Spawning Behavior of Mountain Whitefish and Co‐occurrence of Myxobolus cerebralis in the Blackfoot River Basin, Montana

Abstract

AbstractTo assess the exposure of Blackfoot River mountain whitefish Prosopium williamsoni to the exotic parasite Myxobolus cerebralis, the cause of salmonid whirling disease, we investigated the spawning behavior of 49 adult mountain whitefish and their overlap with M. cerebralis within the Blackfoot River basin, Montana. A majority of the mountain whitefish radio‐tagged in the Blackfoot River migrated upstream (range, 0.1–79.0 km) to spawning sites located primarily in the main stem of the Blackfoot River. Spawning ranged from 31 October in the lower river to 9 November in the upper river and occurred across a range of substrate and channel types. Despite later spawning in the upper river, eggs hatched earlier under the warming influence of groundwater inflows. Here, a majority of wild mountain whitefish fry (65%) tested positive for M. cerebralis infection during the immediate posthatch period of mid‐April. Conversely, mountain whitefish fry from the lower river, downstream of the groundwater influence, showed no detectable infection. June exposure trials using surrogate rainbow trout Oncorhynchus mykiss in nine tributaries supporting mountain whitefish showed M. cerebralis infection rates ranging from 0% to 100% as well as a pattern of high triactinomyxon (TAM) exposure throughout the main‐stem Blackfoot River. For mountain whitefish, the co‐occurrence with M. cerebralis varied spatially across the basin and temporally within the main‐stem Blackfoot River at the most vulnerable early life stages. This variability appears to buffer age‐0 mountain whitefish from infectious conditions across large areas of the basin. However, continuous TAM release from groundwater‐influenced environments coinciding with mountain whitefish hatch and early rearing may impose pathogenic conditions on mountain whitefish in the upper Blackfoot River.