Should we further investigate the cascading effects of introduced fish on insectivorous birds?

We investigated whether the introduction of fish (brown trout, Salmo trutta) in previously fishless boreal lakes could alter the aquatic insect subsidy and affect the nesting population of insectivorous migratory birds, with a particular focus on the pied flycatcher, Ficedula hypoleuca, which is the most represented species. According to our paleolimnological data and bioenergetic model, introduced trout increased the adult midge biomass output from 18.6 to 28.7 kg (+54.6%), which was energetically significant and could have theoretically supported an increase in insectivorous birds nesting around the lake. While the long-term series of insectivorous birds nesting around the lake seemed to increase according to the modeled fish effects, we could not establish a clear link with changes in food availability. Our control (pied flycatchers) showed similar patterns as our treatment, suggesting that this species’ abundance was affected by factors other than fish presence in the lake. The demographic fluctuations and trends of the pied flycatcher at the regional scale seemed to override cascading effects linking introduced fish, emerging midges, and insectivorous birds. We conclude that further studies will be needed to investigate this topic and propose some areas for future research.