Enterocytozoon schreckii n. sp. Infects the Enterocytes of Adult Chinook Salmon ( Oncorhynchus tshawytscha ) and May Be a Sentinel of Immunosenescence

In this work, we describe a new microsporidian species that infects the enterocytes of Chinook salmon. This novel pathogen is closely related to Enterocytozoon bieneusi , an opportunistic pathogen commonly found in AIDS patients and other severely immunocompromised humans.

A non-functional copy of the salmonid sex determining gene (sdY) is responsible for the “apparent” XY females in Chinook salmon, Oncorhynchus tshawytscha

Many salmonids have a male heterogametic (XX/XY) sex determination system, and they are supposed to have a conserved master sex determining gene (sdY), that interacts at the protein level with Foxl2 leading to the blockage of the synergistic induction of Foxl2 and Nr5a1 of the cyp19a1a promoter. However, this hypothesis of a conserved master sex determining role of sdY in salmonids is challenged by a few exceptions, one of them being the presence of naturally occurring “apparent” XY Chinook salmon, Oncorhynchus tshawytscha, females. Here we show that some XY Chinook salmon females have a sdY gene (sdY-N183), with one missense mutation leading to a substitution of a conserved isoleucine to an asparagine (I183N). In contrast, Chinook salmon males have both a non-mutated sdY-I183 gene and the missense mutation sdY-N183 gene. The 3D model of SdY-I183N predicts that the I183N hydrophobic to hydrophilic amino acid change leads to a modification of the SdY β-sandwich structure. Using in vitro cell transfection assays we found that SdY-I183N, like the wildtype SdY, is preferentially localized in the cytoplasm. However, compared to wildtype SdY, SdY-I183N is more prone to degradation, its nuclear translocation by Foxl2 is reduced and SdY-I183N is unable to significantly repress the synergistic Foxl2/Nr5a1 induction of the cyp19a1a promoter. Altogether our results suggest that the sdY-N183 gene of XY Chinook females is non-functional and that SdY-I183N is no longer able to promote testicular differentiation by impairing the synthesis of estrogens in the early differentiating gonads of wild Chinook salmon XY females.

Effects of In-Channel Structure on Chinook Salmon Spawning Habitat and Embryo Production

Adult salmonids are frequently observed building redds adjacent to in-channel structure, including boulders and large woody debris. These areas are thought to be preferentially selected for a variety of reasons, including energy and/or predation refugia for spawners, and increased hyporheic exchange for incubating embryos. This research sought to quantify in-channel structure effects on local hydraulics and hyporheic flow and provide a mechanistic link between these changes and the survival, development, and growth of Chinook salmon Oncorhynchus tshawytscha embryos. Data were collected in an eight-kilometer reach, on the regulated lower Mokelumne River, in the California Central Valley. Nine paired sites, consisting of an area containing in-channel structure paired with an adjacent area lacking in-channel structure, were evaluated. Results indicated that in-channel structure disrupts surface water velocity patterns, creating pressure differences that significantly increase vertical hydraulic gradients within the subsurface. Overall, in-channel structure did not significantly increase survival, development, and growth of Chinook salmon embryos. However, at several low gradient downstream sites containing in-channel structure, embryo survival, development, and growth were significantly higher relative to paired sites lacking such features. Preliminary data indicate that adding or maintaining in-channel structure, including woody material, in suboptimal spawning reaches improves the incubation environment for salmonid embryos in regulated reaches of a lowland stream. More research examining temporal variation and a full range of incubation depths is needed to further assess these findings.

Spatially overlapping salmon species have varied population response to early life history mortality from increased peak flows

In the Pacific Northwest, USA, climate change is expected to result in a shift in average hydrologic conditions and increase variability. The relative vulnerabilities to peak flow changes among salmonid species within the same basin have not been widely evaluated. We assessed the impacts of predicted increases in peak flows on four salmonid populations in the Chehalis River basin. Coupling observations of peak flows, emissions projections, and multi-stage Beverton–Holt matrix-type life cycle models, we ran 100-year simulations of spawner abundance under baseline, mid-century, and late-century climate change scenarios. Coho (Oncorhynchus kisutch) and spring Chinook salmon (Oncorhynchus tshawytscha) shared the highest projected increase in interannual variability (SD = ±15%). Spring Chinook salmon had the greatest reduction in median spawner abundance (–13% to –15%), followed by coho and fall Chinook salmon (–7% to –9%), then steelhead (Oncorhynchus mykiss) (–4%). Our results show that interspecies and life history variability within a single basin is important to consider. Species with diverse age structures are partially buffered from population variability, which may increase population resilience to climate change.

Escape From the Heat: Thermal Stratification in a Well-Mixed Estuary and Implications for Fish Species Facing a Changing Climate

Climate change may cause organisms to seek thermal refuge from rising temperatures, either by shifting their ranges or seeking microrefugia within their existing ranges. We evaluate the potential for thermal stratification to provide refuge for two fish species in the San Francisco Estuary (SFE): Chinook Salmon (Oncorhynchus tshawytscha) and Delta Smelt (Hypomesus transpacificus). We compiled water temperature data from multiple monitoring programs to evaluate spatial, daily, hourly, intra-annual, and inter-annual trends in stratification using generalized additive models. We used our data and models to predict the locations and periods of time that the bottom of the water column could function as thermal refuge for salmon and smelt. Periods in which the bottom was cooler than surface primarily occurred during the peak of summer and during the afternoons, with more prominent stratification during warmer years. Although the SFE is often exceedingly warm for fish species and well-mixed overall, we identified potential for thermal refugia in a long and deep terminal channel for Delta Smelt, and in the periods bordering summer for Chinook Salmon. Thermal stratification may increase as the climate warms, and pockets of cooler water at depth, though limited, may become more important for at-risk fishes in the future.

Southern resident killer whales encounter higher prey densities than northern resident killer whales during summer

The decline of southern resident killer whales (Orcinus orca) may be due to a shortage of prey, but there is little data to test this hypothesis. We compared the availability of prey (Chinook salmon, Oncorhynchus tshawytscha) sought by southern residents in Juan de Fuca Strait during summer with the abundance and distribution of Chinook available to the much larger and growing population of northern resident killer whales feeding in Johnstone Strait. We used ship-based multifrequency echosounders to identify differences in prey fields that may explain the dynamics of these two killer whale populations. Contrary to expectations, we found that both killer whale habitats had patchy distributions of prey that did not differ in their frequencies of occurrence, nor in the size compositions of individual fish. However, the density of fish within each patch was 4–6 times higher in the southern resident killer whale habitat. These findings do not support the hypothesis that southern resident killer whales are experiencing a prey shortage in the Salish Sea during summer and suggest a combination of other factors is affecting overall foraging success.