Testing whether reducing brown trout biomass in peatland lakes increases macro-invertebrate biomass and invertivorous waterbird occurrence

Waterbirds and fish sometimes compete for macro-invertebrate prey. In Scotland, the invertivorous waterbird, the common scoter Melanitta nigra , breeds at oligotrophic lakes with few brown trout Salmo trutta. This study tested whether reducing trout biomass favours this and other invertivorous waterbirds. The study took place in Scotland’s Flow Country, where brown trout occur widely, attracting recreational anglers, though angling has declined. At four small lakes, over 7 years, trout were reduced using 25 m 2 exclosures, and re-introducing traditional angling (including fish removal). Trout, macro-invertebrates and waterbirds were monitored. After angling re-introduction, trout biomass density declined by 56% (95% CLs 13–78%), but there was little lake-level change in combined macro-invertebrate biomass. However, within exclosures, macro-invertebrate biomass increased 4.7-fold (CLs 1.6–14). Analysing invertebrates in eight different groups showed lake-level increases, following angling re-introduction, for two groups (freshwater shrimps Gammarus ; water-surface invertebrates). Gammarus showed the strongest response, increasing sixfold (CLs 2.2–11.6). A combined analysis was performed for the commonest invertivorous waterbirds: common scoter, mallard Anas platyrhynchos , teal A. crecca , greenshank Tringa nebularia and dunlin Calidris alpina . After angling effort increased, occurrence of these species changed little initially, but rose later: 4 years after angling began, odds of occurrence had increased 4.9-fold (CLs 2.2–11). This study supports reducing trout biomass in peatland lakes by encouraging traditional angling, to increase some macro-invertebrate groups and usage by invertivorous waterbirds. Further work should test this across more lakes alongside work investigating the origins (native or stocked) of brown trout populations in the Flow Country.