Abstract
Predatory fishes are a major component of many tropical fisheries, although little is known about their diet and trophic structure, which can hinder effective management. We used stable isotopes δ 15 N and δ 13 C in conjunction with dietary prey items of five fishes ( Lutjanus sebae , Lethrinus punctulatus , Epinephelus areolatus , Epinephelus multinotatus , and Plectropomus maculatus ) to describe the diet and trophic structure across this assemblage. A total of 153 isotope and 87 stomach content samples were collected at two locations that were ≈30 km apart, over two sampling trips, separated by three months. There was clear separation of species’ mean δ 15 N and δ 13 C values in isotopic space; the highest mean δ 15 N was exhibited by E. multinotatus (13.50 ± 0.11 SE) and the lowest was L. punctulatus (11.05 ± 0.13). These two species had the lowest overlap of isotopic niche space, whereas the highest overlap occurred between L. sebae and P. maculatus . δ 15 N increased with fish body size for all species. However, body size was not significantly related to δ 13 C values for any species. There was a notable shift in both δ 15 N and δ 13 C between sampling trips, with δ 13 C being more depleted in the second trip. There was also a difference in δ 13 C between locations for all species, suggesting localised foraging. A multiple tissue comparison for L. sebae indicated positive relationships for both δ 15 N and δ 13 C between dorsal fin and muscle tissue. Mean δ 15 N values were the same for both fin (12.1 ± 0.10 SE) and muscle tissue (12.1 ± 0.09 SE), although δ 13 C was more enriched in fin (-15.6 ± 0.14 SE) compared to muscle tissue (-17.3 ± 0.11 SE). The most common dietary items across species were teleosts and crustaceans, which was consistent with isotope data indicating a reliance on a demersal food web (δ 13 C values ranging from -18 to -15‰). The results from our study indicate a dynamic spatio-temporal trophic structure and diet for commercially important demersal species and highlight the benefits of a multi-facetted sampling approach.