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Chinook salmon population declines span their geographic range with climate hypothesized as a major driver. Concerns of warming freshwater temperatures in their northern range gained urgency during 2019 when a heatwave coincided with premature mortality. This study examined heat stress during the 2019 heatwave compared to subsequent years and described water temperatures in western Alaska to understand the degree to which freshwater temperatures may be a stressor. Heat stress was prevalent among Chinook salmon captured in the 2019 heatwave (Kuskokwim tributaries: 90% in Kwethluk and 63% Takotna river), and variable in subsequent years (~8% to 60% across Kuskokwim tributaries and Norton Sound rivers). A review of water temperature data indicated potentially stressful temperatures (≥18°C) were most common and prolonged in the Yukon River, moderately common and prolonged in the Kuskokwim River, and relatively rare in the Norton Sound region. Water temperatures in 2019 broke several records for overall maximum and frequency of temperatures ≥18°C. Migration water temperatures and heat stress in northern Pacific salmon habitats varies more widely than previously recognized (up to 25°C). 

Abstract Macroalgae and phytoplankton support the base of highly productive nearshore ecosystems in cold-temperate regions. To better understand their relative importance to nearshore food webs, this study considered four regions in the northern Gulf of Alaska where three indicator consumers were collected, filter-feeding mussels (Mytilus trossulus), pelagic-feeding Black Rockfish (Sebastes melanops), and benthic-feeding Kelp Greenling (Hexagrammos decagrammus). The study objectives were to (1) estimate the proportional contributions of macroalgal and phytoplankton organic matter using carbon and nitrogen stable isotopes, (2) determine if macroalgal use affected consumer growth using annual growth rings in shells or otoliths, and (3) describe changes in organic matter use and growth during the Pacific Marine Heatwave (PMH; 2014–2016) in one consumer, mussels. Macroalgae were the major organic matter source (> 60%) to the diet for all three consumers. The relationships between macroalgal contribution and growth were neutral for both fish species and significantly positive for mussels. During the PMH, mussels had a drop (> 10%) in macroalgal contributions and grew 45% less than in other time periods. Simultaneously, the relationship between macroalgal contribution and mussel growth was strongest during the PMH, explaining 48% variation compared to 3–12% before or after the PMH. Collectively, the results suggest that macroalgae is likely more important to cold-temperate nearshore food webs than phytoplankton. Management actions aimed at conserving and expanding macroalgae are likely to benefit nearshore food webs under all climate scenarios and especially during marine heatwaves. 

Abstract Rapid changes in sea ice extent and changes in freshwater inputs from land are rapidly changing the nature of Arctic estuarine ecosystems. In the Beaufort Sea, these nearshore habitats are known for their high productivity and mix of marine resident and diadromous fishes that have great subsistence value for Indigenous communities. There is, however, a lack of information on the spatial variation among Arctic nearshore fish communities as related to environmental drivers. In summers of 2017–2019, we sampled fishes in four estuarine ecosystems to assess community composition and relate fish abundance to temperature, salinity, and wind conditions. We found fish communities were heterogeneous over larger spatial extents with rivers forming fresh estuarine plumes that supported diadromous species (e.g., broad whitefishCoregonus nasus), while lagoons with reduced freshwater input and higher salinities were associated with marine species (e.g., saffron codEleginus gracilis). West–East directional winds accounted for up to 66% of the community variation, indicating importance of the wind-driven balance between fresh and marine water masses. Salinity and temperature accounted for up to 54% and 37% of the variation among lagoon communities, respectively. Recent sea ice declines provide more opportunity for wind to influence oceanographic conditions and biological communities. Current subsistence practices, future commercial fishing opportunities, and on-going oil and gas activities benefit from a better understanding of current fish community distributions. This work provides important data on fish spatial distributions and community composition, providing a basis for fish community response to changing climatic conditions and anthropogenic use. 

Abstract Disentangling the influences of climate change from other stressors affecting the population dynamics of aquatic species is particularly pressing for northern latitude ecosystems, where climate‐driven warming is occurring faster than the global average. Chinook salmon (Oncorhynchus tshawytscha) in the Yukon‐Kuskokwim (YK) region occupy the northern extent of their species' range and are experiencing prolonged declines in abundance resulting in fisheries closures and impacts to the well‐being of Indigenous people and local communities. These declines have been associated with physical (e.g., temperature, streamflow) and biological (e.g., body size, competition) conditions, but uncertainty remains about the relative influence of these drivers on productivity across populations and how salmon–environment relationships vary across watersheds. To fill these knowledge gaps, we estimated the effects of marine and freshwater environmental indicators, body size, and indices of competition, on the productivity (adult returns‐per‐spawner) of 26 Chinook salmon populations in the YK region using a Bayesian hierarchical stock‐recruitment model. Across most populations, productivity declined with smaller spawner body size and sea surface temperatures that were colder in the winter and warmer in the summer during the first year at sea. Decreased productivity was also associated with above average fall maximum daily streamflow, increased sea ice cover prior to juvenile outmigration, and abundance of marine competitors, but the strength of these effects varied among populations. Maximum daily stream temperature during spawning migration had a nonlinear relationship with productivity, with reduced productivity in years when temperatures exceeded thresholds in main stem rivers. These results demonstrate for the first time that well‐documented declines in body size of YK Chinook salmon were associated with declining population productivity, while taking climate into account.