DIURNAL OCCUPANCY OF CREVICES AND OVERHANGS BY FISHES ON THE CAICOS BANK, TURKS AND CAICOS ISLANDS, BRITISH WEST INDIES

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1 BULLETN OF MARNE SCENCE, 51(1): 66-82, 1992 CORAL REEF PAPER DURNAL OCCUPANCY OF CREVCES AND OVERHANGS BY FSHES ON THE CACOS BANK, TURKS AND CACOS SLANDS, BRTSH WEST NDES Stephen Spotte, Patricia M. Bubucis and Gary Adams ABSTRACT The presence of 60 fish species was recorded during daytime visual censuses of 20 large crevices and overhangs over four seasons (711 observations) on the Caicos Bank, Turks and Caicos slands, British West ndies. Data for 26 species were analyzed, We tested five hypotheses: () the number of species occupying sites during the day changes with season, (2) the presence of some species changes with season, (3) associations exist between fish species and site features, (4) interspecific associations among fishes exist, and (5) physical features of the sites affect numbers of species. Tests of all five hypotheses indicated significance at P < Current knowledge of the inshore fishes of the Turks and Caicos slands is contained in a single report published 71 years ago (Nichols, 1921). We describe here the diurnal occupancy of20 isolated crevices and overhangs by 26 fish species on the Caicos Bank. As defined by Ludwig and Reynolds (1988), these sites comprised a series of natural sampling units. The comparative simplicity of the sites permitted us to map their internal dimensions in the genre of artificial reef studies. We also categorized the sessile biota. Differences in dimension, sessile biota, season, and other variables were then analyzed for possible effects on occupancy by fish species. Finally, we analyzed the data for interspecific associations among the fishes recorded. METHODS Study Area. - The Caicos Bank, with an area of several hundred square kilometers, extends southward from the Turks and Caicos slands in the direction of Hispanola. Most of the region is shallow and dotted with small emergent islands or cays. The submerged terrain is flat and featureless except for occasional reefs of coralline algae (Neogoniolithon strictum) and finger coral (Porites porites), limited aggregations of glomerate hermatypic corals (P, astreoides), and expansive beds of turtle grass (Thalassia testudinum). Discrete sites having substantial surface relief are scarce except around the bases of cays; there, weathering has produced extensive networks of undercuts and crevices ranging in complexity from areas of partial shade to labyrinthine passages. Hypotheses. - We tested five hypotheses: () the number of species occupying sites during the day changes with season, (2) the presence of some species changes with season, (3) associations exist between fish species and site features, (4) interspecific associations among fishes exist, and (5) physical features of the sites affect numbers of species. Field Methods. - The sites were overhangs (formed by undercuts) and crevices on the perimeters of seven small cays (Fig. ), each <0.2 ha, located east of Pine Cay (near 21 53'N, 72 05'W). Twenty locations at depths of < 1.5 m were chosen to provide a variety of sizes and shapes. Boundaries of the sampling sites were marked above the high-water mark with orange paint and their locations underwater identified with numbered nylon tags. Fish species observed were recorded on plastiecoated paper. Each site was censused visually by one of us (Spotte) wearing snorkeling equipment, usually nine times eaeh season during 1987 and 1988 for a total of711 observations (summer 180, fall 177, winter 174, spring 180). The time required to census a site depended on its size and complexity, the number of species present, available light, turbidity, and sea conditions, but ranged from 2-6 min. One complete survey of all sites took h. Efforts were focused between 1000 and 1400 h (range to h) when the angle of light was most favorable for seeing into dark recesses. However, much of the Caicos Bank in the vicinity of Pine Cay is inaccessible except at high tide, and censuses were taken with greater regard to tides than time of day. 66

2 SPOTE ET AL.: OCCUPANCY BY ASHES 67 ":" Dellis Cay :.: '-':"",;... ", '. :....,._.',..., r."' 1 /... N t ,,5, )3 C::::,,;, PineCay o Turks Caicos Bank 0 o Caicos o..s. s. 21 <5D ",! Dominican <1- Republic 1 km Figure. Pine Cay and surrounding area. Study sites are at perimeters of cays numbered through 7. Redrawn from "Caicos slands" Series E8ll2 (DOS 309P), Sheet 4, Edition 2-0SD 1984, published by United Kingdom (Ordnance Survey). The sites were measured and sketched in the field (both plan and sectional views) so that physical features could be compared. Areas were calculated by counting grid squares on scale drawings of the sites. Volumes were estimated from the products of either the plan area and mean vertical dimension or the width and mean vertical areas, depending on the shapes of the sites. Vertical dimensions were measured from the floor of the site to the underside of the overhang, or to the water surface if the site was not submerged completely. We also compiled lists of prevalent benthic macroalgae, corals, sponges, and turtle grass at each site. Variables. - Variables were site, season, presence or absence of 26 fish species, number of species observed in a site during a census, and site features. Only data for species seen more than five times during all seasons, and those seen at least once during each season, were retained for analysis. Data for most cryptic species were excluded because their presence could not always be confirmed. Measured or calculated site features were width, plan area, overhang area, volume, horizontal and vertical dimensions, and standard deviations of the horizontal and vertical dimensions. The remaining features recorded were the presence or absence of a sandy floor, turtle grass, Caulerpales algae (Avrainvillea, Caulerpa. Halimeda. Penicillus. or Udotea), Siphonocladales algae (Dictyosphaeria or Valonia), Neogoniolithon strictum. Porites astreoides. P. porites. and sponges, and whether or not access was limited. Data Analysis.-Hypothesis 1: The dependent variable was the number of species in a site during a census; the independent variable was season analyzed in all combinations by Kruskal-Wallis (ANOVA by ranks). Hypothesis 2: The presence of each of the 26 fish species constituted the set of dependent variables; the independent variable was season (ANOVA with Schefte's post hoc test of multiple comparisons). Parametric testing was allowed because of the large number of cases. Hypothesis 3: The presence of each of the 26 fish species constituted the set of dependent variables; the independent variables were each of the site features (Spearman's rank order correlation). We assumed that features remained constant through the seasons. Hypothesis 4: Contingency tables (2 x 2) were constructed for each pair of species from presence data for all censuses based on the simultaneous presence,

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6 SPOTE ET AL.: OCCUPANCY BY ASHES C lall summer.! H! 14 l/) 12 Q) '0 Q) a. 10 l/) Q).0 E::l 6 Z o winter spring 12 l! :r f i '11,.... f i rl 'f i! l: l "! j '.! i ij; ;. f i!! i Z z ; " ; i ; j i; i; i! ii iii j! " ; i! ii!: f..!" '! i!! j! i!:!:;.. ;: Site Figure 2. Mean numbers offish species (±SEM) observed in each site during a season. Vertical lines separate the sites. simultaneous absence, and individual presence of each member of a pair (chi-square test with Spearman's r to identify positive and negative associations). We selected chi square in preference to the numerous indices of association (e.g., Jaccard's, Dice's) for two reasons. First, unlike most indices of association, a probability can be assigned to the chi square. Second, the sites were confined to a small area. Fishes had opportunity to visit any of them, which would make the joint absence of any pair of species relevant (Ludwig and Reynolds, 1988). Hypothesis 5: The dependent variable was the number of species in a site during a census; independent variables were each of the site features (Spearman's rank order correlation). RESULTS We recorded 59 species of teleosts and one elasmobranch representing 26 families (American Fisheries Society, 1991). Of these, data for 34 species of21 families were not used (Table 1). Observations of the 26 species retained for analysis are summarized in Table 2. The number of replicates was sufficiently large to comprise a statistical population. n a species-sample curve (Brower and Zar, 1984) for summer, all but two of the 26 had been recorded by the time each tag had been censused six times. Degree of probability in hypothesis testing (i.e., measures of consistency between data and null hypotheses) are stated at the 0.05 level unless otherwise indicated. Hypothesis 1: Season had a significant effect on the mean number of species observed in a census (summer 7.0, fall 6.2, winter 6.2, spring 6.5). When the sites were tested collectively, interseasonal comparisons showed significance only if summer was included. Examined individually, sites 5, 6, and 11 were unaffected by season, but the rest were affected significantly with no clear pattern. The means

7 72 BULLETN OF MARNE SCENCE, VOL. S, NO., 1992 Site 1 C, 1 1 N t 0.8 m " B,, A,,, 2.3 m 0.7 m Portes pontes frontage.! /. Ị 1.9 m A.-, m Figure 3a. Diagram of plan view of Site. Dashed line is back wall of site, solid line delineates overhang, and dash-dot line is edge of Piporites patch. of the number of species present in each site during each season are shown in Figure 2. Hypothesis 2: Season had a significant effect on the presence of 12 species (Table 3). Significance was not limited to any single season, although summer accounted for the effect most often. Hypothesis 3: Species showed significant associations with site features. Representative sites are diagrammed in Figures 3 and 4, and features of the sites are shown in Table 4. Significant associations of species with these variables are presented in Table 5. Several species were associated positively with large size, limited access, and turtle grass, and negatively associated with Caulerpales algae, N. strictum, and corals. Sites characterized by

8 SPOlTE ET AL.: OCCUPANCY BY FSHES 73 Site 1 c 0.8 m 0.6 m 0.7 m... Porites porites Penicillus B Halimeda Thalassia, Halimeda, Penicillus, Avrainvillea, and sand. A -. Porites porites Thalassia Penicillus 1m Figure 3b. Diagram of sectional views of Site 1.

9 74 BULLETN OF MARNE SCENCE, VOL. 51, NO., 1992 Site 11 ', c '... Thalassa '" frontage _ '. /', Sand floor./ Rock N t 1m A "/ /,- ' / 1m Figure 4a. Diagram of plan view of Site. Dashed line is back wall of site, solid line delineates overhang or rock, and dash-dot line is edge of Thalassia bed. all these features were large, dark, and protected. Species associated with this combination were Abudefduf saxatilis, Acanthurus coeruleus. Diodon hystrix, Epinephelus striatus, Haemulon sciurus, Holacanthus ciliaris, Lutjanus griseus, and L. synagris. Other than their associations with turtle grass, Pomacentrus fuscus (=Stegastes dorsopunicans, negative association) and Malacoctenus triangulatus (no association) also fall within this group. Diodon hystrix, H. sciurus, L. griseus, and L. synagris were associated with large, dark, protected sites. Lutjanus apodus was associated with large sites having neither coral nor Caulerpales algae. Haemulon flavolineatum was associated with large, open sites fronted by reefs of Porites porites. Haemulon parra was associated strongly with open sites of any size, but having deep overhangs. Halichoeres bivittatus and Malacoctenus macropus were associated most strongly with small, protected sites. Pomacentrus leucostictus was associated with small, bright, unprotected sites with Caulerpales algae. Hypothesis 4: Significant interspecies associations were detected for every species (Table 6). Some examples follow. Among the damsel fishes, A. saxatilis was associated positively with P. fuscus and negatively with P. leucostictus. The wrasses Halichoeres radiatus and Thalassoma bifasciatum demonstrated positive association with each other. Hypothesis 5: Significant correlations were detected between the number of species and physical features of the sites. Number of species correlated positively with width, maximum horizontal depth, standard

10 SPOTE ET AL: OCCUPANCY BY FSHES 75 Site 11 c 1.3 m Valonia 0.7 m Thalassia 0.8 m 0.2 m B 0.6 m 0.4 m A Water line Rock 0.55 m Rock - ' 0.2 m m Sand floor 1m Figure 4b. Diagram of sectional views of Site 11.

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14 SPOTE ET AL.: OCCUPANCY BY FSHES 79 deviation of vertical depth, sandy floor, sponges, and limited access; negative correlations were seen with all species of macro algae and P. porites. DSCUSSON The greater number of species present in the sites in summer compared with other seasons might be an anomaly ofthe Caicos Bank; alternatively, it is perhaps similar to regions of the West ndies. Munro et al. (1973) reported that many Caribbean reef fishes (83 species) spawn in winter and early spring when water temperatures are lowest. Settlement presumably would follow, although our unpublished data indicate more juveniles present in winter than in the other seasons. The relationship between species richness and substratum complexity is unclear from the literature. Most studies similar to ours have been conducted on patch reefs or sections of reef fronts on fringing and barrier reefs. These habitats are very different from our sites. According to Molles (1978), substratum complexity does not correlate positively with either species diversity or abundance on patch reefs in the Gulf of California. n most cases reef height showed positive correlation with both factors. Risk (1972) found no significant correlation between fish species diversity and the biological nature of the substratum on patch reefs in the U.S. Virgin slands, nor between total numbers of fishes and either topographic complexity or biological diversity of the substratum. He concluded that "[T]he relationship does not appear to be a simple one of 'more holes, more fish.' " Sale and Douglas (1984) studied structural characteristics of lagoon patch reefs at One Tree sland, Great Barrier Reef. Gross surface area could be used most consistently to predict fish density. This factor, plus the percentage of cover provided by bare or algae-covered rock surfaces, most closely predicted the number of species. The authors concluded that topographic complexity is not a useful predictor of species richness at their study site. Luckhurst and Luckhurst (1978a) found poor correlation between species diversity in the Netherlands Antilles and the percentage of substratum covered by corals, although good correlation was obtained when substratum complexity was used as the reef structural parameter. They concluded that substratum complexity provided diverse shelter sites, which enhanced species richness. On superficial examination, our data appear to predict that the largest sites contained the greatest mean numbers of species (Fig. 2, Table 4), but size alone was not the only factor. Species differences in diurnal activity also appeared to be important. Diodon hystrix, Haemulon flavolineatum, H. parra, H. sciurus, Lutjanus apodus, L. griseus, and L. synagris are principally nocturnal feeders that shelter during the day. Size limitations may explain their association with large sites. Specimens of Diodon hystrix were always large (> 30 cm in length); grunts (Haemulon spp.) and snappers (Lutjanus spp.) observed in the sites were either large (> 20 cm) and solitary or smaller individuals in schools of as many as 200. The only species associated strongly with small sites were relatively small and solitary (Halichoeres bivittatus, < 15 cm; Pomacentrus leucostictus, < 10 cm; Malacoctenus macropus, < 5 cm). Site associations may also have been influenced by behavioral differences. For instance, H. jlavolineatum and H. parra were associated with large, open sites, whereas H. sciurus was associated with large, protected sites. The first two species schooled even as adults and left their sites when disturbed. Larger H. sciurus were solitary. When disturbed, they wedged themselves into crevices or pressed against rocks and other objects, leaving the site only as a last resort. Diodon hystrix was also associated with large, protected sites. The specimens we observed were sol-

15 80 BULLETN OF MARNE SCENCE. VOL. 51. NO Table 6. nterspecific associations among fishes observed in the sites. Only statistically significant pair associations are shown (chi square significance at the P < 0.05 level) with signs from Spearman's r values. The number of census in which a species was present is represented by n (total n = 711). Example: Pomacentrus leucostictus. a damsel fish, was associated negatively with half the species observed in the sites, but positively with the wrasses Halichoeres radiatus and Thalassoma bifasciatum. the parrotfishes Scarus croicensis and Sparisorna viride. and the blenny Malacoctenus rnacropus Abu Aca Aca Arh Cha Dio Epi Hae Hae Hae Species (n) saxa chit coer stip cap; hyst str; jlav parr sciu A budefduf saxatilis (216) Acanthurus chirurgus (118) Acanthurus coeruleus (315) Atherinornorus stipes (62) Chaetodon capistratus (66) Diodon hystrix (59) Epinephelus striatus (37) Haernulon flavolineaturn (234) Haernulon parra (96) Haernulon sciurus (231) Halichoeres bivittatus (40) Halichoeres radiatus (102) Holacanthus ciliaris (24) Lutjanus apodus (450) Lutjanus griseus (133) Lutjanus synagris (98) Malacoctenus rnacropus (109) Malacoctenus triangulatus (49) Mulloidichthys rnartinicus (63) Pornacentrus fuscus (244) Pornacentrus leucostictus (546) Scarus croicensis (220) Sparisorna chrysopterum (51) Sparisorna rubripinne (16) Sparisorna viride (272) Thalassorna bifasciaturn (422) itary and shy, retreating farther into dark recesses and refusing to leave when disturbed. The presence of some species contradicted their behavior on the reefs offshore. Epinephelus striatus is inactive during the day on the Caicos Bank, in contrast with its behavior on offshore reefs in the Turks and Caicos slands. Acanthurus coeruleus is active in daylight, but we always saw many specimens-especially large ones-sequestered in dark, protected sites, sometimes displaying nocturnal coloration. Abudefduf saxatilis, a diurnal species, feeds actively in the water column on offshore reefs. We were surprised to find this species sheltering consistently by day on the Caicos Bank. With observational data it is impossible to determine whether associations of species with site characteristics influence interspecies associations or the reverse, although mutual influences appear to exist. For example, among the damselfishes, P. leucostictus and P.fuscus are aggressively territorial and seem to inhabit different sized sites. Either factor could explain their negative association with each other. Abudefduf saxatilis, which is territorial only when breeding or guarding eggs, is associated with large sites, as is P. fuscus. nformation in Table 6 indicates that both positive and negative interspecific associations exist independently of site association, suggesting that these relationships are confounded. For example, T. bifasciatum was associated positively with H. bivittatus, which is inconsistent with the respective site associations of these species. The significant association of Holacanthus ciliaris with many species is probably

16 SPOTTE ET AL.: OCCUPANCY BY ASHES 81 Table 6. Extended Hal Hal Hoi Lui LU Lu Mal Mal Mul Porn Porn Sea Spa Spa Spa Tho bivi rad; eili apod gris syna macr tria mart fuse leuc ero; chry rub, vir; hi/a an artifact: the same individual appeared in repeated censuses of site 17, which always contained a large complement of species. Sampling problems of this kind are likely to occur whether the data collected are based on presence-absence or species abundance. n either case the results show only that specific sites are available for occupancy by individuals of a species. Many fishes associated with coral reefs and sites such as we describe are home ranging if not strictly territorial (Luckhurst and Luckhurst, 1978a, 1978b; Smith and Tyler, 1972; Smith, 1978), and repetitive censusing of the same individuals is unavoidable. ACKNOWLEDGMENTS Research was supported by the Oakleigh L. Thome Foundation through a grant to S. Spotte. We thank Mr. O. B. Thome, members and employees of the Meridian Club, and the Turks and Caicos slands government for help and encouragement. The project was suggested by C. Lavett Smith. J. W. Atz, R. Nawojchik, C. Recksiek, S. T. Ross, and C. L. Smith reviewed the manuscript. R. Manstan, G. Sirpenski, and J. T. O'Neill provided technical assistance. This is contribution No. 75 of Sea Research Foundation, No. 244 of the Marine Sciences nstitute, and NO.4 of the Turks and Caicos slands Coral Reef Ecology Program. LTERATURE CTED American Fisheries Society Common and scientific names of fishes from the United States and Canada, 5th ed. Spec. Pub. 20, American Fisheries Society, Bethesda. 183 pp.

17 82 BULLETNOFMARNESCENCE,VOL.51, NO., 1992 Brower, J. E. and J. H. Zar Field and laboratory methods for general ecology, 2nd ed. Wm. e. Brown, Dubuque. 226 pp. Luckhurst, B. E. and K. Luckhurst. 1978a. Analysis of the influence of substrate variables on coral reef fish communities. Mar. Bio. 49: and b. Nocturnal observations of coral reef fishes along depth gradients. Can. J. Zool. 56: Ludwig, J. A. and J. F. Reynolds Statistical ecology: a primer on methods and computing. Wiley, New York. 337 pp. Molles, M. e., Jr Fish species diversity on model and natural reef patches: experimental insular biogeography. Ecol. Monogr. 48: Munro, J. L., V. C. Gaut, R. Thompson and P. H. Reeson The spawning seasons of Caribbean reef fishes. 1. Fish Bioi. 5: Nichols, J. T A list of Turk slands fishes, with a description ofa new flatfish. Bull. Am. Mus. Nat. Hist. 44: 21-24, 1 plate. Risk, M. J Fish diversity on a coral reef in the Virgin slands. Atoll Res. Bull. (153): 1-4. Sale, P. F. and W. A. Douglas Temporal variability in the community structure of fish on coral patch reefs and the relation of community structure to reef structure, Ecology 65: Smith, e. L Coral reef fish communities: a compromise view. Environ. Bio. Fish. 3: and J. e. Tyler Space resource sharing in a coral reef fish community. Bull. Nat. Hist. Mus. Los Angeles Co. 14: DATEACCEPTED: March 30, ADDRESSES:(S.S. and P.M.B.) Sea Research Foundation and Marine Sciences nstitute, The University of Connecticut, Noank, Connecticut 06340; (G.A.) Department of Chemical Engineering Technology, Thames Valley State Technical College, Norwich, Connecticut