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Thermal stratification drives movement of a coastal apex predator

Abstract : A characterization of the thermal ecology of fishes is needed to better understand changes in ecosystems and species distributions arising from global warming. The movement of wild animals during changing environmental conditions provides essential information to help predict the future thermal response of large marine predators. We used acoustic telemetry to monitor the vertical movement activity of the common dentex (Dentex dentex), a Mediterranean coastal predator, in relation to the oscillations of the seasonal thermocline during two summer periods in the Medes Islands marine reserve (NW Mediterranean Sea). During the summer stratification period, the common dentex presented a clear preference for the warm suprathermoclinal layer, and adjusted their vertical movements following the depth changes of the thermocline. The same preference was also observed during the night, when fish were less active. Due to this behaviour, we hypothesize that inter-annual thermal oscillations and the predicted lengthening of summer conditions will have a significant positive impact on the metabolic efficiency, activity levels, and population dynamics of this species, particularly in its northern limit of distribution. These changes in the dynamics of an ecosystem's keystone predator might cascade down to lower trophic levels, potentially redefining the coastal fish communities of the future. Temperature is a key environmental factor that, through profound physiological effects, influences the fitness and survival of ectothermic organisms 1–3. Ectothermic organisms are usually adapted to live within a limited range of temperatures, which includes a thermal optimum that maximizes their physiological performance 4. When conditions move away from this optimum, an organism experiences reduced growth, reproduction, foraging, or competitiveness 5, 6. Temperature shifts from the preferred range will thus greatly affect the dynamics of a species, altering the relative abundances of a population and changing the horizontal and vertical distribution of individuals 7–10. In the case of ecosystem keystone species such as apex predators, community-changing herbivores, or structure-forming species, temperature-driven changes in abundance may cause ripple-effects in other levels of the food web, changing the structure and functioning of the ecosystem 11, 12. In the face of global climate change, a great deal of research effort is being expended to characterize the thermal ecology of marine organisms, as it is necessary to understand the trends in ecosystems arising from the warming of the global ocean 6, 13. The ability to move makes fishes much more resistant to environmental change than less mobile or sessile benthic species 14. As with many other mobile animals, marine fishes can readily exploit thermal gradients to regulate their body temperature and increase their metabolic efficiency 15, 16. Movement is thus a good behavioural response with which to infer the thermal ecology of a fish species. Controlled laboratory experiments have shown that fish move across thermal gradients to attain a preferred temperature 17, 18 , and have allowed the researchers to investigate the response of an individual's internal temperature to a fluctuating environment 16. However,
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Eneko Aspillaga, Frederic Bartumeus, Richard Starr, Àngel López-Sanz, Cristina Linares, et al.. Thermal stratification drives movement of a coastal apex predator. Scientific Reports, Nature Publishing Group, 2017, 7 (1), ⟨10.1038/s41598-017-00576-z⟩. ⟨hal-01699033⟩

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