Spawning and early life history of mountain whitefish in the Madison River, Montana
Boyer, Jan Katherine.
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Mountain Whitefish were historically common throughout much of the Intermountain West. However, within the last decade Mountain Whitefish have exhibited population-level declines in some rivers. In the Madison River, Montana, anecdotal evidence indicates Mountain Whitefish abundance has declined and the population is skewed toward larger individuals, which is typically symptomatic of recruitment problems. Recruitment is influenced by factors including reproductive development, spawning behavior, and juvenile distribution. Describing these factors and identifying efficient methods for sampling age-0 fish would form a foundation for investigating mechanisms influencing recruitment. I collected otoliths and gonad samples (n = 147) to characterize fecundity, age-at-maturity, and spawning periodicity. I implanted radio tags in mature Mountain Whitefish (n = 138) and relocated tagged fish in autumn 2012 - 2014. Timing of spawning was determined from spawning status of captured females (n = 85) and from density of eggs collected on egg mats. In spring 2013, I evaluated backpack electrofishing, seining, minnow traps, and lighted minnow traps at sampling sites downstream of Varney Bridge (n = 92). In spring 2014, I seined backwater and channel sites (n = 221) to describe age-0 distribution. Mountain Whitefish in the Madison River were highly fecund (18,450 eggs/kg body weight) annual spawners, and age at 50% maturity was 2.0 for males and 2.6 for females. In 2013 and 2014, spawning occurred between the third week of October and first week of November. Movement varied as a function of spawning behavior, and prespawning movements trended downstream. During spawning, spawning adults and collected embryos were concentrated in the downstream 26 km of the study site, a reach characterized by a complex, braided channel. Of the gears tested, seines were most efficient at sampling age-0 Mountain Whitefish. The downstream reach had the highest catch-per-unit effort of age-0 Mountain Whitefish. Within this reach, age-0 fish were associated with silt-laden backwater and eddy habitats. Maturation and fecundity were similar to other populations, and reproductive development appeared normal, thus factors influencing recruitment probably occur post spawning. Spawning and age-0 rearing sites were concentrated in a small area, thus future work should investigate stressors present in incubation and rearing areas.