The importance of animal components in the diet of adult rabbitfish Siganus fuscescens

The importance of animal matter in the diet of the adult rabbitfish Siganus fuscescens was determined using a combination of composition analysis of the diet and rearing experiments. The gut contents of rabbitfish inhabiting a barren area or a seaweed bed consisted mainly of macroalgae with a smaller concentration of animal matter such as hydrozoas and crustaceans. The carbon/nitrogen stable isotope ratios (δ ¹³C and δ ¹⁵N) of rabbitfish living in the barren area were significantly lower than those of rabbitfish inhabiting the seaweed bed. This difference in the ratios is probably due to the lower concentrations of δ ¹³C and δ ¹⁵N found in crustaceans which are the main food source for rabbitfish inhabiting the barren area as compared to the concentrations of δ ¹³C and δ ¹⁵N provided by algae and other animals as a food source. Rabbitfish reared on brown algae alone showed reduced somatic growth, whereas the fish fed with krill and gammarid exhibited a greater increase in body weight and were found to be in better overall condition. Concentrations of δ ¹³C and δ ¹⁵N in the fish fed only with brown algae showed a small shift during the rearing period. Thus, although the rabbitfish is regarded to be omnivorous with strong herbivorous tendencies, it appears that the inclusion of animal matter in their diet is also important for satisfactory growth.