Ontogenetic changes in the diet of the coral reef grouper Plectropomus leopardus (Serranidae): patterns in taxa, size and habitat of prey

Stomach contents and diet of a wide range of sizes (4.7 to 57.3 cm standard length [SL]I) of the predatory coral trout Plectropomus leopardus (Fam. Se rranidae) collected mostly from the northern Great Barrier Reef during the summers of 1991 and 1992 were examined. Overall, 422 prey in 28 families of fishes were identified of which 3 families, Clupeidae, Pomacentridae and L abridae, represented more than 60% of the diet. Average daily consumption o f prey in P. leopardus was 2.8% of relative body weight and fish of 20 to 4 9.9 mm SL were eaten by all size classes. Juvenile (up to 20 cm SL) and adu lt P. leopardus have distinct diets. The major dietary shift in the type, s pecies composition, length and shape of prey occurs at approximately 20 cm SL, just prior to the onset of maturity. Juveniles consumed a high proporti on of benthic dwelling crustaceans, mostly penaeid shrimps, whereas adults were almost entirely piscivorous. The number of families of fishes in the d iet did not increase with the size of predator as some families were eaten exclusively by juveniles or adults. Abundances of fish families in the diet varied ontogenetically and among size classes in small adults. Predation w as related to size of prey in juvenile and small adults. Up to 35 cm SL, th e range of prey sizes widened with increases in predator length, as larger fish ate larger prey, concomitantly feeding on small fishes, especially sch ooling species. However, the composition of the diet and length of fish pre y did not change in P. leopardus after 35 cm SL and prey did not deepen aft er 45 cm SL. Body depth of prey was more important than length in size-rela ted feeding on 2 common families of fishes with different shapes. In genera l, larger P. leopardus appeared to feed optimally on deep-bodied fish, wher eas juveniles avoided gape limitation by consuming more slender fish. All s ize-related predation in P. leopardus, however, was easily decoupled by the presence of highly abundant small fishes in large visible schools.