Do non-native pumpkinseed Lepomis gibbosus affect the growth, diet and trophic niche breadth of native brown trout Salmo trutta?

Brown trout Salmo trutta Linnaeus, 1758, is a priority species for conservation and management efforts in many European countries. In its native range, interactions with non-native fishes often adversely affect somatic growth rates and population abundances. Consequences of introduced North American pumpkinseed Lepomis gibbosus (Linnaeus, 1758) for native S. trutta were examined in stream stretches with and without L. gibbosus. Data for somatic growth rates and trophic niche breadth (using stable isotope analyses) provided little evidence of L. gibbosus presence being detrimental for S. trutta. Shifts in S. trutta diet at all sites were associated with increased piscivory with increasing body length, with no evidence to suggest that interspecific resource competition with L. gibbosus structured the food web or affected trophic positions. Three years later, and following L. gibbosus removal, data revealed slight shifts in the food web at each site, but these related to shifts in resources at the bottom of the food chain rather than a response to L. gibbosus removal. Consequently, the ecological consequences of L. gibbosus for S. trutta in the study stream were minimal, with S. trutta populations responding more to natural mechanisms regulating their populations than to the presence of this non-native fish species.