Alaska Department of Fish and Game

Objective: Fish stocking is used to manage recreational fisheries and conserve native species; however, stocking success can be reduced if predators consume the stocked fish at high rates. We evaluated how different stable isotopes and predator tissue types influenced estimates of predator reliance on stocked fish. Methods: We measured delta C-13, delta N-15, and delta S-34 stable isotope values in Walleye Sander vitreus, stocked salmonids, and naturally reproduced prey fish before and after large salmonid stocking events in two Wyoming reservoirs. Shifts in the values for delta C-13, delta N-15, and delta S-34 stable isotopes across Walleye tissue types were assessed before, 1 month after, and 8 months after stocking using analyses of covariance, with length as a covariate. The contribution of stocked salmonids to Walleye diets was quantified using stable isotope mixing models in R (simmr) that were determined from Walleye muscle and liver tissues and combinations of stable isotopes. Results: Sulfur stable isotopes differentiated stocked salmonids and natural prey sources for over 8 months poststocking. Liver tissues documented Walleye consumption of stocked salmonoids that was not detected by analyses of muscle tissue alone. Including delta S-34 values increased the precision of the mixing-model outputs and led to more consistent results across modeling approaches that were based on different combinations of stable isotopes. Conclusion: Our results highlight the value of delta S-34 analyses and multiple tissues, with short- and long-term turnover rates, to identify the consumption of stocked fish by predators and describe the fates of stocked fish.

This comprehensive review examines pathogens affecting adult anadromous Pacific salmon (Oncorhynchus spp.) during their terminal freshwater migration and spawning across populations from California through Alaska, including Oregon, Washington, and British Columbia. We systematically reviewed selected pathogens based on their significance to adult salmon health or role in epizootiology, categorizing them by their impact on prespawn mortality (PSM), disease severity, and maternal or ‘egg-associated’ transmission risks to progeny. Our analysis encompasses macroparasites, microparasites, bacteria, and viruses affecting anadromous Pink (O. gorbuscha), Chum (O. keta), Coho (O. kisutch), Sockeye (O. nerka), and Chinook Salmon (O. tshawytscha) and Steelhead Trout (O. mykiss), integrating extensive literature analysis with direct field observations and case studies from representative geographic regions. Understanding pathogen impacts during the spawning life stage is crucial for salmon population sustainability, as the unique semelparous nature of Pacific salmon makes this terminal phase critical for reproductive success and the continuation of these ecologically, economically, and culturally vital species.

Climate change is shifting when animals breed [C. Parmesan, G. Yohe, Nature 421, 37-42 (2003) and S. J. Thackeray et al., Nature 535, 241-245 (2016)], but it is not clear why some populations keep pace with warming while others fall behind [L. D. Bailey et al., Nat. Commun. 13, 2112 (2022) and J. M. Samplonius et al., Glob. Change Biol. 24, 3780-3790 (2018)]. Differences could arise from variation in sensitivity to temperature [L. D. Bailey et al., Nat. Commun. 13, 2112 (2022)] or constraints on the ability to respond to temperature. Without knowing whether populations differ in sensitivity-or in their ability to act on that sensitivity-we cannot identify which are most at risk. Using 1,555 population-years from 123 populations of tree swallows (Tachycineta bicolor), we show that populations have similar sensitivity to local temperature, advancing breeding by about one day per degree of warming. However, northern populations face tighter time constraints and greater exposure to recent warming. Northern populations have advanced laying dates the most, but still experience stronger selection for earlier breeding, especially in warm years; they have also declined most in breeding abundance. These findings suggest that vulnerability to climate change can arise not just from different sensitivity to warming, but from when and where populations can respond effectively. By disentangling sensitivity from timing constraints, our results are consistent with a general mechanism by which even uniformly responsive species can show uneven impacts of climate change across their ranges.

Abstract Prey availability is among the most influential and highly variable determinants of fish growth and freshwater habitat carrying capacity, yet it remains understudied compared to physical habitat variables (Ouellet et al., 2025; Rosenfeld et al., 2014; Weber et al., 2017). We often lack a clear understanding of how much food is available to fishes, how it varies spatially and temporally, and how it influences responses to restoration (Ouellet et al., 2025; Rossi et al., 2024; Wipfli et al., 2010). Drift invertebrates—the primary food source for juvenile salmonids and other drift‐foraging fishes—play a pivotal role in these dynamics. To better understand the spatiotemporal variability of drift invertebrate abundance and biomass across the freshwater range of drift‐feeding salmonids in western North America, we compiled the DRift Invertebrates For salmonids in River Systems (DRIFteRS) dataset. The dataset encompasses 6125 samples of drift invertebrates, and, for a subset of drift samples, associated benthic invertebrate density data, collected from 1360 reaches on 459 unique rivers and streams spanning 55 river basins considered hydrologically independent (i.e., not nested within the same larger watershed) across British Columbia, Canada, and the U.S. states of Alaska, Arizona, California, Colorado, Idaho, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming. Sample sites represent a diverse array of river and stream habitats (e.g., headwater, mainstem, side channel), in watersheds with diverse land uses (e.g., urban, wilderness, agricultural), and disturbance histories (e.g., fire, restoration). Collected between 1997 and 2024, the data span the full calendar year and capture daily and seasonal patterns in drift abundance and biomass densities. When paired with water quality and quantity data as well as remotely sensed environmental landscape data, such as land use/land cover, climate, and disturbance history, channel morphology, and riparian vegetation composition, the DRIFteRS dataset can aid in identifying key drivers of drift invertebrate densities and mean body size and support predictive modeling in unsampled locations and times. The dataset may also be used to analyze aquatic‐terrestrial resource flows, derive prey‐encounter rates and profitability (mean prey size), and inform broader investigations of sit‐and‐wait foraging ecology, especially when paired with data on drift‐foraging predators. For salmonid‐focused applications, the dataset can be integrated into habitat evaluation models, including bioenergetic (e.g., Naman et al., 2019) and life cycle models (e.g., Beechie et al., 2023), to improve estimates of habitat capacity and population dynamics for river‐ and stream‐rearing salmonids. Understanding prey availability dynamics is increasingly important, because rising water temperatures increase salmonid metabolic demands (Crozier et al., 2010). Flow regime transitions (i.e., snow or glacier dominated to rain dominated; Beechie et al., 2013), wildfire frequency and intensity (Hessburg et al., 2021), as well as plant community and phenology shifts (Cleland et al., 2007; Franklin et al., 2016) are all predicted to change with rising temperatures and are potential drivers of terrestrial and aquatic invertebrate prey quality and availability in lotic systems. These insights can ultimately inform restoration prioritization and design, helping managers consider food resource implications when evaluating restoration priorities and match habitat improvement to food supply. The data are released under the Creative Commons Attribution 4.0 International license and may be reused with attribution.

Conventional thought holds that in formerly glaciated areas straying of anadromous fish from nearby unglaciated areas established contemporary salmon populations. An additional explanation for patterns of salmon life-history diversity and population structure derives from isolation of populations in proglacial lakes. We evaluate evidence for these potentially complementary hypotheses in chum salmon from two previously glaciated North American regions: the southern Alaska Peninsula/upper Cook Inlet and the Salish Sea of northwestern Washington and southern British Columbia. Some chum salmon populations in the southern Alaska Peninsula are genetic outliers compared with other nearby populations, while Salish Sea chum salmon populations have greater region-wide genetic divergence and lower gene diversity. Within-population genetic diversity and among-population divergence in both study areas support a hypothesis of salmon persistence relying on cryptic isolation and freshwater-resident (trout-like) life histories in proglacial lakes. We find that ice age adaptation of salmon to a trout life history helps explain aspects of contemporary population structure and life-history diversity.