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Temperature effects on fish production across a natural thermal gradient
Authors:Eoin J O'Gorman  Ólafur P Ólafsson  Benoît O L Demars  Nikolai Friberg  Guðni Guðbergsson  Elísabet R Hannesdóttir  Michelle C Jackson  Liselotte S Johansson  Órla B McLaughlin  Jón S Ólafsson  Guy Woodward  Gísli M Gíslason
Institution:1. Department of Life Sciences, Imperial College London, Berkshire, UK;2. Institute of Life and Environmental Sciences, University of Iceland, Reykjavík, Iceland;3. The James Hutton Institute, Aberdeen, UK;4. Norwegian Institute for Water Research (NIVA), Oslo, Norway;5. Institute of Freshwater Fisheries, Reykjavík, Iceland;6. Centre for Invasion Biology, Department of Zoology and Entomology, University of Pretoria, Gauteng, South Africa;7. Department of Bioscience, Aarhus University, Silkeborg, Denmark;8. Institut National de la Recherche Agronomique (INRA), UMR 1347 Agroécologie, Dijon, France
Abstract: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 underlying mechanisms if we are to improve our ability to predict future trends. Here, we used a natural warming experiment in Iceland and quantitative theoretical predictions to investigate the success of brown trout as top predators across a stream temperature gradient (4–25 °C). Brown trout are at the northern limit of their geographic distribution in this system, with ambient stream temperatures below their optimum for maximal growth, and above it in the warmest streams. A five‐month mark‐recapture study revealed that population abundance, biomass, growth rate, and production of trout all increased with stream temperature. We identified two mechanisms that contributed to these responses: (1) trout became more selective in their diet as stream temperature increased, feeding higher in the food web and increasing in trophic position; and (2) trophic transfer through the food web was more efficient in the warmer streams. We found little evidence to support a third potential mechanism: that external subsidies would play a more important role in the diet of trout with increasing stream temperature. Resource availability was also amplified through the trophic levels with warming, as predicted by metabolic theory in nutrient‐replete systems. These results highlight circumstances in which top predators can thrive in warmer environments and contribute to our knowledge of warming impacts on natural communities and ecosystem functioning.
Keywords:natural experiment  Arctic  Hengill  freshwater     Salmo trutta fario     PIT tag  mark‐recapture  ecosystem services
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