P. A. H. Westley | Geology Reports

USGS2022

Premature mortality observations among Alaska’s Pacific salmon during record heat and drought in 2019

Widespread mortality of Pacific salmon Oncorhynchus spp. returning to spawn in Alaska coincided with record-breaking air temperatures and prolonged drought in summer 2019. Extreme environmental conditions are expected to happen more frequently with rapid climate change and challenge the notion that Alaska could indefinitely provide abundant, cool freshwater habitat for Pacific salmon. A total of 110 geographically widespread opportunistic observations of premature mortality (carcasses) were collected from a variety of sources. Premature mortalities were documented for Pink Salmon Oncorhynchus gorbuscha , Sockeye Salmon O. nerka , Chum Salmon O. keta , Chinook Salmon O. tshawytscha , and Coho Salmon O. kisutch . Additionally, observations of Pink Salmon returning to spawn in Prince William Sound streams in 2019, obtained from systematic aerial surveys conducted annually, revealed low migration success in 87% of rain-driven streams ( n = 30), 52% of snow-driven streams ( n = 65), and only 18% of glacier-driven streams ( n = 11). Salmon mortality observations were consistent with death due to heat stress resulting from high water temperatures or drought caused hypoxia and stranding. Developing a better understanding of how broad-scale climate patterns manifest at the stream scale can help us determine whether a major shift in Pacific salmon productivity is underway and inform fisheries management plans to better mitigate future risks.

Alaska↗

USGS2021

Assessing potential stock structure of adult Coho Salmon in a small Alaska watershed: Quantifying run timing, spawning locations, and holding areas with radiotelemetry

Run timing and spatial locations of spawning habitats are often used to identify stocks for conservation planning or management of salmonid fishes. Although complex stock structure is most common within large watersheds with diverse habitats, even small drainages can produce multiple co-occurring spatially or temporally isolated populations or “stocks.” This project sought to address the potential existence of stock structure of Coho Salmon Oncorhynchus kisutch in a small coastal watershed on Kodiak, Alaska that supports vital subsistence and recreational fisheries and is currently managed as a single stock. We radio-tagged a total of 348 adult Coho Salmon upon freshwater entry into the Buskin River across three spawning seasons (2015–2017) and tracked in-river movements to the final locations where mortality signals were recorded. We identified two primary spawning habitats within the system: main-stem and lake tributaries, with 54% (range of 47% to 61%) of tagged fish with determined fates tracked to main-stem river spawning areas and 46% (range 39% to 53%) presumably spawning in small tributaries of the 1-km 2 Buskin Lake at the headwater of the watershed. Despite distinct spatial differences in spawning locations, main-stem and tributary spawners did not differ in migration timing into freshwater (difference in run timing of main-stem versus tributary spawners = 1 d) nor body size (main-stem mean body length, mideye to tail fork = 625 mm, tributary mean = 613 mm). Unexpectedly, we determined nearly 70% of all Coho Salmon spent at least some time in Buskin Lake, including 54% of main-stem spawners, suggesting a potential role of Buskin Lake as an important staging habitat for premature migrating adult Coho Salmon who enter freshwater in advance of final maturation. We also identified areas consistently used for holding prior to spawning that could be used in spatial management planning and during times of necessary conservation to ensure integrity of the stock for the future.

Alaska↗