Dietary lysine requirement for juvenile Snake River Cutthroat Trout <i>Oncorhynchus virginalis behnkei</i>

ABSTRACT Objective Rocky Mountain Cutthroat Trout Oncorhynchus virginalis are valued in recreational fisheries. Efforts to culture these fish are impeded by the lack of diets formulated for Rocky Mountain Cutthroat Trout. The nutritional requirements of Rocky Mountain Cutthroat Trout are not completely known, leading to the use of Rainbow Trout O. mykiss diets that cause reduced growth performance in Rocky Mountain Cutthroat Trout. This study was conducted to determine the optimal dietary lysine requirement of cultured juvenile Snake River Cutthroat Trout O. virginalis behnkei. Methods Six diets were formulated using practical ingredients and purified amino acids to contain graded levels of lysine (1.81, 2.37, 2.90, 3.49, 4.03, and 4.61% on a dry matter basis). The diets were isonitrogenous, isolipidic, and isoenergetic on a dry matter basis. The essential amino acid profile of the diets matched the whole-body profile (except lysine) of Rocky Mountain Cutthroat Trout. Twenty-one 145-L tanks supplied with 15°C spring water in a flow-through system were each stocked with 60 fingerlings (average body weight = 15.7 ± 0.09 g) in a complete randomized design. Diets were hand fed to triplicate tanks of fish to apparent satiation three times daily for 10 weeks. Results Fish that were fed the lowest lysine level had significantly lower weight gain and daily growth index but had a higher feed conversion ratio than fish fed the other lysine levels. Fish that were fed diets with 2.90–4.61% lysine grew as well as fish fed the commercial control diet, with similar feed conversion ratios. Among several nonlinear regression models that were used to determine lysine requirements, the three-parameter logistic regression model provided the best fit to the daily growth index data (adjusted R2 = 0.804). Conclusions Optimal growth rates of juvenile Rocky Mountain Cutthroat Trout can be achieved with 2.37–2.90% total lysine in the diet.