Contributions of Stocked and Naturally Reproduced Rainbow Trout to the Deerfield Reservoir Population

Deerfield Reservoir, located in the Black Hills of South Dakota, is supplemented annually with approximately 12,000 rainbow trout Onchorynchus mykiss. Recently, juvenile rainbow trout of unknown origin were collected in the primary tributary to Deerfield Reservoir, indicating that natural reproduction is occurring.  Our objectives were to 1) assess a method to distinguish between rainbow trout of wild and hatchery origin using analysis of scale growth patterns and 2) estimate the proportions of wild- and hatchery-origin fish that were sampled during the annual lake survey.  Logistic regression analysis was used to develop a predictive model based on scale measurements for known wild and known hatchery fish.  The model was highly significant (P=0.0001), rho squared was 0.166 (i.e., rho = 0.41).  The model correctly classified 60.4% of the input data.  Our predictive logistic regression equation was then used to calculate the probability that each of the 26 unknown (i.e., unclipped) fish in the standard lake survey were of hatchery or wild origin.  The calculated probability values for our 26 unknown fish ranged from 0.05 (relatively certain to be a wild fish) to 0.93 (relatively certain to be a hatchery fish); 13 were classified as wild fish and 13 as hatchery fish.