Analysis and comparison of age-structured assessment models for two Pacific herring populations

Abstract

Substantial research has been devoted to identify causes for decline the of Prince William Sound (PWS) Pacific herring in the early 1990's because of the proximity to the 1989 Exxon Valdez oil spill (EVOS). A potential source for decline has been identified with the isolation of disease in the PWS population. There have been limited investigations of PWS Pacific herring population dynamics related to other stocks in the Gulf of Alaska. The objective of this thesis was to compare observations and age-structured assessment (ASA) model results between PWS and Sitka Sound Pacific herring. Data conflicts were evaluated in the PWS ASA model and indicate that hypotheses about natural mortality in the four years subsequent to EVOS depend on the type and weighting of population indices. In Sitka, the ASA model was used to show that time-dependent natural mortality can be estimated. Comparison between PWS and Sitka indicated that age structure and recruitment have been comparable, but abundance indices and weight-at-age data have not been similar after 1993. The differences identified in this thesis between PWS and Sitka imply uniqueness in natural mortality and condition within each Pacific herring population.