Variation in size-at-age between native cutthroat and introduced brown trout in allopatry and ­sympatry: implications for competitive interaction


otoliths, size-at-large, introduced species, competitive exclusion, sympatry


Brown trout Salmo trutta have been introduced into aquatic ecosystems throughout the western United States and have been implicated in the extirpation of many native cutthroat trout Oncorhynchus clarkii populations. We document patterns of size-at-age among multiple populations of brown trout and cutthroat trout in allopatry and sympatry. Comparisons were based on data from 48 allopatric populations (9 brown trout and 39 cutthroat trout) and 2 sympatric populations gathered from published reports and field collections. Brown trout are significantly larger than same-age cutthroat trout at all ages in both sympatry and allopatry. Size-at-age was also significantly negatively influenced with increasing latitude and elevation. The size-at-age for both species is reduced when in sympatry, but the reduction is more pronounced for brown trout. Cutthroat trout may be able to persist in the presence of brown trout in these instances because of the limited competitive ability of brown trout due to the reduced size, which may be a result of sub-optimal conditions of the habitat. The fact that brown trout are unable to exclude cutthroat trout when growth is restricted suggests that size is an important mechanism of competitive exclusion of cutthroat trout by brown trout.

Original Publication Citation

J.E. Rasmussen, M.C. Belk, E. Habit, D.K. Shiozawa, R.D. Hepworth, and A. Anthony. 2011. Variation in size-at-age between native cutthroat and introduced brown trout in allopatry and sympatry: implications for competitive interaction. Aquatic Biology 13:285-292.

Document Type

Peer-Reviewed Article

Publication Date



Aquatic Biology




Life Sciences



University Standing at Time of Publication

Full Professor