Date of Award
2026
Document Type
Honors Thesis (Open Access)
Department
Colby College. Environmental Studies Program
Advisor(s)
Amanda Gallinat
Second Advisor
Cait Cleaver
Abstract
The timing of sockeye salmon spawning migration is crucial for ecosystem function, fishery management, and the reproduction of salmon populations in Bristol Bay, Alaska. In other systems, salmon are shifting their migration timing earlier in response to warming ocean temperatures and changes to offshore wind patterns. Further, the timing of catch and escapement may not be shifting at the same magnitude, indicating potentially harmful selection on the population from the fishery. In other parts of Alaska, freshwater conditions have been warming more significantly than ocean temperatures, potentially leading to selection for early migration. To examine shifts in migration phenology over time, the environmental drivers of these shifts, and the impact of timing on recruitment, we used publicly available daily escapement data (1955-2025), daily catch data (1985-2025), yearly age structure, and yearly recruitment data from the Alaska Department of Fish and Game, as well as yearly average spring SST and June wind data from the International Comprehensive Ocean-Atmosphere Data Set. For 7 rivers and their 5 associated fishing districts in Bristol Bay, we examined how timing is shifting linearly over time for the onset (10%), median (50%), and end (90%) of passage. We used linear mixed-effects models to determine the impact of environmental and demographic drivers on timing. We used linear regressions to quantify the impact of arrival timing on recruitment in each of the 7 rivers. As a general pattern, we found the season of escapement to be lengthening, such that the onset of arrival is shifting earlier, while the end of arrival is shifting later. While the end of catch is shifting later, the onset of catch is not shifting earlier in the same pattern as escapement. We found that the onset of catch and escapement were earlier in years with greater westward winds driving favorable northward currents. In years with warmer SST, we see a lengthening of the season of escapement and later end to catch that both align well with our observed shifts over time. Years with older average age of the run led to earlier onset and median timing of catch only, likely due to fishery selection where older fish are more likely to be caught. In the Wood River, years with earlier onset and median timing of escapement had significantly greater recruitment. The Wood River is shifting significantly earlier over time, more so than all but one other river in this study. These results provide strong evidence for fisheries-induced selection on arrival timing, longer seasons being driven by warming ocean temperatures, and selection for earlier timing in a population shifting earlier over time.
Keywords
Environmental Science, Fisheries, Salmon, Ecology, Selection, Oncorhynchus nerka
Recommended Citation
Goldrick, Jeremy, "The Drivers of Shifting Sockeye Salmon Migration Phenology in Bristol Bay, Alaska" (2026). Honors Theses. Paper 1545.https://digitalcommons.colby.edu/honorstheses/1545