The evaluation of pellet type-preference and dietary soy sensitivity in Snake River cutthroat trout (Oncorhynchus clarkia)
Many sub-species of cutthroat trout are considered threatened or endangered in the United States. This has created an emphasis to transition from Rainbow trout to this lesser domesticated salmonid species; however, rearing cutthroat trout on commercially available rainbow trout diets has resulted in decreased performance in the hatchery. Research in other species has shown characteristics such as size, shape, color, and movement all affect interest in feed, while taste and texture influence consumption. A recent trend of decreasing fishmeal and increasing plant protein levels in salmonid diets has raised the question of how less domesticated species tolerate alternative ingredients. Therefore, the objectives of this study were to examine whether pellet-type preference affects the growth of juvenile cutthroat trout, and to examine how inclusion of soy-based protein sources affect the growth of this species, in hopes to develop a diet that improves the success of cutthroat trout hatchery culture. Two feeding trials were conducted. In the first, Westslope and Snake River cutthroat trout were fed a single experimental formulation manufactured to display four different pellet characteristics (floating, sinking, semi-moist, and flake). Consumption, weight gain and survival were compared. Results demonstrated a significant effect based on pellet type (P=0.008) but not stocking density (P=0.0950) on weight gain. Fish fed the flake grew significantly less than those fed other dietary treatments, with a 74% percent increase in weight gain (P=.0001). To address the effects of inclusion of dietary soybean meal and soy protein concentrate on growth efficiency, Snake River cutthroat trout were fed a practical-type formulation with 0, 5, 10, 15 or 30 % inclusion of dietary soybean meal or soy protein concentrate. Consumption, weight gain, nutrient retention and survival were compared. Final fish weight was altered by dietary soy inclusion level (P=0.0001) but not type (P=0.779). Snake River cutthroat fed the highest level of soy protein inclusion (30%) were significantly larger than fish fed other diets (P=0.0001). Additional research is needed to address the effects of soy on gut health and validate these results. Results from these studies can help hatchery managers choose appropriate feed types for culturing cutthroat trout in hatcheries.