Poststocking Survival and Myxospore Evaluation of Whirling Disease‐Resistant Rainbow Trout Strains

Abstract

The ensuing spread of whirling disease following its initial introduction in the United States in the late 1950s caused drastic declines in populations of Rainbow Trout Oncorhynchus mykiss. The German Rainbow (GR) strain has strong resistance to Myxobolus cerebralis, the parasite causing whirling disease, but also has an extensive history as a domesticated food-production fish, resulting in traits that are presumed to be maladaptive for survival in the wild. The wild Harrison Lake Rainbow (HAR) strain exhibits reduced infection severity compared with other wild Rainbow Trout strains, but resistance is lower than the GR strain and was considered insufficient to meet management goals. Prior research suggests that crosses of GR with susceptible strains create offspring with Myxobolus cerebralis resistance and survival rates intermediate to the parental strains in lotic environments, but analysis on lentic environments with larger predators has been limited. We evaluated survival rates and infection severities of fingerling Rainbow Trout stocked into a lentic environment from four different strains with theoretically varying resistances to Myxobolus cerebralis: GR, HAR, and an F 1 [GR × HAR(50:50)] and F 2 [GR × HAR(75:25)] crossed strain between the GR and HAR strains. Using Seber dead-recovery models, we found that both the HAR and GR × HAR(50:50) strains had the highest survival rate, followed sequentially by the GR × HAR(75:25) and GR. Whirling disease resistance, as estimated from infection probability and severity, increased with increasing GR strain genetic background. To reduce spore loads and increase resistance in managed Rainbow Trout populations, we suggest stocking strains like the GR × HAR(50:50) that have respectable whirling disease resistance and high survival rates, enabling wild recruitment with increased cost-efficiency of stocking.