Changes in the Daily Ration Size of Pacific Saury, Cololabis saira, with Seasons and Body Sizes

Abstract

Daily ration size of Pacific saury, Cololabis saira, for each season was estimated from the diurnal variation in the weight of gut contents. In all seasons, the weight of gut contents reached the maximum value around sunset, then decreased during the night, and reached the minimum value around sunrise. Thus, Pacific saury are concluded to feed during the day and ceased feeding during the night. Daily ration sizes in wet weight estimated from Eggers’ formula for saury standardized to 300mm in knob length were 7.99, 9.03, 3.11, and 3.67g for spring [May and June, the Oyashio (food-rich subarctic water)-Kuroshio (food-poor subtropical water) transitional waters (Transitional)], summer (July, the Oyashio and the Transitional waters), autumn (September-November, the Oyashio and the Transitional waters), and winter (January and February, the Kuroshio waters), respectively. The daily ration size increased with body size (210-349mm) in spring and summer, in contrast it remained at an approximately constant level in autumn and winter. The possibility of seasonal differences in the relationships between daily ration size and ambient food density is also suggested.