Interannual variation in diets of walleye pollock in the Doto area, in relation to climate variation

Abstract

Interannual variation in the diet of walleye pollock Theragra chalcogramma along the southeastern coast of Hokkaido Island (Doto area) was analyzed using >15 000 specimens col- lected in the periods 1989�1992 and 1995�2011, in relation to oceanographic variables. The area experienced a warm period from 1998 to 2002, which was preceded and followed by cold periods. An apparent shift in diet occurred between the first cold period and the warm period. Appendic- ularians (larvaceans) became an important summer prey for small-sized pollock (≤300 mm SL) in addition to copepods and euphausiids, which were also important before the onset of the warm period. In medium- and large-sized pollock, an abrupt decrease in mesopelagic fishes (mainly myctophid Diaphus theta) was observed in stomach contents 3 yr after the 1997/1998 climate regime shift, possibly reflecting a recruitment failure of D. theta. Appendicularians, cannibalism and anchovy Engraulis japonica compensated for the disappearance of mesopelagic fishes from the diet. During autumn and winter, a similar shift from micronekton (mainly D. theta and firefly squid Watasenia scintillans) to euphausiids (exclusively Euphausia pacifica) and cannibalism was found between 2000 and 2001. Although diet changes lagged the onset of the warm period, con- dition factor increased immediately after the regime shift. This inconsistency may be attributable to the bioenergetic response of juvenile pollock, which attain better growth at warmer tempera- tures. Thus, in the Doto area, the effect of climate variation on the survival of pollock seemed to be limited, due to their flexible feeding behaviors.