The shift in commercial diet formulations for Rainbow Trout Oncorhynchus mykiss toward formulations that include more plant ingredients, specifically an increased inclusion of soy products, may have negative implications for less‐domesticated trout species fed these modern diets. Therefore, the objective of this study was to examine the effects of increasing dietary soybean meal and soy protein concentrate inclusion on growth efficiency and intestinal health of Cutthroat Trout O. clarkii. To achieve this objective, a feeding trial was conducted with juvenile Snake River Cutthroat Trout O. clarkii behnkei fed a practical type formulation that included 0, 5, 10, 15, or 30% dietary soybean meal or soy protein concentrate. Feed and consumption, weight gain, proximate composition, intestinal health, and survival were compared. Juvenile Snake River Cutthroat Trout (initial weight, 28.1 ± 1.0 g/fish [mean ± SD]) were stocked at 20 fish per tank in 200‐L tanks with three replicate tanks per diet. Fish were fed their respective diets for 10 weeks. Final fish weight was affected by dietary soy inclusion level (P = 0.0001) but not type (P = 0.7790), and no interaction was observed (P = 0.6019). Snake River Cutthroat Trout fed the diets with the highest level of soy protein inclusion (30%) were significantly larger than fish fed all other diets and had a final average fish weight of greater than 130 g (P = 0.0001). Feed conversion ratios were higher in fish fed diets with 0% and 5% soy inclusion than in fish fed the 10% or 30% soy inclusion diets (P = 0.0044). No significant effect of soy inclusion level (P = 0.0825) on feed intake was observed. Increased inflammation and decreased vacuolization, however, was observed in histological samples of the intestine of Cutthroat Trout fed the 30% soybean meal and soy protein concentrate diets. Additional research is necessary to determine whether the intestinal pathology observed can predispose Cutthroat Trout to pathogenic disease and/or negatively affect growth with extended feeding.

Hatchery-cultured cutthroat trout Oncorhynchus clarkii fed some commercially available rainbow trout feeds display slow growth and increased mortality. Feed characteristics such as buoyancy and texture alter feed acceptance in some fish species, but their effects have not been adequately addressed in cutthroat trout. Therefore, the objective of this study was to examine whether feed structure and behavior preferences explain the decreased hatchery performance of juvenile cutthroat trout. To achieve this, we conducted two feeding trials in which we fed Westslope cutthroat trout O. clarkii lewisi and Snake River finespotted cutthroat trout O. clarkii behnkei a single diet formulation manufactured to display four different characteristics (floating, sinking, semimoist pellets, or a flake feed) and compared consumption, weight gain, and survival. In the first feeding trial, we stocked Westslope cutthroat trout (initial weight 11.3 g ± 0.5 g) at 20 fish/tank. We used two different sizes of tanks, with four replicate small tanks (54-L) and two replicate large tanks (96-L) per feed type. Results of the first trial demonstrated a significant effect of feed type but not tank size on weight gain of Westslope cutthroat trout with no interaction. Westslope cutthroat trout fed the flake feed gained less weight than did fish fed any of the other feed types. Feed conversion ratio was affected by both feed type and tank size with no interaction. In a second feeding trial, Snake River cutthroat trout (initial weight 19.5 g ± 0.5 g) were stocked at 20 fish/tank in 96-L tanks with four replicate tanks per feed type. Results of the second trial demonstrated that Snake River cutthroat trout fed the flake feed grew less, had higher feed conversion ratio, elevated hepatosomatic index, and reduced muscle ratio compared with fish fed the other feeds. Results demonstrate that flake feeds are not adequate for cutthroat trout at this life stage. However, additional research is needed to address other culture-related limitations because only minor differences between fish fed other feed types were detected.