Voss, N. S., R. Al-Chokhachy, A. J. Sepulveda, C. E. Verhille, M. P. Ruggles, and A. V. Zale. 2022. The importance of phenology and thermal exposure to early life history success of non-native Smallmouth Bass in the Yellowstone River. Transactions of the American Fisheris Society 151:527-542. DOI: 10.1002/tafs.10364
Abstract
Knowledge of the potential spread of an introduced species is critical to direct management efforts to where they will provide the greatest benefits. The Smallmouth Bass Micropterus dolomieu, is an example of a fish that has been introduced globally, often spreads following introduction, and has substantial predatory impacts on fish assemblages. We evaluated Smallmouth Bass spread in the free-flowing Yellowstone River, Montana, where this non-native fish has expanded from warmer, downstream sections of river into colder, upstream sections. We hypothesized that progressively colder upstream climates would limit age-0 growth and overwinter survival in the Yellowstone River, thereby curbing potential upstream spread. We documented the phenology, growth, and overwinter survival of age-0 Smallmouth Bass across a gradient of temperature and population densities. Age-0 individuals were rare or absent throughout the uppermost upstream distribution of adults, suggesting that something limits or discourages reproduction farther upstream. However, we found no support for colder water temperatures alone as the limiting factor. Age-0 body size at the onset of winter did not differ significantly among the uppermost 200 km of their distribution. Rather, the earlier hatching timing exhibited by some age-0 individuals in colder, upstream sections allowed them to experience longer growing seasons than many individuals in warmer, downstream sections. This counter-intuitive hatching phenology mediated much of the negative effect of colder upstream climates on growth. Furthermore, evidence of successful overwinter survival and simulations of age-0 starvation mortality indicated that age-0 individuals at the upstream extent of their distribution successfully recruited to the age-1 year-class in four consecutive years. Taken together, our results suggest that Smallmouth Bass have not yet reached the thermal limit of their upstream distribution in the Yellowstone River, and that further spread will probably be driven by complex interactions between density dependence, spawn timing, and ambient thermal and hydrologic regimes.