Spatial distributions of Pacific herring, Clupea pallasi, and walleye pollock, Theragra chalcogramma, in Prince William Sound, Alaska

Nursery areas of Pacific herring, Clupea pallasi, and walleye pollock, Ther agra chalcogramma, were identified by using acoustic surveys in October 199 5, and in March and July 1996 in Prince William Sound, Alaska. Pacific herr ing and walleye pollock were aggregated in the east-northeast and west-sout hwest areas. Juvenile Pacific herring spent the first two years of their li ves isolated within bays. Water temperatures within bays were cooler in the summer and warmer in the winter compared with temperatures measured along coastal passages and the open coast. Although these temperature differences were small and based on point observations their accumulative effect over the season may be considerable. The school structure of juvenile Pacific he rring varied seasonally. Newly recruited age-0 Pacific herring were tightly aggregated, forming a few, dense schools within a single-size cohort, in s hallow water at the heads of bays in July. Age-0 Pacific herring were still aggregated within bays in shallow water in October, but the school structu re was less cohesive and size cohorts began to mix. School structure and di stribution completely changed in March as age-0 Pacific herring moved away from the shores into deeper water and spread out forming sparse shoals of m ixed-size cohorts. Juvenile Pacific herring joined the adult schools after their second winter, leaving as new recruits entered the bays. Juvenile wal leye pollock also aggregated within bays and were spatially segregated from adults. However, there appeared to be little overlap between Pacific herri ng and walleye pollock juveniles because they occupied different portions o f the water column within these bays.