Vertical position, specific gravity and swimming ability of Pacific cod Gadus macrocephalus yolk-sac larvae reared at four temperatures

Vertical position and movement rate were measured in the yolk-sac larvae of the Pacific cod Gadus macrocephalus at different rearing temperatures (3, 5, 7, and 9 A degrees C) and an identical temperature of 5 A degrees C in experimental columns during the initial 3 days post-hatch (dph). Larval specific gravity was also measured through sinking velocities. Yolk-sac larvae swam upward and initially (0 dph) remained in the upper section of the experimental column through intermittent upward swimming, and gradually moved to near-neutral buoyancy at 3 dph at corresponding rearing temperatures. Rearing temperature significantly affected the swimming ability: at 0 dph, vertical movement rates measured at 5 A degrees C were low in the yolk-sac larvae reared at warm temperatures. However, no such difference was observed at 3 dph. The difference in vertical movement rate may have resulted from the temperature-mediated body sizes (standard length) and yolk-sac sizes (yolk-sac volume) at hatching. Specific gravity did not change for the varying temperatures during the initial 3 days, but decreased as ontogeny progressed. Therefore, we suggest that Pacific cod yolk-sac larvae achieve a demersal-pelagic habitat transition through upward swimming and increasing buoyancy.