VARIATION IN CONDITION OF RAINBOW-TROUT IN RELATION TO FOOD, TEMPERATURE, AND INDIVIDUAL LENGTH IN THE GREEN-RIVER, UTAH

We examined how condition (weight at length) of rainbow trout Oncorhyn chus mykiss varied in relation to availability of drifting invertebrat es and temperature at two locations over four seasons in the Green Riv er, Utah. Food availability (daytime drift density) varied more than 1 7-fold across sites and seasons, and rainbow trout experienced an 11 d egrees C range in mean monthly temperature. Both rainbow trout gut ful lness and condition increased as joint, nonlinear functions of increas ing food availability and increasing temperature. Variation in conditi on decreased with fish size, although condition of intermediate sized fish was most strongly related to variation in food and temperature. O bserved relationships between rainbow trout condition, drift abundance , and temperature were qualitatively consistent with bioenergetic mode ls that predict fish growth should vary as a joint function of food an d temperature. However, the inferred temperature optima for rainbow tr out seemed to be significantly higher than predicted. Our results supp ort a growing body of evidence that stream trout may be frequently foo d-limited in nature.