment location. The species distribution characterized by dominance of Ostrea angasi. G3: 8 of 11 among these groups is probably indicative of larval plates from Pandora Reef characterized by heavy settlement availability across the shelf. Two of the remaining of Pinctada maxjma. G4: 8 of 14 plates from Myrmidon Reef groups (Gl, G5) contained plates which had undergone characterized by dominance of Ostrea folium. G5: 6 of 14 plates from Myrmidon plus 1 each from Pandora and Bowling transplantation. Any changes in species associations Green Bay. G6: plates from Bowling Green Bay with Patro within these 2 groups from that of the original Brewer australis. G7: a few plates transplanted to Pandora and Bowl- plates are most likely due to the effects of the new ing Green Bay plus 2 plates from Bowling Green Bay transplant environment. (Fig.2, G6). Patro australis occurred only on these plates. Low species equitability, combined with the Recruitment patterns presence of only 6 species, resulted in the lowest species diversity index of all initial settlement locations The hypothesis that larvae are equally available (Hb' = 0.1432). across the shelf can be evaluated by considering the Group 3 (Fig. 2) totally comprised settlings plates initial recruitment of oysters on the plates suspended at from Pandora Reef (8 out of 11 replicates). Many of the each of the 4 sampling locations (Fig. l; Table 2). species which occurred at the offshore reef locations Groups 2 through 4 produced by the cluster analysis were also found here (Table 2). This large number of contained only replicate samples collected after the species produced a relatively high diversity index (Hb' initial recruitment period from Bowling Green Bay, = 0.5818). Pinctada maxima settled only on Pandora Pandora and Myrmidon respectively. Initial settlement plates and in high abundances. The recorded values plates from John Brewer Reef clustered with the major- were, however, a gross underestimate of the true ity of plates transplanted from John Brewer Reef to the abundance of this species due to its poor attachment to other reefs or back to John Brewer (Fig. 2, Gl). the plates. When the racks were recovered, the entire Bowling Green Bay plates lacked several species structures were covered with thousands of P. maxima found at the other locations (Table 2). Nine out of 14 and the plates appeared as black spheres. Unfortu- plates from this location form a group (Fig. 2, G2) nately, as they came out of the water practically all of characterized by the dominance of Ostrea angasi (Jbl = the P. maxima fell off, even before they could be 0.1841), a species which occurred only at the 2 near- photographed, much less counted. The densities of P. shore locations. Three of the remaining plates, all from maxima were calculated from the volume of a sphere, the same mooring, formed another distinct group assuming slightly different sphere diameters. The real 80 Mar. Ecol. Prog. Ser. 54: 75-90, 1989

densities on all but 3 plates ranged from 1 to 7000 P. maxima per 500cm2. The 3 exceptions were plates apparently dominated by bryozoans and compound tunicates. The densities for the other species are prob- ably valid as we saw no evidence that they were lost with the loosely attached P. maxima. Bivalve densities at Myrmidon (F = 6.2 ind, 500 cm-') were significantly lower than at the other reef locations (Tables 2 and 3). The densities were so low that the plates were recorded as essentially bare when re- trieved. Despite the low settlement densities, the number of species was second only to Pandora (Table 2). Eight of the 14 replicate settlement plates from myrmidon clustered together in a distinct group (G4; Fig. 2). The species association in this group was domi- nated by Ostrea folium (Jbl = 0.45) which occurred in densities 1 to 2 orders of magnitude greater than the other species. The remaining Myrmidon plates clus- tered with 1 plate from each of Pandora and Bowling Green Bay (G5). Bivalve densities on the Myrmidon plates in this group were very low (3 ind. 500cm-'1. This clustering is probably based on joint absence - all they have in common is the absence of most species. Biologically, G5 is an artifact and the 6 Myrmidon replicates could just as well be clumped with the other 8 replicates, and the plates from Pandora and Bowling Green could be retained with their original clusters. Oyster densities were greater at John Brewer and Pandora Reefs than at Myrmidon Reef and Bowling Green Bay (Table 3), Recruitment densities for 7 of the oyster species differed significantly among locations. The 3 species which did not significantly differ were relatively rare species. While there is no consistent pattern for the 7 species, neither the ANOVA nor the cluster analysis showed significant mooring effects at a given site and the patterns seem to represent on onshore-offshore gradient. Summarizing, the initial recruitment was variable, as might be expected in settlement studies, but there were marked trends towards a low density of a diverse assemblage of species on the outer shelf, a high density of many species in the central and inner reef habitats and a low density, low diversity recruitment at the nearshore non-reef habitat.

Effect of transplantation

The cluster analysis suggested that transplanting plates from Brewer to the other locations did not, in the majority of cases, affect the species associations pre- sent on those plates. All of the initial Brewer plates, all Brewer controls (plates returned to Brewer), all plates transplanted to Myrmidon, 50 O/O of those transplanted

to Bowling Green Bay and 40 O/O of those transplanted Dayton et al.. Oyster settlement on the GBR 81

to Pandora clustered together in a distinct group (Cl, cantly higher densities offshore (Brewer and Myrmi- Fig. 2). The abundances of most species were highest in don) than inshore (Pandora and Bowling Green Bay), this group with Pinctada maculata, a species which and Ostrea had significantly higher densities at Brewer initially recruited in large numbers on Brewer plates than at inshore locations. Most of the Pinctada taken to (Table 2), contributing most to the characterization of Bowling Green Bay died. The analyses on individual this particular bivalve association. This group also had species generally supported these findings. As we have the highest species diversity index of all groups (Hb' = presumably eliminated any new recruits from the 0.9961) due both to species richness (16 species) and a transplant analyses by excluding individuals below a high evenness component (Jbl= 0.8273).The remain- set size, these results do indicate different survivorship ing plates transplanted to Pandora and Bowling Green at the different locations. Bay clustered together with a small number of the initial Bowling Green Bay inshore plates (G? in Fig. 2). The species diversity index was also high for this group Growth data (Hb' = 0.9439) due again to both species richness (15 species) and even proportioning of individuals anlong All the size data for each individual species and each species (Jb' = 0.8026). However, species abundances genus has been collated into 2 tables. The basic pa- were much lower than in Group 1 (mean density = 50 rameters (sample size, mean, median and mode) as ind. 500~m-~)and no one species contributed excep- well as the results from the frequency analyses and tionally in defining the association. The change in mean size analyses are presented in Table 5 for Initial species associations in this group from that found on recruits and Table 6 for subsequent transplants. the initial Brewer Reef plates is probably due to differ- After the initial recruitment period, in all cases where ential mortalities among species. Increased mortality in enough data existed for frequency analyses, there were the more offshore species may account for the substan- significant differences in size distributions among loca- tial drop in bivalve densities between this group and tions (Table 5). The results from analyses of mean sizes Group 1. support these findings. Mean sizes of Ostrea and Cras- The ANOVA analysis of the density of surviving sostrea diminished with distance from the mainland; transplanted oysters is presented in Table 4. This however, Pandora often had larger individuals than the analysis suggests that the effect of transplant location other reef locations. Mean size for Pinctada was signifi- on the density of surviving bivalves differed for each of cantly greater at Brewer than at the other reef loca- the 3 genera. Crassostrea showed no significant differ- tions. ence among transplant locations, Pinctada had signifi- The mean size of all transplanted Ostrea and Pinc-

Table 4.Density of oysters (no. per 500 cm2) surviving translocation from Brewer to Myrmidon, Pandora, Bowling Green Bay or returned to Brewer. The 12 most abundant species and the totals for each genus are listed. Only Ostrea spp. with elliptical areas between 250 and 1100 mm2,Crassostrea spp. with elliptical areas between 300 and 1100 mm2, and Pinctada spp. with a hinge length between 20 and 45 mm were considered. ANOVA. analysis of variance, SNK: Student-Newman-Keuls multiple range procedure for comparing means. Standard deviation in parentheses. Remaining codes as in Table 3

Species Myrmidon Brewer Pandora Bowling Transfor- ANOVA SNK (n= 9) (n=8) (n=10) Green Bay mation (n=8)

Ostrea folium 4.9 (4.44) 11.0(9.11) 2.7 (4.64) 1.7 (1.55)Log,,(Y+l) ' ' BR > MY, PA, BG 0.nomades 5.9(6.55) 2.7(2.66) 1.9(2.10) 1.4(1.05)None ns - 0. trapezina 10.0 (10.05)15.2 (8.14) 6.6 (6.84) 2.4 (3.95)None ' BR>BG 0. angasi 0 (0) 0.1(0.32) 0.08(0.32) 0.1 (0.32)None ns - Crassostrea commercialis 4.1(3.71) 3.4 (2.81) 2.4 (1.84) 2.1(3.08) None ns - C. echinata 0 (0) 0.16(0.45) 0.1 (0.32) 0 (0) None ns - C. tuberculata 2.3 (2.30) 0.7 (1.45) 1.3 (1.67) 0.6 (0.84)None ns - C. sedea 3.8(3.71) 2.9(4.17) 2.5(4.75) 2.3(3.30)None ns - Pinctada maculata 10.6 (7.01) 15.6 (10.82) 2.7 (4.36) 1.6 (2.30)L0g,~(y+1) ' BR, MY > PA, BG P. furcata 4.1(2.53) 3.3(1.87) 1.6(2.64) 0.5(1.0)None m,. MY, BR>BG or MY > PA, BG P. albina sug~llata 8.5(6.73) 6 9 (4.56) 2.8 (3.52) 5.8 (5.76)None ns - P chemnitzae 3.2(1.87) 18(.0) 2.5(4.05) 0.2(0.45)None ns - All Ostrea 20.8(17.81) 29.0 (12.21)11.2 (10.91) 5.6 (4.40)Loglo(Y+l) " BR> PA, BG All Crassostrea 10.2(7.03) 7.2 (6.59) 6.2 (6.35) 5.1 (4.86)None ns - All Pinctada 26.5 (8.4) 27.6(11.03) 9.5 (12.21)8.1 (7.93)None .., BR,MY > PA, BG Table 5. Average sizes of oysters after the initial settlement and survival period, analysis of size frequency and ANOVA's on mean size among sites. Ostrea and Crassostera size is area of an ellipse (mm2);Pinctada size is hinge length (mm). N number of individuals; A. multimodal distribution (>3), remaining codes as in previous tables

Species Myrrnldon Brewer Pandora Bowling Green Bay Frequency analyses ANOVA's on mean size (I-way une\,en) N Medn Median Mode N Mean ,Median Mode N Mean Median Made N Mean Mnd~an Mode No.01 G- Slg. Trans- Sig. SNK (SDI (SDI (SDI (SDI classes slalisl~c forrna- Lion

OsLiea folrurn 70 153.9 129.5 139.9 73 255.7 207 9 139 9 77 382 33 341.6 279.7 0 - - 6 72.8 None" " ' PA>BR>MY (106.7) (208 01 (189.81 O nomddes 9 368.9 327 1 188.7 2 166.2 166.2 71.9 7 330.8 297 7 305 4 0 (232.2) (120.2) 305 4 (175.0) 538.9 0 lrdpezina 4 148.3 171.3 168 7 I6 189.2 130.2 25 37 441.3 433 3 456 2 12 374.6 317 7 25 J 85 None" '' PA> BR>MY (64.41 (163 6) (151 8) (314 9) 0. dngdsi 0 - - 0 - - 21 501.8 473.3 482.5 140 232.2 165.4 179.1 . 25 3 ' None "' PA>BG 147.3 (16.1) (14.6) AU Oslred 83 178.9 146.8 147 3 91 242.1 164 121 397.4 372.7 294.6 13 350.2 305.4 " ? 80'' None* "' PA> BR, MY (139 9) (200.0) 282.3 (179.61 (341.1)

Crassoslred 5 1566 104 5 141 1 9 273.9 313 52 485.2 477.7 4234 3 364 9 383.1 G Log,,(Yl " ns co171nlercinl~s (104 8) (I10.71 (257.6) (142 3) C. echrnars 1 198.7 199.7 198.1 5 260.8 222 1 A 1 447 447 4587 0 - - - (0) (200.1) (01 C L~be~~uldLd 1 226.4 226.4 219 3 0 6 456.9 279.8 219.3 8 146.8 105.8 52.7 (01 (371.01 (102.41 C sedea 10 148.4 93 6 18.4 5 310.7 299.1 283.5 31 451.8 453.7 416 1 0 - - \/V PA. BR> MY (130.01 (52 11 (I89 7) PA>BR>MY All Crdssoslrea 16 156 5 112.5 300 7 14 287 0 306.1 300 7 89 471 6 454.3 451 1 11 206.3 154 6 150 4 \/ Y ... (115.91 (93.41 (242.5) (147 6) Pinclddd 2 10.5 10.5 8.2 37 16.5 15.3 15.1 5 9 8 9.8 8.2 0 - None" " BR> PA rnaculald (2.3) 11 6 (5.3) (0.71 P furcdla 3 10.6 11.1 12.37 21 22.4 22 23.7 1 10 1 10.1 10.6 0 - None "' BR>MY (1.71 14 31 17 3 10) P dlbirla s~lgiiidld 2 11.2 11.2 10 6 9 18 6 18.5 17 3 0 - - 3 224 24.4 A l0 3) (7 07) (7.21 P rarchdrldrum 1 63 6.3 5.: 0 1 34.3 34.3 34.3 l 20.6 20.6 21 1 None (01 (0) ("1 P margrililem - - 1 37 37 69 8.3 8.4 7 1 8.7 8 7 7.9 (01 (1.18) (0) P nlaxirnd - 0 - - 417 89 9 7 0 - - (1 48) All Pincladd 10 8.14 10.0 10.6 68 18.9 18.2 15.9 76 88 8.6 10 6 5 19 3 20.6 26 4 2 123.2 ' None" " ' BR>tvlY, PA (4 71 (6.21) (3.27) (7.81 :able 6. Average sizes of oysters survivors translocated from Brewer. Codes as in previous tables

Species Myrmidon Brewer Pandora Bowl~ngGreen Bay Frequency cln.rlyses ANOVA s on 1r:ca::

size (I-way uneven) h' Mean Median Mode N Mean Medan Mode N Mean Med~an Mode N Mean Median Mode Noof G- Slg. Trans- S SNK ISD) (SDI (SDI (SDI classes sldl~sttc forrna- lion

OsLrea lolium 57 358.3 339 365.3 104 453.8 386.3 365 3 42 390 24 374.6 365.3 17 379.0 296.5 250 4 ? 18.2 None,' "' BR:.MY (83 29) (184 45) (143 56) (142 151

.A c 0.nonrades 75 368.5 348.6 371.6 26 507 2 433.7 371 6 29 366.3 321 8 2504 16 359.3 340.8 2504 ns Nonce "' ns (98 7 1) (220.51) (1 12.68) (107.59) 0 uapczina 124 426.2 078.8 369 9 163 502.0 444.1 370 103 485 5 433.8 370 30 531.9 512.9 370 ns Cog,,,(\l " BG. BR. PA>MY (157.53) (196.3) (196.09) (217 20) 488.7

AU Oslrea 256 394.2 361.9 371.6 293 485.4 426 2 371.6 174 442 7 394.8 371 6 63 446.8 376.7 250.4 None' "' BR>MY (131.53) (195 17) (179.85) (191.90) Crassoslrea 47 419.6 360.5 300.2 32 430.4 407.4 404.7 31 621.1 587 8 404.7 23 516 2 486.3 300.2 L09 ,c,( V) " ' PA>RG>BR>MY commercialis (122.75) (11 1.57) (203 131 1509.2) (19580)

C. lubercdata 27 455.8 391.6 406.8 8 451 7 355.8 304.9 16 500 0 4 17 3 304.9 7 396 7 41 1.3 304.9 I .J ns None (176 43) (215 56) (199 13) (51 07) 406 8 C. sedea 44 41jj 7 424.4 403.4 29 542 0 462.8 403.4 32 524.6 470 403.4 27 531.2 477.7 403.4 8.8 ns None (l>s.44) (216.42) (163.34) (176.84) All Crassoslrea 118 452.5 394.5 404.7 69 479.8 421 7 404.7 79 557 5 513.7 404 7 57 508.6 444.6 404 7 6 28.9 None P% .BR. MY (150.26) (1 80 66) (191.90) (178.03)

Pincfada 124 27.4 26.9 23.2 139 29.9 298 29.7 40 30.8 317 32.9 18 27.3 28.5 29.7 4 38.8 ' %nzr "' PA. BR> MY. BC; maculala (4.60) (5 27) (4.27) (3.95) .- P. furcala 47 27.9 27.4 27.0 31 32.0 32.3 33.9 20 31 8 31.7 34.0 5 25 3 21.3 20.1 a None "'BR,PA>MY>BC (4.68) (4.47) (6.43) 16 161 P dblnd 95 28.6 28 7 29.9 69 29 9 30.5 29.9 38 31.8 31.7 33.2 64 27 5 26.1 26.6 i.4 None "' PA>.AR>BG sug~llala 15 08) (5.32) 33.2 (4 65) (4 871

P. chen~nrlzae 36 27 3 26 4 23.7 18 31 2 31 3 30 0 33 31.2 31.3 30.0 3 30.6 30.7 300 9 0 ns None " ns (4 68) (4.50) (5 081 (1.15) All Pinclada 327 21.9 27.4 23.5 272 30.3 30.6 30.5 139 31.2 31.5 33.9 113 27.8 28.2 27 0 .J2 None "' PA>BR>MY. BC; (4.77) (5.34) (5 06) (4.73) 84 Mar. Ecol. Prog. Ser 54: 75-90, 1989

tada spp, and of Crassostrea commercialis differed sig- CROSS-REEF TRANSPLANTS nificantly among. locations, but the other 2 Crassostrea spp. did not differ (Table 6). The largest bivalves were found at either Brewer or Pandora. Ostrea were gener- ally largest at Brewer and smallest at Myrmidon. These differences were always significant. Most Crassostrea and Pinctada spp, were largest at Pandora. Only 2 species, Crassostrea sedea and , were larger at Brewer than Pandora but these differences were not significant. In summary, the size distribution of the initial recruits increased from the outer Myrmidon site to the inner Pandora Reef, which in turn had larger mean sizes than Bowling Green Bay. As they relate to relative growth rates these conclusions are based on the untested assumption of comparable settlement dates. This prob- lem is eliminated with the transplant experiment of plates cultured at John Brewer Reef which presumably were composed of similarly sized individuals. The transplant oysters tend to corroborate these patterns as Fig. 3. Dendrogram from classification analysis of all replicate they too show strong trends in which the outer shelf site uncaged and caged plates from the fore-, m~d-and back-reef is marked by the slowest growth rates and Pandora and locations and, from the sand area and Acropora treatment (N=40),for the 18 bivalve species encountered during the John Brewer Reefs generally have the highest rates. study. G1: all caged plates on fore-reef and sand locations, 80 % caged plates from mid-reef, and 60 % caged plates from the back-reef. G2: non-caged and 20% caged plates from Cross-reef experiment mid-reef. G3. caged and non-caged plates from under Acro- pora, and 80% non-caged front reef plates. G4: non-caged plates from back reef and sand locations Within l d of transplanting oysters onto Brewer Reef the effects of fish predation on non-caged plates were Group 2 included all the non-caged and 20 % of the very noticeable. Of non-caged plates, only those under caged mid-reef plates. It was associated with Group 1 Acropora appeared unaffected. At the fore-reef, back and had the next highest density (48.9 ind. 500~m-~) reef and sand sites, all but the tougher Crassostrea and and number of species (13). The fact that these oysters a few Ostrea had been removed and the plates dam- fared well relative to Groups 3 and 4 supports the initial aged by fish grazing. The exposed plates on the sand, observation that the mid-reef site had less fish preda- probably because they were very conspicuous, had the tion. most dramatic l d fish effect. Although the effects of Branching Acropora may provide habitat refuges for fish predation were much less obvious at the mid-reef oysters by successfully excluding large predatory fish location than at the other 3 locations, many of the more in a manner similar to cages. The lack of fish predation vulnerable Pinctada were missing. on non-caged plates under the Acropora compared to After 2 mo on Brewer Reef the effects of first day fish the back reef and sand habitats was evident after only predation were still apparent as caged plates had much 24 h. After 2mo all plates, regardless of treatment higher oyster densities than exposed plates (Figs. 3 and (caged and non-caged), were covered with similar 4; Table 7). The mid-reef plates which were initially species associations and clustered together in the one affected little by exposure to fish predation now group (G3 in Fig. 3). Similar oyster assemblages in both showed a marked difference in oyster densities treatments indicates similar mortality pressures, again between caged and non-caged plates. One group dis- suggesting successful exclusion of large predators by tinctly different from the others was formed by all the branching Acropora. However, this group also con- caged plates from the fore-reef and sand locations, tained 4 out of 5 non-caged plates from the reef front, 80 O/O of the caged plates from the mid-reef location and had a mean density (31.2 ind. 500cm-~),less than 60% of the caged plates from the back reef location that of plates from which fish predators had artificially (G1 in Fig. 3; Sandland & Young 1979a,b). This group been excluded (G1 in Fig.3) and poor survivorship contained the most species (15) and had the highest (Table 7; Fig.4). Generally, caged plates had higher oyster density (81.8 ind. 500~m-~)of all groups pro- densities of survivors; however, the densities of all the duced by the cluster analysis. Crassostrea tuberculata Ostrea species and 50% of the Crassostrea species was present in especially high densities. were lower on the caged plates from under the Acro- Dayton et al.. Oyster settlement on the GBR 85

Crossostrea echlnata P~nctadafurcala 1 r

, Ostrea nomades

N

5 10, Crossostrea sedeo

All Ostreo species 30,

AIl Crassostreo species 30 1 .I[

All Pinclada species

Reef Mid Bock Sond Acroporo \ front reef reef oreo thicket Open \ -----• Closed \ rC . \

Fig. 4. Mean density per 500cm-~of survivors within the most abundant bivalve species subjected to the 2 experimental treatments (caged and uncaged) at each of .. J Rat M#d Bock Sond Acmpom the 5 locations (fore-reef, mid-reef, back-reef, sand area and Acropora front reef reef oreo thicket thicket) across Brewer reef

pora thicket than on those not caged. As expected from Finally, Group 4 represents the non-caged plates their apparency and relative vulnerability to fish, this from the back reef and sand locations. These plates had was not the case for the Pinctadaspecies for which the lowest mean density of 14.5 ind. 500cm-~. This densities were always higher on the caged plates. represents the most intense fish predation observed Density of survivors was generally lowest on non- after l d and these plates were subsequently subject to caged plates from these 2 locations. intense fish predation.

Table 7. Mean densities (ind. per 500 cm2) and proportions of living Ostrea, Crassostrea and Pinctada species in each of the 2 treatments at the 5 locations across John Brewer Reef. Most dead oysters had been drilled or killed by flatworms

Location Caged Not caged No. of Density Prop. 95 % conf. No. of Density Prop. 95 % conf. species alive limits species alive limits

Fore-reef 13 79.3 0.32 0.283-0.367 7 26.8 0.16 0.112-0.228 Mid-reef 14 81.8 0.54 0.500-0.589 12 53.5 0.61 0.554-0.660 Back-reef 14 60.2 0.58 0.527-0.628 11 26.2 0.50 0.418-0.576 Sand 12 86.3 0.72 0.679-0.757 9 16.6 0.61 0.506-0.703 Acropora 11 36.6 0.17 0.123-0.224 1 34.8 0.37 0.309-0.442 86 Mar. Ecol. Prog. Ser. 54: 75-90, 1989

The cage experiments thus suggest that fish preda- flatworms and other benthic predators associated with tion is a general and important source of mortality for the rubble did not occur. The uncaged plates on sand young oysters. However, it is not the only source were however conspicuous to fish and most of their because a large proportion of dead oysters from all oysters were quickly killed. The density and proportion locations had been drilled by gastropods, and others of live bivalves was highest on the caged plates from were observed being killed by flatworms (Fig. 5). This the sand area and lowest in plates from the reef front and Acropora thicket locations (Table 7; Figs. 4 and 5). CAGED OPEN The proportion of drilled shells was lowest on the caged plates from the sand area and the non-drilled mortality here seemed to be from flatworms. Of the Ostrea spp., 0. folium appeared to be the most robust; of the Crassostrea spp., both C, echinata and C. tuber- data consistently had the highest su~vorship.The survivorship data for Pinctada spp. (Fig.4) are com- promised because dead individuals fell off the plates. Thus Groups 3 and 4 were lumped together (Fig. 3) by intense predation - by fish on Group 4 and inverte- brates on Group 3 under the Acropora. One interesting observation in this study is that the 3 main genera have different Life history patterns. Ostrea spp. have thin shells and, when young, seem particu- larly vulnerable to predation, but some individuals of each species were observed to be brooding well- developed eggs within the 2mo maximum time be- tween settlement and collection. Crassostrea spp, did not appear to have well developed gonads, and their age of maturation is presumably greater than Ostrea spp. At this time the Pinctada valves had not even attached to the substratum and casual observations of older individuals on various reefs suggested that they are considerably older before becoming reproductive. Thus the 3 genera appear to have different life history patterns which merit study.

DISCUSSION

The scale of resolution may be the single most impor- tant component of an ecological research program because it predetermines the questions, the pro- cedures, the observations and the results. In fact, much IPP' drilled of the structure of almost all ecological communities is a Fig. 5. Percent mortality and proportion drilled of recovered 'shadow' of past disturbance and/or recruitment oysters from the cross reef experiment. These data do not events; this is especially true in coastal marine systems show mortality which resulted In loss of valves (such as from in which most of the contributing species are charac- fish or crabs). Because the top valves were often drilled but terized by variable mortality and episodic recruitment. lost, it underestimates mortality from crabs and gastropods These species often have long-lived individuals and was especially true on the cross-reef experiments slowly declining populations, with episodic recruitment where the plates were placed on the substratum, but it pulses producing long-term spatial and temporal pat- was even true for plates on the off-reef moorings where terns (Dayton 1984). The importance of episodic some mortality was observed from gastropods and flat- recruitment is well recognized for pelagic fisheries worms which had settled with the bivalves. where research often focuses on large (1000's of km) Our most direct means of testing the importance of and mesoscale (100's of km) oceanic patterns, and it benthic invertebrate predation on young oysters was has been an issue of continuing importance in the coral the racks of plates placed on the sand where , reef fish literature (Doherty & Williams 1988). Interest- Dayton et al.. Oyster settlement on the GBR 87

ingly, despite an early recognition of its importance to and strong dominance, there was a remarkably high sessile or sedentary species (Thorson 1946, Coe 1956), species richness (l? species of the 24 bivalve species only recently have benthic researchers returned to this differentiated in the study) at the outer shelf site. The problen~.Probably in almost all cases where an ade- mid-shelf John Brewer Reef had 16 of the 24 species quate time series exists, one finds that there are impor- differentiated in this study and the greatest equitabil- tant annual fluctuations in recruitment; this is true even ity. Furthermore, with the exception of an anomalous for barnacles (Connell 1985). Finally, these details, settlement of one species at Pandora Reef, John Brewer often initially unknown, determine the proper scale of Reef had the highest densities. The inshore Pandora study. Reef plates had a speciose (20/24 bivalve species), Study of this important phenomenon has been dif- relatively dense and equitable recruitment, but were ficult because both spatial and temporal recruitment subsequently inundated by a massive settlement of are influenced by multiple biological and physical fac- Pinctada maxima, most of which fell off the plates tors representing several scales. For example, for before they could be recovered. All of the P. maxima demersal or sessile species with planktonic larvae, the were almost identical in size, suggesting that this availability of the larvae usually involves large-scale episodic settlement was almost instantaneous. This dispersal (Parrish et al. 1981, Dayton & Tegner 1984, implies that competent larvae arrived in a species Power 1984, Williams et al. 1984), while settlement swarm with a diameter e 0.5 km - the minimum moor- patterns involve microscale transport and behavior ing separation at that site. Finally, the coastal non-reef often dealing with boundary layers (Butman 1987), Bowling Green Bay plates had 13/24 species but were chemical clues (Crisp 1955), microtopography (Wethey strongly dominated (an order of magnitude higher than 1986, Carleton & Sammarco 1987) and even predation the next most dense) by Ostrea angasi, an apparent or mortality from pre-existing individuals. Once the specialist found almost exclusively at Bowling Green larvae are settled, there are various survivorship pat- Bay. The bivalve densities at Bowling Green Bay were terns over several time scales resulting from competi- relatively low, but the plates at this site were also tive and/or predatory pressures (Peterson 1982). heavily encrusted with several unidentified species of Finally, all these patterns are complicated by various compound ascidians and stinging hydroids and it is rates of site-specific growth and fecundity which are possible that these organisms interfered with the settle- unpredictable over long-term temporal scales making ment of bivalve larvae. this a very difficult problem to study with long-lived Summarizing, each of the 4 regions had a charac- organisms. One alternative to a long-term study is to teristic association of bivalves. The outershelf Myrmi- increase spatial scale in the hope that a large-scale don plates were apparently exposed to a relatively very short-term study may uncover the mechanisms causing low abundance of larvae of many species of which long-term variability. In addition there are apparent there was a strong dominant, Ostrea folium. Excepting cross-shelf differences in populations of established the episodic settlement of Pinctada maxima at Pandora, corals (Done 1982), soft corals (Dinesen 1983), sponges the midshelf Brewer Reef was characterized by the (Wilkinson 1986), calcified green algae (Drew 1983), highest densities of a speciose collection of bivalves. and fishes (Williams 1982). It is of interest to know The inshore Pandora Reef was similar to Brewer but whether these regional differences result from larval had more species and lower densities, and the coastal dispersal and/or differences in the water itself in addi- Bowling Green Bay plates had a low density and diver- tion to in situ reef differences. This study evaluates sity and a strong dominance of Ostrea angasi. patterns of settlement, su~vorshipand growth of oys- ters in 4 cross-shelf regions of the Central Great Barrier Reef. The fundamental objective was to elucidate Growth and survival which scales are most appropriate to the study of recruitment of invertebrates with pelagic larvae. The sizes of the bivalves at the time of the recovery of the plates show a clear increase in mean size from the outer shore to the nearshore Pandora Reef. The Settlement patterns assumption that the growth rates are reflected by the mean size assumes that, in general, the larvae settled at There were clear regional differences between the approximately the same time; we cannot test this recruitment observed on the plates. The plates at Myr- important assumption. If it is true, the growth trend midon Reef on the outer shelf had by far the lowest parallels the apparent gradient in primary production densities, and these were dominated by Ostrea folium (Revelante & Gilmartin 1982). which was almost an order of magnitude more abun- The structure of many benthic communities is often dant than any other species. Despite the low density thought to be maintained by post settlement survivor- 88 Mar Ecol. Prog. Ser. 54: 75-90, 1989

ship (Paine 1980), although there is a growing appreci- from the cross-reef experiment is the importance of ation of the importance of an earlier focus on recruit- predation by benthic invertebrates. That is, we fully ment events (Thorson 1966). The characteristic struc- expected fish predation to be important as it certainly ture of the Brewer plates which were transplanted to is, but the study has further shown that young bivalves other areas was maintained (Fig. 2) despite sometimes are also exposed to extremely strong predation being in very different habitats. At the level of resolu- pressure from gastropods, flatworms, and tion of the cluster analysis, this suggests that these crabs. This was demonstrated by the transplants onto bivalve assemblages area determined more by recruit- the sand habitat which had much higher survivorship ment events than by subsequent survivorship patterns. when protected from fish, but even here many of these This seems to be in agreement with an emerging oysters were hlled by invertebrates which apparently generalization of coral (Sammarco 1983) and also of settled as larvae with the bivalves. The Acropora reef fish studies (Mapstone & Fowler 1988). Even our experiment suggests that fish may also impact some of observation of a heavy synchronous settlement of Pinc- these benthic invertebrate predators. Under the Acro- tada maxima is reminiscent of the observation of Victor pora the caged and open plates had approximately (1986) and Williams (pers, comm.) that several species equal but low survivorship which was similar to that of of fish settle synchronously over remarkable distances. the back reef uncaged plate exposed to fish predation. However, the more specific ANOVA analysis did show One explanation is that the fish predation on back reef species and generic level trends (Table 4), especially uncaged plates was approximately replaced by for Pinctada and Ostrea spp. which had significantly invertebrate predation under Acropora where these higher mortality at the 2 inshore stations. predators may have increased with their reduced fish The growth patterns of the transplanted bivalves predation. The higher survivorship under the back reef were interesting. The mean size of all Ostrea and cage may reflect both fish exclusion and a general Pinctada spp. and of Crassostrea commercjalis differed reduction of benthic invertebrates as compared with significantly among all locations. Myrmidon Reef the Acropora thicket. This interplay between guilds of tended to have the lowest growth rates with Ostrea predators deserves research. spp, being largest at John Brewer Reef. But most Cras- sostrea and Pinctada spp. were largest at Pandora Reef. Bowling Green Bay had low survival and low growth Caveats rates. While the gradient of productivity is usually considered to be higher inshore, data are rare and The main thrust of this project was to evaluate re- sometimes equivocal (Revelante & Gilmartin 1982), gional cross-shelf differences in a environ- and the strength of this assumption often may rest on ment. In addition, preliminary work addressed differ- the dramatic contrast in water clarity (low in Bowling ences over a single reef. It is not a community study Green Bay and exceptionally high at Myrmidon) rather and no attempt was made to relate the observations than real measurements of productivity. Despite these with the natural history of adult bivalves on the reefs. patterns in water clarity, nutrients and productivity can These bivalves appear relatively rare and they are very be very low at nearshore stations (J. A. Hansen pers. cryptic. In nature their settlement and survivorship comm.), and furthermore the nearshore productivity is would be very different from that observed on plates, driven by the outflow of local rivers which respond to and a proper study of bivalves on coral reefs would be rainfall (Sammarco & Crenshaw 1984). There may be fascinating but very difficult. Our objective here was to years between flooding episodes during which time describe regional differences for representative plank- coastal waters may be nutrient starved. Similarly, the tonic larvae with regard to their dispersal, and subse- productivity regimes of central and offshore areas are quent to their settlement, their growth and survivor- affected by different physical processes (Williams et al. ship. In general there are marked differences between 1984). Our data corroborate the conclusion of Williams each of the outer, mid and inner shelf reefs and the et al. and suggest that the inshore Bowling Green Bay coastal non-reef Bowling Green Bay site. is a very different regime representing suboptimal con- This project was restricted in space (only one reef or ditions for these oysters. site per cross-shelf zone) and time (only one year), but Logistic restraints did not allow adequate replication the results do parallel those of the more intensively of the cross reef fish predation study, but results of studied reef fishes (Doherty & Williams 1988) and cor- these preliminary transplant experiments suggest als (Sammarco & Andrews 1988). In addition to the interesting within-reef differences in the intensity of many potential problems associated with restricted fish predation. Predation appears to be relatively low at spatial and temporal scales, one important artifact of the reef flat site, in agreement with the observations of studies such as this is that the recruitment was on Russ (1984a, b). Perhaps the most important lesson plates held away from natural reefs. This was done Dayton et al.: Oyster settlement on the GBR 89

deliberately because we wanted to study regional Connell, J. H (1985). The consequence of variation in initial differences in larval availability not impacted by reef- settlement vs post-settlement mortality in rocky intertidal communities. J. exp. mar Biol. Ecol. 93: 1145 associated planktivores; nevertheless, the interpreta- Crisp, D. J. (1955).The behavior of barnacle cyprids in rela- tion of such data must recognize that such 'off-reef' tion to water movement over a surface. J. exp. Biol. 32: recruitment patterns are from larvae which have not 569-590 been exposed to the 'wall of mouths' of reef-associated Dayton, P. K. (1984).Processes structuring some marine com- planktivorous predators (Hamner et al. 1988). Bivalve munities: are they general? In: Strong, D. R. Jr., Simberloff. D., Abele. L. G., Thistle, A. B. (eds.) Ecological com- larvae are small and may or may not escape such munities: conceptual issues and their evidence. Princeton predators, but this issue is not addressed in this paper. University Press, Princeton, NJ, p. 181-197 Given the observed variation in recruitment and the Dayton, P. K..Tegner, M. J. (1984).The importance of scale in relatively high mortality to the oysters transplanted to commun~tyecology: a kelp forest example with terrestrial John Brewer Reef, it is very clear that in any natural analogs. In: Price, P. W., Slobodchikoff, C. N., Gaud, W. S. (eds.) A new ecology: novel approaches to interactive situation such bivalve populations are regulated by systems. John Wiley & Sons, New York, p. 457-481 both recruitment and post-settlement mortality. This Dinamani, P. (1971). Identification of oyster species competing implies that these types of reef populations will be very with rock oysters for settlement space. New Zealand difficult to model (Warner & Hughes in press). Ministry of Agriculture and Fisheries. Fisheries Research Division Information Leaflet No. 1 Dinesen, 2. D. (1983). Patterns in the distribution of soft corals Acknowledgements. Most of the field work was done in across the central Great Barrier Reef. Coral Reefs 1: extremely difficult weather conditions and could not have 229-236 been accomplished without Ray McAllister's skillful develop- Doherty, P. J., Williams, D. Mc.B (1988).The replenishment of ment and assembly and reassembly of the mooring devices coral reef fish populations. Oceanogr. mar. Biol. A. Rev. 26: and the professional support of J. Futcher and the crew of the 487-551 RV 'Lady Basten' W. Ellery, J. Hardman and V Svenson Done, T J. (1982). Patterns in the distribution of coral com- made the work possible with diving and logistic support. R. munities across the central Great Barrier Reef. Coral Reefs McDonald provided computer support and A Dartnall 1: 95-107 arranged for the flgures. We are grateful to G. MacNaughton, Drew, E. A (1983). Halimeda biomass, growth rates and J. Small and J. Collingwood for superb shop support. We are sediment generation on reefs in the central Great Barrier grateful to T Done, M. Furnas, D. McKinnon, A. Mitchell, A. Reef Province. Coral Reefs 2: 101-110 Robertson, H. Sturmey, C. Wilkinson, and D. M. Williams for Hammer, W M,, Hauri, I. R. (1981). Effects of island mass: continued stimulating discussions and help and T Done, E. water flow and plankton pattern around a reef in the Great Crissman, N. Holland and 2 referees for reviewing the manu- Barrier Reef Lagoon, Australia. hmnol. Oceanogr. 26 (6): script. Many thanks to L.S. Jackson for patient and excellent 1084-1102 typing support. P K.D, is very grateful to John Bunt, then Hamner, W. M,, Jones, M. S, Carleton, J. H., Hauri, I. R., Director of AIMS, for setting up this program and to Acting Williams. D. McB. (1988). Zooplankton, planktivorous fish, Director R. Bradbury and J. Baker, the present Director, for and water currents on a windward reef face: Great Barrier support which made it possible. 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This article was presented by Professor N. D. Holland, Revised version accepted: March 2, 1989 La Jolla, California, USA