Temperature effects on fish production across a natural thermal gradient

Global warming is widely predicted to reduce the biomass production of top predators, or even result in species loss.Several exceptions to this expectation have been identified, however, and it is vital that we understand the underlyingmechanisms if we are to improve our ability to predict future trends. Here, we used a natural warming experiment inIceland and quantitative theoretical predictions to investigate the success of brown trout as top predators across astream temperature gradient (4–25 °C). Brown trout are at the northern limit of their geographic distribution in thissystem, with ambient stream temperatures below their optimum for maximal growth, and above it in the warmeststreams. A five-month mark-recapture study revealed that population abundance, biomass, growth rate, and produc-tion of trout all increased with stream temperature. We identified two mechanisms that contributed to theseresponses: (1) trout became more selective in their diet as stream temperature increased, feeding higher in the foodweb and increasing in trophic position; and (2) trophic transfer through the food web was more efficient in the war-mer streams. We found little evidence to support a third potential mechanism: that external subsidies would play amore important role in the diet of trout with increasing stream temperature. Resource availability was also amplifiedthrough the trophic levels with warming, as predicted by metabolic theory in nutrient-replete systems. These resultshighlight circumstances in which top predators can thrive in warmer environments and contribute to our knowledgeof warming impacts on natural communities and ecosystem functioning.The authors are supported by grants awarded by NERC (NE/L011840/1 and NE/I009280/2), the Royal Society (RG140601), the British Ecological Society (4009-4884), the Fisheries Society of the British Isles, the Grand Challenges in Ecosystems and the Environment initiative at Imperial College London, the Scottish Government Rural and Environment Science and Analytical Services (RESAS), the Salmonid Fisheries Management Fund in Reykjavik, and Assistantship and Research Funds from the University of Iceland (GMG2006, GMG2007).Peer ReviewedRitrýnt tímari