Exxon Valdez Oil Spill Restoration Project Final Report Investigation of Pacific Herring (Clupea pallasii) Stock Structure in Alaska Using Otolith Microchemistry and Heart Tissue Fatty Acid Composition Restoration Project 070769 Final Report

Understanding the structure of Alaska’s Pacific herring stocks is relevant to how these exploited populations should be assessed and managed. We evaluated the capability of heart tissue fatty acid profiles and otolith microchemistry to identify the stock of origin for herring sampled from fourteen spawning aggregations associated with seven putative herring stocks throughout Alaska (e.g., Sitka, Prince William Sound, Kamishak, Kodiak, Togiak, Kuskokwim Bay, and Dutch Harbor). Fatty acid profiles were determined by performing transesterification and fatty acid chromatography on purified lipids from whole hearts. Otolith microchemistries were measured (ppm) using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Across-year comparisons were made to evaluate the temporal stability of fatty acid biomarkers that successfully discriminated putative stocks at multiple spatial scales within-years. Cross-validation of discriminant functions and nMDS with ANOSIM revealed that considerable shifts in fatty acid composition occurred for most stocks across short (1 year) and long (4-5 year) time periods. Temporal shifts also made it difficult to identify stock of origin for a putative mixed-stock sample collected during winter. Otolith edge microchemistries were significantly different between some putative stocks at multiple spatial scales, but nMDS and ANOSIM suggested that this difference was infinitesimal. We used heart tissue fatty acid profiles to discriminate Alaskan herring stocks at relatively fine spatial scales when we built models from data collected within the same annual spawning period, but we were unsuccessful discriminating unknown samples (e.g., from mixed stock fisheries) collected outside the spawning period, or in other years.