Environmental Factors in Funka Bay, Hokkaido, Affecting the Year Class Strength of Walleye Pollock, Theragra chalcogramma

Abstract

The 1995-year-class of walleye pollock Theragra chalcogramma was dominant in and around Funka Bay. To understand the formation of dominant year classes of pollock in this area, annual changes in monthly fluctuation of density of pollock larvae, prey concentration at the initial feeding stage, and hydrographic conditions were investigated from 1991 to 1998. Pollock larvae occurred mainly from January to February. Warm water temperatures at 10m depth in the bay, where pollock larvae were considered to be abundant, were observed from January to early February before the intrusion of the Oyashio Coastal Water (cold water). Thus the relatively warm water temperature in early winter appears to be necessary to produce the dominant year class of pollock in Funka Bay. From 1991 to 1998, no cold water temperature affecting the survival of larval pollock was observed. The densities of copepod nauplii as a primary food for larval pollock at the initial feeding stage were relatively high from January to February in 1991 (7.8-18.1 inds. l^-1), 1994 (13.1-15.2 inds. l^-1), 1995 (12.3-15.5 inds. l^-1), and 1996 (13.8-17.2 inds. l^-1), but the dominant year class occurred only in 1995. Therefore, the high density of copepod nauplii does not determine the year class strength of pollock in this area. These results suggest that the abundance of food for pollock larvae at the first feeding stage were enough to survival in early winter in this area. Pollock larvae and juveniles (7-30mm in total length) mainly feed on small sized copepodites of Pseudocalanus spp., whose density was high from March to April and then decreases in early June. At that time, juveniles larger than 30mm in total length initiate to feed on larger sized copepodites of Neocalanus plumchrus. It is considered that pollock larvae hatching in March do not grow enough to feed on N. plumchrus in June. From this information, it is inferred that the spawning strategy of pollock in this area is concerned with the food conditions for juveniles at a transition period of food organisms. In Funka Bay, water temperature in early winter and food conditions for juvenile pollock might control the year class strength of pollock in and around Funka Bay.