We found insufficient evidence to conclude that seminatural treatment (SNT; i.e., rearing in camouflage-painted raceways with surface and underwater structures and underwater feeders) of juvenile Chinook salmon Oncorhynchus tshawytscha resulted in higher survival indices than did optimum conventional treatment (OCT; i.e., rearing in concrete raceways with surface feeding) for the specific treatments and environmental conditions tested. We reared spring Chinook salmon from fry to smolt in paired raceways under the SNT and OCT rearing treatments for five consecutive years. For four to nine SNT and OCT raceway pairs annually, we used passive integrated transponder, coded wire, and visual implant elastomer tags to compare survival indices for juvenile fish from release at three different acclimation sites 340-400 km downstream to passage at McNary Dam on the Columbia River, and for adults from release to adult return to Roza Dam in the upper Yakima basin. The observed differences in juvenile and adult survival between the SNT and OCT fish were either statistically insignificant, conflicting in their statistical significance, or explained by significant differences in the presence of the causative agents of bacterial kidney disease in juvenile fish at release.

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.

The relationship between pH and acute toxicity of free cyanide (i.e. HCN + CN−) and dissolved sulfide (i.e. H2S + HS− + S2−) to the fathead minnow (Pimephales promelas) at 20 °C was determined for pH values ranging from about 6.8 to 9.3 and 6.5 to 8.7, respectively. The 96-h median lethal concentrations (LC50s) of free cyanide and molecular HCN were little different and fairly constant within the pH range 6.8–8.3. Beyond this, to pH 9.3 the values diverged markedly, with the free cyanide LC50s increasing and the HCN LC50s decreasing. Except for some increase with rise of pH from about 6.5 to 7.1, the 96-h LC50 concentrations of molecular H2S decreased linearly and by approximately fourfold as the pH increased from 7.1 to 8.7. The 96-h LC50 concentrations of dissolved sulfide in corresponding equally toxic solutions increased logarithmically as pH increased from 6.5 to 8.7. Acute toxicities to fathead minnows of free cyanide and dissolved sulfide do not depend entirely on the concentration of ambient molecular HCN or H2S. Change in toxicity of the molecular forms with pH was evaluated by two gill-permeability theories. The likelihood of certain chemical changes occurring at the gill surface or the possible penetration of the gill by both molecular and anionic forms present in the test solutions at different pH's was evaluated. Increased apparent toxicity of molecular HCN or H2S with elevated test pH is believed to result from the CN− and HS− anions penetrating the gill epithelium, though less readily than do the molecular forms, and enhancing the toxicities of these solutions as the pH increases. Key words: toxicology, hydrogen ion concentration, bioassays, lethal limits, gill-permeability theories