Date of Award

2026

Document Type

Honors Thesis (Open Access)

Department

Colby College. Biology Dept.

Advisor(s)

Kyle Coblentz

Abstract

Climate change is shifting the timing of biological events globally, but the mechanisms linking environmental variation to phenological changes are poorly understood in many systems, particularly those where individuals undergo ontogenetic habitat shifts. Here, I examined the join influence of climate-driven environmental variability and species interactions migration timing and body size at migration in juvenile sockeye salmon (Oncorhynchus nerka) in Lake Aleknagik, Alaska, using a multi-decadal dataset (1967–2024). Migration timing varied widely among years and was best explained by a combination of ice-out phenology, epilimnetic temperature, and fish densities in littoral habitats. Juvenile sockeye salmon migrated earlier in years of earlier ice-out, warmer offshore conditions, and higher conspecific densities, while migration was delayed at higher interspecific fish densities. In contrast, zooplankton phenology showed strong interannual variability but weak directional change, while zooplankton abundance showed limited and taxon-specific associations with migration timing. Body size at migration also varied interannually. It was primarily associated with littoral temperature and conspecific density, with warmer conditions resulting in larger individuals and higher densities resulting in smaller individuals at migration. Together, these results demonstrate that migration phenology and body size at this ontogenetic transition emerge from interacting abiotic and biotic processes rather than temperature alone. Phenology and growth are partially decoupled responses to shared environmental drivers, with implications for how climate change may reshape life-history timing and organismal state across trophic levels in seasonally constrained ecosystems.

Keywords

Oncorhynchus nerka, phenology, climate change, ontogenetic shift, freshwater ecosystems

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