Impacts of invasive stream salmonids on native fish : using meta-analysis to summarize four decades of research

Salmonid fishes are among the most frequently introduced organisms. We included 58 papers to meta-analysis to assess the effects of introduced stream salmonids on native salmonids. We also explored whether the responses of native species depended on the type of study. Introduced salmonids had negative effects on the foraging rate, abundance and survival of native salmonids, which also altered their habitat use in the presence of invad- ers. Brown trout appeared to be the 'worst' alien species (strongest impact on native fish). Negative effects were most pronounced when several introduced species were present. Moreover, the magnitude of the impact was related to the study type: the observed impacts were stronger in laboratory streams than in field enclosures or in natural streams. Our results indicate that introduced salmonid species may have little effect on native fish in some areas, but may have substantial effects in other parts of their range.