Historical returns of coho salmon Oncorhynchus kisutch to the Yakima River basin were estimated to range from 45,000 to 100,000 fish annually but declined to zero by the 1980s after decades of overexploitation of fishery, water, and habitat resources. In 1996, the Yakama Nation and cooperators initiated a project to determine the feasibility of reestablishing a naturally spawning coho salmon population in the Yakima River. The project explored the feasibility of successful coho salmon recolonization in the Yakima River by introducing stocks that had been reared in hatcheries for multiple generations. After 10–20 years of outplanting, we compared data for adult returns of known natural origin (i.e., returns from parents that spawned in the wild) and returns from hatchery releases. We found that fish of natural origin returned at a significantly larger size than those of hatchery origin. The mean egg mass and mean egg size of natural‐origin females were greater than those of hatchery‐origin females, but the differences were statistically significant for only one of three sample years. Natural‐origin adults returned 2–9 d later and spawned 5 d later than their hatchery‐origin counterparts. Preliminary indices of smolt‐to‐adult survival for natural‐origin fish were 3.5–17.0 times the survival indices of hatchery‐origin fish. The number of returns to the historical spawning habitats in upriver areas generally increased. Spawning surveys demonstrated the existence of robust and sustainable spawning aggregates in various locations in the basin. Hatchery releases from the local brood source (Yakima River returns) had significantly higher smolt‐to‐smolt survival than releases from out‐of‐basin (non‐Yakima River) hatchery broodstock, but some of these observed differences in survival may be partially attributable to differences in smolt size. We concluded that hatchery‐origin coho salmon with a legacy of as many as 10–30 generations of hatchery influence demonstrated an ability to reestablish themselves in the Yakima River (i.e, as a naturalized, nonnative population) after as few as 3–5 generations of outplanting in the wild.
Native inland populations of rainbow trout Oncorhynchus mykiss gairdneri, particularly resident populations, often hybridize with introduced populations of the widely cultured coastal form of the species, O. m. irideus. The inland and coastal subspecies differ genetically from each other by allozyme polymorphisms at the lactate dehydrogenase (LDH‐B2*) and superoxide dismutase loci (sSOD‐1*) that can be detected using protein electrophoresis. Fewer laboratories, however, are now using allozyme technology, and most genetic studies from wild organisms are now being conducted using DNA rather than protein analyses. We have identified the single‐nucleotide polymorphism (SNP) differences responsible for the protein variations by sequencing the complementary DNA for the LDH‐B2* and sSOD‐1* genes in a large number of individuals whose genotypes were also determined by protein electrophoresis. The genetic differences causing the allozyme polymorphisms have been converted into SNP allelic discrimination assays. This should allow simple, efficient tests to be conducted in a large number of laboratories as an aid to assessing the level of hybridization between inland and coastal rainbow trout. It should also allow DNA studies to be more directly related to previous allozyme studies. High variability was also found at other sites in the superoxide dismutase gene.
The construction and operation of dams represents one of the most significant anthropogenic impacts to the aquatic environment of freshwater ecosystems and includes changes in flow, temperature, water chemistry, sedimentation, and nutrient delivery. Despite the substantial changes caused by dams, we have a limited understanding of how dams influence important rate functions of fish, including growth rates. This study measured the growth rates of Rainbow Trout Oncorhynchus mykiss from successive captures of individually marked fish over seven annual increments within four river sections downstream of Libby Dam on the Kootenai River, Montana. We modeled the influence of hydropower‐related environmental variables on Rainbow Trout length and weight growth rates using linear mixed‐effects models. The top models predicting annual length and weight growth rates contained measures of water chemistry (ratio of total N to total P [N:P]) during the growing season, winter substrate coverage by the diatom Didymosphenia geminata, and an interactive term between winter D. geminata coverage and fish size at tagging. Winter D. geminata coverage and N:P were negatively correlated with annual growth rates, but the interactive term indicates that the influence of winter D. geminata coverage disproportionally affects smaller fish more than larger fish. We hypothesize that N:P and D. geminata are influencing Rainbow Trout growth rates through lower‐trophic‐level impacts. Top Rainbow Trout length and weight growth models explained 94.6% and 92.2%, respectively, of the annual variability in growth rates, of which 87.7% and 76.2%, respectively, were attributable to fixed effects. An experimental nutrient addition study and robust trophic monitoring efforts in the Kootenai River downstream of Libby Dam would be an effective means of independent corroboration of these study results. If successful, nutrient addition may be an effective management strategy to improve annual Rainbow Trout growth rates, mitigating for the nutrient retention occurring in the large reservoir upstream of Libby Dam.
Stream habitat changes affecting primary consumers often indirectly impact secondary consumers such as fishes. Blooms of the benthic algae Didymosphenia geminata (Didymo) are known to affect stream macroinvertebrates, but the potential indirect trophic impacts on fish consumers are poorly understood. In streams of the Kootenai River basin, we quantified the diet, condition, and growth rate of species of trout, char, and sculpin. In 2018, macroinvertebrate taxa composition was different between a stream with Didymo and a stream without, but trout diets, energy demand, and growth rates were similar. Trout abundance was higher in the stream with Didymo, but the amount of drifting invertebrates was higher in the stream without. In 2019, we surveyed 28 streams with a gradient of coverage. Didymo abundance was correlated only with the percentage of aquatic invertebrates in trout diets and was not related to diets of char or sculpin or condition of any species. Thus, we found no evidence for a trophic link between Didymo blooms and the condition or growth of trout, char, or sculpin in mountainous headwater streams.
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