title: Temperature effects on fish production across a natural thermal gradient.
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.
authors.affiliation: Department of Life Sciences, Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot, Berkshire, SL5 7PY, UK.
authors.forename: Eoin J
authors.initials: EJ
authors.lastname: O'Gorman
authors.affiliation: Institute of Life and Environmental Sciences, University of Iceland, Askja, Sturlugata 7, Reykjavík, 101, Iceland.
authors.forename: Ólafur P
authors.initials: ÓP
authors.lastname: Ólafsson
authors.affiliation: The James Hutton Institute, Aberdeen, AB15 8QH, UK.
authors.forename: Benoît O L
authors.initials: BO
authors.lastname: Demars
authors.affiliation: Norwegian Institute for Water Research (NIVA), Gaustadalléen 21, Oslo, N-0349, Norway.
authors.forename: Nikolai
authors.initials: N
authors.lastname: Friberg
authors.affiliation: Institute of Freshwater Fisheries, Keldnaholt, Reykjavík, 112, Iceland.
authors.forename: Guðni
authors.initials: G
authors.lastname: Guðbergsson
authors.affiliation: Institute of Life and Environmental Sciences, University of Iceland, Askja, Sturlugata 7, Reykjavík, 101, Iceland.|Institute of Freshwater Fisheries, Keldnaholt, Reykjavík, 112, Iceland.
authors.forename: Elísabet R
authors.initials: ER
authors.lastname: Hannesdóttir
authors.affiliation: Centre for Invasion Biology, Department of Zoology and Entomology, University of Pretoria, Hatfield 0026, Gauteng, South Africa.
authors.forename: Michelle C
authors.initials: MC
authors.lastname: Jackson
authors.affiliation: Department of Bioscience, Aarhus University, Silkeborg, Denmark.
authors.forename: Liselotte S
authors.initials: LS
authors.lastname: Johansson
authors.affiliation: Department of Life Sciences, Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot, Berkshire, SL5 7PY, UK.|Institut National de la Recherche Agronomique (INRA), UMR 1347 Agroécologie, 17 rue Sully - BP 86510, Dijon, 21065, France.
authors.forename: Órla B
authors.initials: ÓB
authors.lastname: McLaughlin
authors.affiliation: Institute of Freshwater Fisheries, Keldnaholt, Reykjavík, 112, Iceland.
authors.forename: Jón S
authors.initials: JS
authors.lastname: Ólafsson
authors.affiliation: Department of Life Sciences, Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot, Berkshire, SL5 7PY, UK.
authors.forename: Guy
authors.initials: G
authors.lastname: Woodward
authors.affiliation: Institute of Life and Environmental Sciences, University of Iceland, Askja, Sturlugata 7, Reykjavík, 101, Iceland.
authors.forename: Gísli M
authors.initials: GM
authors.lastname: Gíslason
mesh_terms: D000818:Animals; D004032:Diet; D017753:Ecosystem; D020387:Food Chain; D007055:Iceland; D013696:Temperature; D014337:Trout
keywords: Arctic; Hengill; PIT tag; Salmo trutta fario; ecosystem services; freshwater; mark-recapture; natural experiment