Characterisation of salmonid food webs in the rivers and lakes of an irish peatland ecosystem

Peatlands are being degraded by the combined impacts of land use and climate change. Carbon stored in peat is a key constituent of aquatic food webs in rivers and lakes of humic catchments, and changes in the downstream transport of this allochthonous carbon may have considerable implications for the production of Atlantic salmon and brown trout. Understanding the food web of these keystone species is therefore crucial to their conservation in a changing world. Here, we use a combination of stomach content analysis (SCA) and stable isotope analysis (SIA) to characterise the diet of juvenile salmonids in aquatic habitats of a typical Irish peatland catchment (Burrishoole). SCA showed that Diptera, Ephemeroptera, Plecoptera and Trichoptera were the main components of the diet of juvenile salmonids. Daphnia were the primary prey item in salmon smolt stomachs. The average stable isotope signature of salmonids was 9.26 ± 0.87‰ δ¹⁵N and-25.6 ± 1.99‰ δ¹³C, but differed between species, age class and habitat (river vs lake). Salmonids were supported by a wide range of carbon energy sources, with δ¹³C increasing as fish moved downstream out of the headwater rivers and into a large downstream lake.