Relationship between plantation forest and brown trout growth, energetics and population structure in peatland lakes in western Ireland

• Investigation associations of conifer plantations on brown trout within blanket bog catchments.
• Lakes with forested catchments had elevated nutrient levels but pH did not differ.
• Trout population structure differed between reference and forested lakes.
• Trout were larger in lakes with large areas of conifer plantations within their catchment.
• Trout appeared thermally stressed, particularly in forested lakes due to high temperatures.

Much of the conifer plantation forest in Ireland is on peat soils. Plantations on this soil type are known to pose the greatest risk to degrading water quality by increased sedimentation, acidification and heavy metal accumulation. Peat soils are also known to leach phosphorus (P), nitrogen (N) and dissolved organic carbon (DOC) as a result of forestry operations. In moderation, such nutrient enrichment may have positive trophic impacts in oligotrophic freshwater systems such as those typical of peat catchments in western Ireland.In this study, the water chemistry and brown trout (Salmo trutta) populations of peat-land lakes were investigated to assess the associations of conifer plantation forest on the growth, energetics and population structure of brown trout. We conducted this study over a three-month period in the summer of 2010 by comparing brown trout populations and water chemistry in lakes with three distinct catchment land uses: (i) unplanted blanket bog, (ii) moderate levels of conifer plantation forest (30–40% of catchment afforested), and (iii) high levels of conifer plantation forest (80–90% of catchment afforested).Changes in hydrochemistry associated with conifer plantations resulted in elevated concentrations of N, P and DOC, but no change to pH, with increasing levels of plantation forest within the catchment. Whereas there was no consistent trend in brown trout density between land uses, highest densities were recorded in lakes with afforested catchments. Trout populations in lakes with afforested catchments were dominated by younger fish, primarily 1+ (year old) and 2+ (2 year old) individuals with some 0+ (young of the year) trout present, compared to control lakes, which were largely dominated by 2+ and 3+ (3 year old) individuals.Older age classes of trout had larger body sizes in lakes with high levels of plantation forest within the catchment relative to the other lakes, indicating higher empirical growth rates, likely due to the trophic enrichment effects of forestry. Brown trout specific growth models that incorporate the potential confounding influence of different temperature regimes, showed no consistent relationship between growth and forest cover over the study period. Food consumption models indicated that trout in all sites were energetically challenged during the summer when sampling took place. Discrepancies between the observed body size and estimated growth of trout in lakes may potentially be due to (a) a significant amount of growth occurring outside of the summer study period and/or (b) unusually elevated temperature regimes during the study period, particularly in the afforested sites.No negative impacts of conifer plantation forests on brown trout populations were recorded. However, forest managers may wish to minimise felling coupe within peatland plantations as felling operations may exacerbate nutrient and/or heavy ion input into aquatic systems in such catchments.