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RESEARCH ARTICLE
Contents Vol 61(9)

Demographic analysis of the pelagic thresher shark, Alopias pelagicus, in the north-western Pacific using a stochastic stage-based model

Wen-Pei Tsai A C , Kwang-Ming Liu B D and Shoou-Jeng Joung A
+ Author Affiliations
- Author Affiliations

A Department of Environmental Biology and Fisheries Science, National Taiwan Ocean University, Keelung 202, Taiwan.

B Institute of Marine Affairs and Resource Management, National Taiwan Ocean University, Keelung 202, Taiwan.

C Current address: Institute of Oceanography, National Taiwan University, Taipei 106, Taiwan.

D Corresponding author. Email: kmliu@mail.ntou.edu.tw

Marine and Freshwater Research 61(9) 1056-1066 https://doi.org/10.1071/MF09303
Submitted: 3 December 2009  Accepted: 2 April 2010   Published: 23 September 2010

Abstract

The pelagic thresher shark, Alopias pelagicus, like most large sharks produces a low number of offspring (two embryos per litter). However, there is no accurate information on its population status. To improve the accuracy of population simulations, a stochastic stage-based birth-flow model was constructed to assess the stock status of the pelagic thresher in the north-western Pacific. Based on the best biological information available, its life history was represented as four stages: neonates, juveniles, subadults, and adults. Results indicated that, without mortality from fishing, the stock would clearly increase (mean annual population growth rate (λ) = 1.058 year–1, 95% CI = 1.014–1.102 year–1). When current fishing mortality was taken into account, a decrease in population was indicated (λ = 0.979 year–1, 95% CI = 0.921–1.030 year–1) with a projected reduction of 34.3% over 20 years. These results suggest that the stock is overexploited. Simulations using various management measures showed that the population will remain steady under these protection options. However, this species is extremely vulnerable to overexploitation and is especially sensitive at the juvenile and adult stages, implying that nursery closures or size limit management measures are urgently needed to ensure the sustainable utilisation of the stock.

Additional keyword: stock assessment.


Acknowledgements

We would like to thank Dr Chi-Lu Sun of National Taiwan University and Dr Sheng-Ping Wang of National Taiwan Ocean University for their constructive suggestions. We acknowledge Nanfangao and Chengkung Fishermans Associations for providing the longline shark catch and weight data. We also thank three anonymous reviewers for their helpful comments, which greatly improved this manuscript. This study was financially supported by the National Science Council of the Republic of China under Contract No. NSC96-2313-B-019-008 and NSC97-2313-B-019-002-MY2.


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