Cohort strength and body size in co-occurring salmonids in a small stream network: variation in space and time

Trout and salmon commonly coexist in stream networks. Exploring similarities and differences among species can help explain coexistence and invasive ability. Here, we describe spatial distribution, cohort strengths and size-at-age of three co-occurring species in a small stream network. Spatial distributions varied dramatically among species; native brook trout (Salvellinus fonti-nalis) occupied all stream reaches, naturalized brown trout (Salmo trutta) were found in the mainstem and lower portions of tributaries and fry-stocked Atlantic salmon (Salmo salar) were limited to the mainstem. Size-at-age also differed among species, Atlantic salmon were consistently the smallest, brook trout were intermediate in size and brown trout were the largest. Despite size differences, mean lengths of brook trout and brown trout were highly correlated among years. Cohort strengths varied considerably across years but were also highly correlated for the two trout species, suggesting strong environmental control on cohort strength and a reduced role for species interactions. At low densities, we observed strong negative effects of density on body sizes and weaker effects otherwise. Overall, these results suggest differences in spatial distribution combined with similarities in response to environmental variation contribute to species coexistence in this small steam network.