1Japan Sea-Farming Association Akkeshi Station, Akkeshi, Hokkaidou 088-1108, 2Japan Sea-Farming Association Notojima Station, Notojima, Ishikawa 926-0216, Japan
The horizontal distribution of Pacific cod Gadus macrocephalus juveniles and their diet were examined in the northern part of Nanao Bay and its vicinity from April to May in 1999. In the bay, cod juveniles ranging from 24mm to 56mm in total length (TL) were collected with a beam trawl net on the bay bottom where the temperature was ≤12°C from late April to early May. In a commercial set net located just outside the bay, cod juveniles ranging from 39 to 64mm in TL were found at the temperature ranging from 11 to 13°C in early and mid May. Most juveniles might grow up to 40-50mm TL and migrate outside by mid-May and the upper limit of inhabiting temperature would be about 12°C. As for the diet, cod juveniles in the bay fed mainly on zooplankton, mostly copepods. As juveniles of ≥45mm TL fed on some demersal prey items, such as gammarid amphipods and natantia juveniles, the diet shift from pelagic prey to demersal prey would start at this fish size.