Daily rhythm and seasonal variation of feeding habit of Pacific saury (Cololabis saira) in relation to their migration and oceanographic conditions off Japan
Seasonal and diel variation of feeding activities of Pacific saury (Cololabis saira) were examined following their annual migration cycle in North Pacific waters adjacent to Japan in relation to oceanographic conditions, especially abundance and composition of zooplankton collected in surrounding waters. A high gut fullness and low degree of digestion of the gut contents were frequently observed during the dusk period during the 24-h cycle for all seasons. In spring and summer saury were distributed in the Kuroshio–Oyashio transitional region and the Oyashio (cold current) area, where they mainly fed on the euphausiid, Euphausia pacifica, and had a higher gut fullness than the other seasons. Their feeding activity decreased in fall, and increased in winter, the spawning season, although they were distributed in an area of poor food conditions in the Kuroshio warm current area. Their distribution in the Kuroshio–Oyashio transitional region and the Oyashio area allow utilization of adult euphausiids, E. pacifica which is a significant food source.