ESR 29:239-254 (2016)  -  DOI: https://doi.org/10.3354/esr00714

How much sea turtle bycatch is too much? A stationary age distribution model for simulating population abundance and potential biological removal in the Mediterranean

Paolo Casale1,2,*, Selina S. Heppell

1Department of Biology and Biotechnologies ‘Charles Darwin,’ University of Rome ‘La Sapienza,’ Viale dell’Università 32, 00185 Rome, Italy
2Centre for Ecology & Conservation, University of Exeter, Cornwall Campus, Penryn, Cornwall TR10 9FE, UK
3Department of Fisheries and Wildlife, Oregon State University, Corvallis, OR 97331, USA
*Corresponding author:

ABSTRACT: Mediterranean populations of loggerhead Caretta caretta and green sea turtles Chelonia mydas are subject to several anthropogenic threats, with documented mortality from incidental capture in fishing gear. However, how such mortalities actually affect the populations is uncertain without an estimate of population size. We derived a theoretical demographic structure for each species in the Mediterranean, assuming a stationary age distribution in a stable population with constant proportions of turtles in each life stage, using distributions of age-specific vital rates. We incorporated uncertainty into the main vital rate parameters to identify a likely order of magnitude of turtle abundance in different life stages. Through this approach, we aim to (1) provide a rough estimate of all population stage classes, particularly the juvenile classes that are most subject to fisheries interactions, (2) provide an estimate of reproductive life span, (3) identify and review the key demographic parameters, and (4) identify the priority gaps in our information in need of further investigation. The range of population abundance estimates from the models constructed with uncertainty (95% CI) was 0.81-3.38 million loggerheads and 0.26-2.21 million green turtles, Mediterranean-wide. When we calculated the potential biological removal for the segment of the population at risk of fisheries capture, our estimates were comparable to or lower than the estimated bycatch levels in fisheries. Although the model assumes a stable population and provides only a rough estimate of abundance, these results suggest that the current bycatch level should be regarded as unsustainable for Mediterranean turtle populations.


KEY WORDS: Loggerhead turtle · Caretta caretta · Green turtle · Chelonia mydas · Demography · Incidental capture


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Cite this article as: Casale P, Heppell SS (2016) How much sea turtle bycatch is too much? A stationary age distribution model for simulating population abundance and potential biological removal in the Mediterranean. Endang Species Res 29:239-254. https://doi.org/10.3354/esr00714

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