National Fisheries University, Shimonoseki, Yamaguchi 759-6595, Japan
We conducted detailed analyses of gastrointestinal contents on the adult rabbitfish Siganus fuscescens occurring in a Sargassum bed in order to evaluate the dietary importance of corticated large brown macroalgae. Fish specimens were collected by spear gun in the daytime within two mixed-species Sargassum beds at Futaoi Island. Large brown macroalgae accounted for 45% in summer and 74% in autumn in gravimetric percentage composition of the stomach contents, and were dominated by several species of Fucales such as Myagropsis myagroides, S. patens, and S. piluliferum, suggesting selective foraging by the rabbitfish. Animal components accounted for 18% in summer and 14% in autumn in the stomachs, consisting primarily of sessile animals, and large gelatinous zooplankton found in the intestinal contents was also regarded as an important animal component. In addition, the ingested abundance of small epiphytic crustaceans or sessile animals showed no correlation with large brown macroalgae found in the stomach contents. The rabbitfish appears to forage in seaweed beds targeting large brown macroalgae themselves, although the fish may ingest some animal components as an important nutritional source.