Elevated temperature is considered an important factor in the decline of the threatened bull trout Salvelinus confluentus, but the thermal requirements of this species have not been defined. We used the acclimated chronic exposure (ACE) method to assess the upper thermal limits and growth optima of bull trout fed daily to satiation over test temperatures ranging from 8 degreesC to 28 degreesC during 60-d trials. Survival of age-0 bull trout was at least 98% at 8, 10, 12 14, 16, and 18 degreesC, but 0% at 22, 24, 26. and 28 degreesC after 60 d. The predicted ultimate upper incipient lethal temperature for these trout was 20.9 degreesC. Peak growth, as estimated by regression analysis, occurred at 13.2 degreesC (95% confidence interval, 10.9-15.4 degreesC). Feed consumption declined significantly (P < 0.001) at temperatures greater than 16 degreesC, and fish held at temperatures of 22 degreesC and above did not feed. Feed, lipid, and protein efficiencies were similar at 8-18 degreesC but declined significantly (P < 0.001) at 20 degreesC. Our results corroborate field investigations suggesting that bull trout have among the lowest upper thermal limits and growth optima of North American salmonids. The slower acclimation times and long-term duration of the ACE method resulted in a more realistic measure of thermal tolerance in natural situations than would have been obtained with traditional methods and afforded sufficient time for sublethal differences in growth rate, feed consumption, and feed efficiency to become apparent.

Benthic macroinvertebrates with elevated concentrations of metals were collected from the Coeur d'Alene (CDA) River, Idaho, pasteurized, and fed to cutthroat trout Oncorhynchus clarki in the laboratory from start of feeding until 90 d posthatch. Invertebrates were collected from two sites known to contain elevated concentrations of metals: near Pinehurst in the South Fork of the CDA River and at Cataldo, approximately 5 km below the confluence of the South Fork and the North Fork. Invertebrates collected from a relatively clean site in the North Fork were used as a reference diet. We performed measurements of fish health that indicate reduced fitness of fish fed the South Fork and Cataldo diets. Effects measured were reduced feeding activity, increased number of macrophage aggregates and hyperplasia of cells in the kidney, degeneration of mucosal epithelium in the pyloric caecae, and metallothionein induction. These effects would likely reduce growth and survival of fish in the wild. Vacuolization of glial cells were also observed in fish fed the Cataldo diet. Metals in the water often exacerbated the histological effects observed. Although the invertebrates collected near Cataldo had lower concentrations of arsenic (As), cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn) than the invertebrates from the South Fork, fish fed the Cataldo diet had equally high or higher concentrations of all metals except As by day 44. The Cataldo diet also caused the most deleterious effects on survival and growth. These findings are especially important for early life stage fish, whose diet consists wholly of benthic macroinvertebrates. Therefore, fish feeding on invertebrates in the CDA River below the Bunker Hill smelting complex are at risk of reduced fitness.