Contrasting impacts of conifer forests on brown trout and atlantic salmon in headwater streams in Ireland

Salmonid populations were sampled in 64 first- and second-order headwater streams throughout the Republic of Ireland in 2008. Streams were selected from neighbouring sub-catchments dominated either by moorland or conifer plantations, across two soil types—peat soil and mineral soil. A range of chemical variables were measured on three occasions over the year. Brown trout and Atlantic salmon populations were quantified on a single occasion for density and biomass in each stream. Data were analysed to test the null hypothesis that neither land use nor soil type had a significant impact on fish or chemical metrics in the streams. Streams draining afforested catchments had a significantly lower pH, ΔpH and elevated TDOC, total phosphorus, total monomeric aluminium, ammonia and colour. Streams draining peat soil sub-catchments had significantly lower minimum pH and TON, and higher suspended solids, TDOC, ammonia, aluminium and colour. While density and biomass of brown trout were somewhat lower in afforested catchments, the difference was not significant. Densities and biomass of juvenile Atlantic salmon, however, were significantly lower in streams draining afforested catchments. Atlantic salmon also had a much lower threshold tolerance to deleterious forestry-mediated water chemistry than trout. Forestry-mediated acidification of streams apparently remains a severe threat to Atlantic salmon populations, despite the reduction of airborne sulphate pollutants, posing challenges to forest and fisheries managers.