Rainbow Trout Spawning Characteristics and Relation to the Parasite Myxobolus cerebralis in the Missouri River, Montana

Abstract

The myxosporean parasite Myxobolus cerebralis is responsible for significant declines of rainbow trout (Oncorhynchus mykiss) populations in several western states, including Montana. Despite a high prevalence of the parasite in Montana’s Missouri River, there have been no apparent impacts to the rainbow trout population. This study examined long-term M. cerebralis monitoring data from the Missouri River system below Holter Dam and evaluated rainbow trout spawning characteristics such as migration distance, spawning location, site fidelity and amount of spawning in the Missouri River and tributaries over three years in an attempt to explain why the population has not declined in the presence of M. cerebralis. Over 13 years of monitoring, a mean 5.3 percent of rainbow trout handled during population estimates had clinical signs of M. cerebralis infection. In experiments using sentinel fish 53 percent of the spawning habitat had high severity of M. cerebralis, 38 percent had low to moderate severity, and 9 percent had no infection. Radio telemetry showed spawning locations varied among years and tagged fish lacked spawning site fidelity. The distance that radio-tagged rainbow trout migrated to spawning locations was significantly different among river sections of the study area. Twenty-eight percent of the spawning redds were found in the Missouri River and 72 percent in the tributaries. Relative to previous studies, we found less tributary spawning and an increase in Missouri River spawning, where M. cerebralis infection severity is lower. Our findings suggest that diverse spawning behaviors may contribute to rainbow trout population stability by spreading the risk of M. cerebralis impact over spawning locations that have a broad range of infection severity.