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

Abstract

To 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.

Reproductive Ecology, Spawning Behavior, and Juvenile Distribution of Mountain Whitefish in the Madison River, Montana

“…Prespawning movements in the Madison River were similar in distance and speed to Mountain Whitefish movements reported in other rivers (Liebelt 1970;Pettit and Wallace 1975;Pierce et al 2012;Benjamin et al 2014), but net direction of movements differed. Most Mountain Whitefish in the Madison River moved varying distances (1.0-68.1 km) downstream during the prespawning and spawning periods, and longer movements were observed for fish moving downstream than for fish moving upstream.…”

“…Most Mountain Whitefish in the Madison River moved varying distances (1.0-68.1 km) downstream during the prespawning and spawning periods, and longer movements were observed for fish moving downstream than for fish moving upstream. Mountain Whitefish moved similar distances (<1-80 km) in other rivers (Pettit and Wallace 1975;Pierce et al 2012;Benjamin et al 2014). However, migratory Mountain Whitefish typically moved upstream to main-stem or tributary sites before spawning in the Methow River, Washington (Benjamin et al 2014), Yellowstone River, Montana (Liebelt 1970), Blackfoot River, Montana (Pierce et al 2012), and Clearwater River, Idaho (Pettit and Wallace 1975).…”

“…Macroscale habitat features (e.g., braiding and valley width) appear to be the best explanation for spawning site locations, because we did not find evidence of adult selection for mesoscale habitat features (e.g., depth, substrate, water velocity) or any relationship between spawning sites and winter water temperatures. We are not aware of other studies that have statistically tested spawning habitat selection, but observations of Mountain Whitefish spawning in a wide variety of depths, substrates, and velocities (Brown 1952;Stalnaker and Gresswell 1974;Thompson and Davies 1976;Pierce et al 2012) suggest that spawning Mountain Whitefish do not show strong selection for mesoscale habitat features in other rivers. Age-0 Mountain Whitefish were associated with slow-velocity, silt-laden habitats (e.g., backwaters, eddies, beaver ponds), and these habitats were most common in braided reaches of the Madison River.…”

“…The exact mechanisms for declines are often unknown. Studies have documented mechanisms of Mountain Whitefish mortality including whirling disease (Schisler 2010;Pierce et al 2012), high temperatures (Boyd 2008;Brinkman et al 2013), pollutants (Brinkman et al 2008;Quinn et al 2010), decreased discharge, and entrainment (Kennedy 2009). However, little is known about the effects of these stressors on populations (but see IDFG 2007; Kennedy 2009).…”

Gear Comparison for Sampling Age‐0 Mountain Whitefish in the Madison River, Montana

“…Understanding the gear or gears that are most efficient (i.e., highest CPUE and lowest variability) at sampling age-0 Mountain Whitefish is necessary to design costeffective and informative studies. Age-0 Mountain Whitefish have been sampled using seines (Brown 1952), backpack electrofishing (Stalnaker and Gresswell 1974), and dip nets (Pettit and Wallace 1975;Pierce et al 2012). These methods successfully collected fish for growth, diet, and disease studies, but the relative efficiency of these gears has not been compared.…”