Trends in juvenile sockeye salmon rearing capacity, stock specific growth performance, and estuarine habitat use in the Chignik watershed, Alaska

Abstract

Coastal watersheds provide a network of interconnected heterogeneous habitatsaccessible to mobile species. Multiple populations can exploit alternative habitats, buffering overall abundances against perturbations in climate and ecosystem processes. In the Chignik Lakes watershed, Alaska, sockeye salmon (Oncorhynchus nerka) populations exhibit a diversity of juvenile life history strategies, exploiting diverse freshwater and estuarine rearing habitats. In 2018, Chignik sockeye salmon stocks experienced a catastrophic collapse, threatening the viability of commercial and subsistence harvest. The overall goal of this thesis was to investigate how changes in juvenile sockeye salmon rearing capacity, growth performance, and habitat use within the Chignik watershed may have contributed to the 2018 fishery disaster. We compiled and analyzed multiple decades of habitat quality data to explore long-term trends in the freshwater rearing capacity and growth performance of juvenile sockeye salmon in iii the years preceding the 2018 collapse. We identified increasing water temperatures in a shallow lake, and more stable conditions in a deeper lake. Zooplankton prey quality increased in response to both bottom-up and top-down food web dynamics. We observed strong effects of competition and density dependence in sockeye salmon populations throughout the watershed. Although we detected no evidence that freshwater habitat quality has declined, the data suggest that high early life stage mortality in brood years of the 2018 collapse likely contributed to poor adult returns. Using Single Nucleotide Polymorphisms, we successfully assigned individual juvenile sockeye salmon to two distinct populations. We reenforced previous findings that body condition reflects the productivity of an individual’s rearing habitat, regardless of stock of origin. Additionally, we observed multiple stocks exploiting estuarine habitat for juvenile rearing for the duration of the summer growing season. Together, our results suggest habitat heterogeneity and population diversity buffer sockeye salmon populations in the Chignik watershed. By exploiting alternative habitats, multiple populations are able to achieve sufficient growth despite variability in local habitat quality.