1Faculty of Agriculture, Kochi University, Nankoku, Kochi 783-8502, 2Graduate School of Kuroshio Science, Kochi University, Nankoku, Kochi 783-8502, Japan
We examined the seasonal variation of lipid content and ecological traits including gonad index of Shimizu-Saba, a population of spotted mackerel, caught off the Cape of Ashizuri. Sampling was conducted from December 2005 to November 2006. Fifteen fishes were collected in each month and were classified into 3 size classes: middle size 400-600 g (M), large size 600-900 g (L), extra large size 900 g or over (XL) (5 fishes were sampled from each size class in each month). The gonad index increased from December 2005 and attained the peak earlier in order of the size class (XL: January, L: February, M: March). Lipid content of the dorsal muscle showed a common seasonal change in the three size classes, decreasing from winter to spring and increasing from autumn to winter. The first decrease in lipid content of M and XL sizes was observed at the time of maturity and the early spawning season, respectively. After the spawning season, the lipid content increased. Comparing the rate of lipid given in autumn-winter with that in spring-summer, the former clearly showed higher content, suggesting that