Abstract
In the present study, we examined decadal changes in the diets of the surface migratory myctophid fish Myctophum nitidulum (50.3-78.8 mm standard length) in the Kuroshio region of the western North Pacific over a period of 18 years, from 1964 to 1999. Samples were collected with a neuston net within 1 m of the surface, at night, from January to March each year. Myctophum nitidulum fed mainly on crustacean zooplankton, such as euphausiids, copepods and amphipods, in the study years from 1964 to 1975 and from 1991 to 1999. However, Japanese sardine larvae were the most or one of the most common prey for 7 of the 8 years studied between 1976 and 1990, accounting for 20.2-61.8% (mean (± SD) 36.1±16.6%) of the total weight of M. nitidulum stomach contents. The Japanese sardine population exploded between 1976 and 1990; sardine larvae were correspondingly abundant in the Kuroshio region. The feeding habits of M. nitidulum seemed to change according to changes in the composition of prey species in its habitat. We estimated that M. nitidulum consumed between 4.8±2.9 and 10.3±6.2 mg wet weight/m3 (0.24±0.14-0.52±0.31 individuals/m3) of sardine larvae per night in the Kuroshio region during the 7 years between 1976 and 1990.
