Impacts of stocked trout on prey communities in small boreal foothills lakes

Background/Question/Methods The stocking of lakes with non-native trout is an established management activity that can create recreational fisheries.  However, significant ecological impacts of stocked trout have been documented, including altered prey communities, extirpations of vulnerable native populations of macroinvertebrates and amphibians, changes in behaviour and habitat use, and subsequent alterations in ecosystem function. In lakes, however, dramatic impacts are not universal, and seem particularly concentrated in naturally fishless, oligotrophic and/or alpine lakes.  In contrast to alpine lakes, however, small lakes in the boreal foothills of west-central Alberta that are stocked with trout are warmer, more productive, and contain dense populations of macrophytes and hardy forage fishes. Since small-bodied fishes can themselves affect macroinvertebrates and amphibians, and ecosystem resistance to disturbance can be related to productivity and availability of refuges, we hypothesized that effects of stocked trout would be muted in these boreal-foothills lakes. We censused amphibians, forage fishes, and littoral macroinvertebrates in a set of stocked and unstocked lakes over a 3-year period to assess impacts of non-native trout on native communities.

Results/Conclusions We found no evidence that wood frog populations differed in abundance, body size, or metamorphosis patterns between fish-bearing stocked and unstocked lakes. In a naturally fishless (and unstocked) lake, however, adult and metamorph wood frogs were both more abundant and larger than in lakes containing fish. Size structures of minnows suggested that size-limited predation by trout was occurring, and forage fishes were more benthic, more concentrated inshore, and/or less diurnal in the presence of trout. These effects did not, however, translate into significant population-level impacts; densities of forage fishes (fathead minnow, brook stickleback, dace spp.) did not differ between stocked and unstocked lakes. Likewise, littoral macroinvertebrate assemblage composition did not differ consistently between stocked and unstocked lakes, while differences in the abundances or sizes were found in relatively few taxa. Our comparisons suggest that there were limited impacts on native communities of the lakes into which trout were stocked. Greater productivity, structurally complex littoral zones, and impacts of native fishes may buffer communities in these foothills lakes from severe impacts observed elsewhere.