FEEDING ECOLOGY OF JUVENILE MASU SALMON ONCORHYNCHUS-MASOU IN THE COASTS OF HOKKAIDO WITH SPECIAL REFERENCE TO STOMACH CONTENTS

Feeding ecology of juvenile masu salmon, Oncorhynchus masou, collected by commercial set nets, was studied in the coastal waters of Hokkaido between 1991 and 1993. Stomach fullness indices varied among sampling dates and two areas sampled (Japan Sea and Pacific Ocean). The stomac h contents of juveniles collected along the coasts of the northern Jap an Sea were composed entirely of sand lances (Ammodytes personatus), w hich were 1/4-1/2 of the body lengths of the juvenile salmon. In contr ast, stomach contents of the juveniles collected along the coast of th e Pacific Ocean included a variety of organisms, and size ratios of pr ey to juveniles were lower than those in the northern Japan Sea. The r esults of comparisons of juvenile stomach contents between the Japan S ea and Pacific Ocean indicate that juvenile masu salmon occupy differe nt ecological niches in the two areas. Production of juveniles in the northern Japan Sea may depend on the abundance of sand lance.