DIETARY LYSINE REQUIREMENT OF JUVENILE SNAKE RIVER CUTTHROAT TROUT
Cutthroat trout Oncorhynchus clarkii are native to the western United States. Currently, many subspecies of cutthroat trout (CTT) are raised in the state and federal hatcheries and private farms for stock enhancement programs and recreational fishing. In an effort to develop a suitable diet for raising CTT as fast as rainbow trout, a study was conducted to determine the optimal dietary lysine requirement of juvenile Snake River cutthroat trout (O. clarkii behnkei).
Six diets using practical ingredients and purified amino acids were produced to contain graded levels of lysine (1.81%, 2.37%, 2.90%, 3.49%, 4.03% and 4.61%, dry-matter basis). The diets were isonitrogenous (46% crude protein), isolipidic (18% crude fat) and isoenergetic (22 MJ/kg) on dry-matter basis. Essential amino acid profile of diets matched that (except lysine) of the whole-body CTT. A commercial trout diet was used as the control. Groups of 60 fingerlings (average body weight, 15.7 g) were stocked into 21 145-L tanks each supplied with 15 °C spring water in a flow-through system. Each diet was hand-fed to randomly assigned triplicate tanks of fish to apparent satiation twice daily for 10 weeks. At the end of the growth trial, fish samples were collected for proximate and amino acid analyses, and condition factor. Remaining fish were used for determining lysine digestibility in a diet pooled from the experimental diets.
Fish fed the lowest lysine level had significantly lower weight gain and daily growth index but higher feed conversion ratio (FCR) than fish fed other lysine levels (P<0.05). Fish gained 33.2 g-45.1 g with FCRs of 1.29 to 1.67. Fish fed diets with 2.90%-4.61% lysine grew as well as the fish fed the commercial control diet with similar FCRs. Fish fed the two lowest levels of lysine appeared to have lower whole-body crude protein but higher crude fat levels than fish fed the other diets. Among several non-linear regression models, the 3-parameter logistic regression model was the best fit to the daily growth index data (adjusted-R2 =0.84, Figure 1). Dietary lysine requirement of juvenile cutthroat trout at 95% of the asymptote was 2.39%.