Size related dietary shifts of Epinephelus marginatus in a western Mediterranean littoral ecosystem: an isotope and stomach content analysis

Abstract

The diet of the dusky grouper Epinephelus marginatus , from the Balearic Islands, western Mediterranean, consisted primarily of crustaceans, molluscs and fishes, but diet composition varied with body size. The smaller dusky groupers (<300 mm L T ) fed primarily on crustaceans, and particularly on brachyurans, which accounted for 46% of the prey identified. As dusky groupers grew, cephalopods became increasingly important and constituted 10 to 40% of the prey identified in subadult and adult specimens. The largest dusky groupers fed primarily on fishes that represented 40·9% of prey identified. These shifts in diet were accompanied by a positive selection of increasingly large prey and by an expansion of trophic niche. δ 15 N values of dusky grouper white muscle ranged from 8·8 to 13·1% and 71% of the variation in δ 15 N was explained by differences in dusky grouper size. δ 13 C values ranged from – 17·9 to – 15·9%, and no significant body size effect on δ 13 C was detected. Stomach content and 13 C values indicated that from 1 year old, the diet of E. marginatus was based on the benthic food web. The enrichment in δ 15 N registered from juvenile to large males was c . 3·8%. Overall, there was good agreement between gut content and stable isotope data indicating that the latter may be useful as a tool in trophic studies in marine protected areas where sampling for fishes is not allowed.