Horne, John KMcGowan, David W2018-04-242018-04-242018-04-242018McGowan_washington_0250E_18292.pdfhttp://hdl.handle.net/1773/41793Thesis (Ph.D.)--University of Washington, 2018To support ecosystem-based management of Alaska’s marine resources, there is a need to improve our understanding of how climate-related perturbations in ocean conditions and long-term warming affect distributions and abundances of planktivorous fish that function as a mid-trophic link within marine food webs. Capelin (Mallotus villosus) is an important small pelagic fish species in boreal-Arctic marine ecosystems. Spatial and temporal changes in capelin distributions affect their availability as prey to piscivorous seabirds, marine mammals, and commercially important fish. Compared to Atlantic populations, there is limited information describing fluctuations in capelin distributions and abundances in the Alaskan North Pacific. This dissertation examined environmental influences on distributions and relative abundance of age-1+ capelin over the Gulf of Alaska (GOA) continental shelf to assess variability in capelin biomass and availability to predators. Acoustic, oceanographic, and trawl sampling were conducted in summer and fall of 2011 and 2013 as part of the Gulf of Alaska Integrated Ecosystem Research Program to characterize spatial and temporal variability in distributions of capelin and other forage fish species over the central (CGOA) and eastern (EGOA) shelf and slope. Environmental factors that influenced occurrence and density of capelin in summer 2013 were identified at spatial resolutions associated with systematic sampling at discrete stations and continuous sampling along transects. Additional data from an independent fisheries-oceanographic survey were used to investigate effects of temperature on spatiotemporal variability in capelin distributions and densities during a period of warm and cold years between 2000 and 2013. GOA capelin have concentrated over the shelf south and east of the Kodiak Archipelago in the CGOA since at least the mid-2000s. Distributions were influenced by increased vertical mixing and bathymetry. Model results indicated that over shallow submarine banks (< 100 m bottom depth) capelin concentrated in areas with relatively higher water column stratification associated with enhanced primary production. In deeper troughs (≥ 100 m), capelin were most likely to occur in waters between 8 and 9° C, concentrated in areas that were less stratified and associated with higher production. Mean densities of capelin were not directly related to interannual differences in temperature. Observed differences in distributions and relative abundance of capelin between the western (WGOA) and CGOA regions were attributed to either displacement of capelin along the shelf, expansion of distributions during warm years and contraction during cold years, or spatiotemporal differences in capelin mortality due to shifts in predator distributions. Interannual variability in the relative abundance of capelin in the WGOA was correlated with the winter Pacific Decadal Oscillation (PDO) index lagged by 1 year. This suggests that GOA capelin abundance may be influenced by climate-related processes linked to North Pacific sea surface temperature anomalies that affect capelin survival during their first year of life.application/pdfen-USnonecapelindistributionfisheries acousticsforage fishGulf of AlaskaspatiotemporalEcologyAcousticsNatural resource managementFisheriesThesis