Evidence for trophic differences between live and bycatch oceanic juvenile loggerhead sea turtles
The loggerhead sea turtle Caretta caretta is a vulnerable migratory species that spends its first years of life in the open sea. During this developmental phase, loggerheads can be found foraging in the epipelagic zone of the waters surrounding the Madeira Archipelago, providing a rare opportunity to gather information on the ecology of its oceanic developmental stage. In this study, we characterized the isotopic niche of these juveniles, using stable isotope analysis. We assessed two groups of turtles, turtles captured alive (n = 24) and turtles captured as bycatch on local longlines (n = 12), and explored whether animals caught in the local fishing gear represented a random sample of the population, or whether there is some evidence for a specialized foraging behaviour. We found that turtle bycatch had a significantly higher mean stable nitrogen isotope value in whole blood (δ15N = 8.5 ± 0.6‰ SD) compared to the group of turtles captured alive in the same period (δ15N = 7.6 ± 0.5‰ SD), indicating that they had a different diet. While there was a tendency for turtle bycatch to be slightly larger, we found no effect of body size on δ15N values. We propose a distinct foraging behaviour strategy hypothesis, with a group of turtles being more susceptible to interactions with fisheries and thus having a higher mortality risk, which should motivate the implementation of existing guidelines to reduce sea turtle bycatch.
We thank the efforts of all people involved in the fieldwork and in the laboratory work, and the collaboration of the local fishers. This study was conducted under the license of Madeira’s Natural Park and the research was funded by the Portuguese Foundation for Science and Technology (FCT) through the project OceanWebs: Trophic links in open oceans: combining spatial data, dietary and functioning of subtropical pelagic ecosystems (PTDC/MAR-PRO/0929/2014), the strategic projects UID/MAR/04292/2013, IF/00502/2013/CP1186/CT0003 and UID/AMB/50017/2013 granted to MARE and CESAM, respectively, and European Regional Development Fund, within the PT2020 Partnership Agreement and Compete 2020.
This study was funded by the Portuguese Foundation for Science and Technology through the project OceanWebs: Trophic links in open oceans: combining spatial data, dietary and functioning of subtropical pelagic ecosystems (PTDC/MAR-PRO/0929/2014), and the strategic projects UID/MAR/04292/2013, IF/00502/2013/CP1186/CT0003 and UID/AMB/50017/2013 granted to MARE and CESAM, respectively, and the European Regional Development Fund; PT2020 Partnership Agreement and Compete 2020.
Compliance with ethical standards
Conflict of interest
Author CR declares that she has no conflict of interest. Author ARP declares that she has no conflict of interest. Author PC declares that he has no conflict of interest. Author TD declares that he has no conflict of interest. Author JPG declares that he has no conflict of interest.
All procedures were approved by the Animal Welfare Body of the Faculty of Sciences, University of Lisbon (Statement 2/2016) and license 0421/000/000/2016 issued by the competent governmental agency (Direção Geral da Alimentação e Veterinária), and followed recommended guidelines (Bolten 1999, NOAA 2008). No animal experiments were conducted for the purpose of this study.
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