Effects of Temperature on the Growth, Food Consumption, and Thermal Tolerance of Age‐0 Nimbus‐Strain Steelhead

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Abstract
Although steelhead (anadromous rainbow trout Oncorhynchus mykiss) have the widest natural distribution among Pacific Coast salmonids, their populations have declined because of habitat loss, habitat alteration, and overexploitation. Because juvenile steelhead can be exposed to high summer water temperatures in streams and in California hatcheries that rely on surface water sources, we compared the food consumption, growth, and temperature tolerance of Nimbus‐strain steelhead (an introgressed breeding stock in the American River) held at 11, 15, and 19°C in replicated laboratory experiments. Although food consumption rate showed no statistically distinguishable difference among temperatures, the growth rate was higher at 19°C than at 11°C or 15°C, providing evidence that food conversion efficiency in juvenile steelhead is higher at the warmer temperature. Age‐0 steelhead showed a clear thermal acclimation effect, as their critical thermal maximum increased from 27.5°C for 11°C‐acclimated fish to 29.6°C for fish acclimated to 19°C. We conclude that fish culturists could use the higher growth rate and food conversion efficiency at 19°C to increase the size of their age‐0 steelhead. We also suggest that it may be possible to confer greater thermal tolerance on age‐0 steelhead by rearing them at elevated temperatures for a short while, but we caution that it would still be necessary to ultimately rear the fish in cooler water to ensure successful smoltification.