Changes in the insect emergence at the water–air interface in response to fish density manipulation in high altitude lakes.

Emerging insects are important nutrient vectors for lake-to-land linkages, but they are affected by fish predation. We manipulated the fish density in four high altitude lakes in connection with a brook trout eradication project. We used a paired benthic and surface funnel trap design to test whether fish density affects abundance, size, and proportion of aquatic insects reaching the surface in the pelagic zone. The study was complemented with benthic macroinvertebrates and fish diet data. Due to sample size, we focused on midge pupae, which rely on water surface tension (water–air interface) to emerge. Conspicuous insects emerging along the shoreline (water–land interface) were rarely collected in the traps, even if they rapidly recovered through the eradication. There was a positive relationship between fish density and midge abundance, and negative relationship for size and proportion reaching the surface. The increase in the emergence of midges in the presence of fish supports several other studies, but we have provided evidence that this pattern exists in conjunction with a size reduction even when fish consume a substantial amount of midges. The inclusion of small Diptera emergence at the water–air interface is a likely keystone in understanding fish impact on insect subsidy.