February 1st to 5th 2016
Olhão, Portugal
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Demography of the bigeye thresher shark Alopias superciliosus in the Atlantic Ocean: Application of Leslie matrices with incorporated uncertainties

Oral Presentation
Natural Resources
Wednesday, February 3, 2016 -
17:00 to 17:15

Fernandez-Carvalho, J. 1 Coelho, R. 2 Erzini, K. 3 Santos, M.N. 4

11. Centre of Marine Sciences (CCMAR), University of Algarve, Campus de Gambelas, 8005-139 Faro, Portugal 2. Portuguese Institute for the Ocean and Atmosphere (IPMA)
21. Centre of Marine Sciences (CCMAR), University of Algarve, Campus de Gambelas, 8005-139 Faro, Portugal 2. Portuguese Institute for the Ocean and Atmosphere (IPMA)
3Centre of Marine Sciences (CCMAR), University of Algarve, Campus de Gambelas, 8005-139 Faro, Portugal
4Portuguese Institute for the Ocean and Atmosphere (IPMA)

The bigeye thresher shark, Alopias supercilious is an oceanic pelagic shark, occasionally caught as bycatch in pelagic longline fisheries targeting tunas and swordfish. It is particularly vulnerable to fishing pressure, with overexploitation occurring even at low levels of fishing, due to their slow growth rates, low fecundity (two to four pups) and migratory nature crossing both national and international waters. A stochastic population dynamics model was developed for this species using age-structured Leslie matrices. Uncertainties were incorporated in the fecundities and survivorship parameters using Monte Carlo simulation. The estimated population growth rate was very low (λ = 1.008, 95% CI = [0.9968, 1.0189]), with this being one of the lowest value known for shark populations, highlighting the very low growth potential of this species. Elasticities for the matrices were estimated and it was observed that the matrix elements with higher elasticities were the survivorship parameters for the juvenile stages. This pattern is typical of the larger sized and slower growing shark species. If conservation efforts are to be applied to specific components of this species they should be focused mainly on the survivorship of the juveniles. The estimated λ for bigeye thresher are extremely low and any added source of mortality to this population will likely result in declines, since even under stable condition the population only grows at a rate of ~1% per year. This study presents new and important information about the population dynamics of the bigeye thresher in the Atlantic, highlighting its high vulnerability to fishing pressure which can be used to assist fishery managers to adopt more informed and efficient conservation measures for this species in the Atlantic.

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