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

Abstract

Nursery areas of Pacifi c herring, Clupea pallasi, and walleye pollock, Theragra chalcogramma, were identifi ed by using acoustic surveys in October 1995, and in March and July 1996 in Prince William Sound, Alaska. Pacifi c herring and walleye pollock were aggregated in the east-northeast and west-southwest areas. Juvenile Pacifi c herring spent the fi rst two years of their lives isolated within bays. Water tem- peratures 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 consider- able. The school structure of juvenile Pacifi c herring varied seasonally. Newly recruited age-0 Pacifi c herring were tightly aggregated, forming a few, dense schools within a single-size cohort, in shallow water at the heads of bays in July. Age-0 Pacifi c herring were still aggregated within bays in shallow water in October, but the school struc- ture was less cohesive and size cohorts began to mix. School structure and dis- tribution completely changed in March as age-0 Pacifi c herring moved away from the shores into deeper water and spread out forming sparse shoals of mixed-size cohorts. Juvenile Pacifi c her- ring joined the adult schools after their second winter, leaving as new recruits entered the bays. Juvenile walleye pol- lock also aggregated within bays and were spatially segregated from adults. However, there appeared to be little overlap between Pacifi c herring and walleye pollock juveniles because they occupied different portions of the water column within these bays.