Seasonal changes in dietary composition of the herbivorous fish Kyphosus bigibbus in southwestern Japan

Kyphosus bigibbus is an herbivorous fish, and its relatively large body size and schooling behavior may have a significant effect on macroalgae in southwestern Japan. However, the feeding habits of K. bigibbus are poorly understood. We investigated seasonal changes in stomach contents of K. bigibbus in Nishisonogi Peninsula, Nagasaki Prefecture, in southwestern Japan. In this area, large brown algae (i.e., Sargassum and annual kelp) grow abundantly only in spring and early summer; thereafter, from late summer to winter, its biomass becomes negligible. Dietary analyses revealed that K. bigibbus consumed exclusively benthic macroalgae, regardless of the depletion of large macroalgae for a half-year, and that carnivory or detritivory was not evident in any season. When large brown algae were abundant, K. bigibbus fed primarily on these algae, and to the lesser extent on some species of small brown algae (Scytosiphonaceae). During summer to winter, when large brown algae were absent, the main diet consisted of Padina arborescens and Zonaria diesingiana, and even the holdfast of Eckloniopsis radicosa was ingested. Scytosiphonacean species and fleshy red algae were important diets in late winter to early spring, when large sized brown algae were juveniles.