Assessing the effect of nutrient enrichment on the diet of stream
salmonids: a novel approach
Conor T. Graham†*, Simon S.C. Harrison* & C. Harrod˟
*School of Biological, Earth & Environmental Sciences, University College Cork, Ireland.
˟School of Biological Sciences, Queens University Belfast, UK & Instituto de Investigaciones
Oceanológicas, Universidad de Antofagasta, Chile.
†
Email: grahamconor@gmail.com
Abstract
While Atlantic salmon feed almost exclusively on aquatic prey, brown trout in
nutrient depauperate streams rely greatly on terrestrial subsidies. Moderate nutrient
enrichment however, can stimulate the supply of aquatic invertebrate prey items
within streams and impact considerably on instream food web dynamics.
The brown trout is a more generalist and opportunistic predator than the Atlantic
salmon and is considerably more aggressive. We investigated if brown trout modify
their diet in response to nutrient-mediated increases in the supply of aquatic prey
resources across a gradient of nutrient enriched rivers, and hence increase dietary
competition between the two sympatric salmonids.
However, calculating reliable estimates of the relative importance of allochthonous
versus autochthonous material in aquatic food web has proved challenging. The
application of carbon and nitrogen stable isotope analyses has proved valuable in this
quantification but remain problematic. Here we use a novel combination of gut
contents and stable isotope of carbon, nitrogen and hydrogen to assess the reliance of
stream salmonids on these two energy sources.