Identification of spawning areas and the influence of environmental variation on freshwater migration timing and in-river movements of adult coho salmon in the Buskin River, Alaska

Abstract

The timing of freshwater entry by anadromous salmonids varies markedly among species and populations within species and is frequently used as an indicator of local adaptation to sitespecific patterns of selection. Although complex stock structure is most often associated with large watersheds that have extensive habitat diversity, even small drainages can produce multiple co-occurring stocks that differ in migratory timing. In addition, migration timing can be influenced by within-year environmental conditions experienced by migrating individuals en route to spawning sites, staging near the river mouth in the ocean, or within the river itself. Each stage of migration through both freshwater and saltwater could be altered based on climatic drivers and how each individual fish reacts to these stressors. The objective of this thesis was to assess the potential for stock structure in Coho Salmon within a small coastal watershed on Kodiak Island, Alaska by 1) identifying important differences in spawning and holding locations associated with run timing, length, and stream life between main stem and tributary spawners, 2) quantifying the influence of large-, intermediate-, and local-scale climate variables on freshwater entrance timing and in-river movements. To address the first objective, fish were tracked to their spawning locations using acoustic telemetry in three spawning seasons (2015-2017). I detected no statistically or biologically meaningful differences in body size (length, mm) or migration timing into the river between main stem and tributary spawning fish. Unexpectedly, I found that a large portion of fish (80%) utilize the lake during their in-river migration suggesting the lake may represent critical staging habitat for adult Coho Salmon prior to spawning. I also identified holding habitat throughout the river that both spawning groups consistently used across years that also appears to be important to premature migrating Coho Salmon. In Chapter Two, I analyzed 33 years of freshwater entrance timing data and utilized radio tags to track in-river movement to quantify the influence of precipitation and temperature on total distance moved and probability of moving. Despite marked variation among years, I found no evidence of a temporal trend in entrance timing based on escapement counts, which contrasts with other recent examples throughout Alaska reporting changes in run timing. The strongest influence on timing of freshwater entry was ocean sea surface temperature, where cold temperatures delayed entry up to 11 days. Within-river movements were positively related to precipitation and temperature, confirming local traditional knowledge in this system, and consistent with life history patterns of Coho Salmon. The primary messages of this thesis are that i) any within-watershed stock structure is unlikely to be differentially affected by harvest or management given overlapping run timing, body size, and use of main stem holding areas; future population genetics analyses would be an obvious and illuminating next step to assess the extent to which main stem and tributary spawners are reproductively isolated groups; ii) both main stem and tributary spawners use Buskin Lake as holding habitat prior to spawning, and thus assumptions that fish that enter the upper watershed are destined to spawn in headwater tributaries are invalid, which in turn limits the utility of enumerating adult passage into the lake for escapement-based management, iii) adult freshwater entrance timing is highly variable but not changing systematically through time, though the extent to which the variation in timing reflects environmental response vs. uncertainty in the counts at the weir is unknown, and iv) low precipitation and warm temperatures suppress movement and result in protracted use of main stem and lake habitats for holding, which may put some individuals at risk to angler harvest or, in extreme events, potentially low dissolved oxygen environments. Spatial management that restricts fishing in locations of known primary holding habitats may be an option to reduce probability of mortality and stress in years of low adult abundance.