Similarity and Diversity Among Coral Reef Fish Communities: A Comparison between Tropical Western Atlantic (Virgin Islands) and Tropical Central Pacific (Marshall Islands) Patch Reefs

The fish assemblages of groups of large natural coral patch reefs in the tropical western Atlantic and Pacific oceans were censused visually during the summers of 1976 and 1978. Thirty-one reefs were in the northeastern Caribbean Sea (25 at St. Croix and 6 at Anegada) and 15 reefs were located at the southern end of Enewetak Atoll, Marshall Islands. The degree of similarity among the faunas in each group of reefs (using a similarity index based on the log of individual abundances) was correlated with environmental parameters of the reefs in both regions (complexity of surface topog- raphy, reef height, reef area, and position with respect to the main reef, currents and grassbeds). The mean degree of similarity among all the fish faunas in each area was the same at the two major sites (0.61 at Enewetak and 0.62 at St. Croix) as was the similarity among the faunas of the most uniform subset of reefs in each area (0.68 at Enewetak and 0.73 at St.. Croix). These comparable levels of similarity suggest similar levels of predictability in the composition of reef fish faunas in the two areas, in contrast to previous studies on very small natural and artificial structures, which suggested a low level of predictability among Pacific reefs (Sale 1978). The discrepancy between our results and those of previous workers in the Pacific is partly a result of differences in the sizes of the reefs studied, being several orders of magnitude larger in the present case. Reef fish diversity was greater at Enewetak (mean number of species per reef = 93; H' = 5.38) than at St. Croix (mean number of species per reef = 64; H' = 4.58) but equitability values were the same in both areas (0.82 and 0.81, respectively). At both sites fish species diversity was positively correlated with reef surface com- plexity, projected reef surface area and reef height, but the correlation was higher at St. Croix for all parameters. This was probably attributable to the greater structural similarity of reefs at Enewetak. At Enewetak there was also a correlation between H' and proximity to the main barrier reef. Major differences in the trophic composition of these fish faunas based on available published dietary in- formation indicate that differences between the two sites are accounted for primarily in the proportions of diurnal planktivores and nocturnal general invertebrate feeders. These differences were probably the result of major environmental differences between the two sites: a high oceanic influence at Enewetak, resulting in abundant zooplankton, and the presence of dense grassbed (harboring abun- dant invertebrates; Randall 1963) surrounding the reefs at St. Croix.