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Estimates of juvenile and adult raggedtooth shark (Carcharias taurus) abundance along the east coast of South Africa

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Abstract:

A Cormack–Jolly–Seber model was developed to estimate abundance, survival, and probability of capture for juvenile (<1.8 m total length, TL) and adult (>1.8 m TL) raggedtooth sharks (Carcharias taurus) along the east coast of South Africa. Model estimates were adjusted to account for tag loss, nonreporting, and after release mortality. The model was constructed using mark–recapture data from the Oceanographic Research Institute and Port Elizabeth Museum cooperative tagging programs from 1984 to 2004. The adjusted estimate for juvenile survival was 0.56 and that for adult survival was 0.89. The adjusted estimate of probability of capture for juvenile sharks ranged from 0.06 to 0.17, while that for adult sharks was from 0.02 to 0.04. The mean annual abundance of juvenile sharks was 6800 (coefficient of variation, CV = 13%) and adult sharks 16 700 (CV = 9%). The accumulated effect of tag loss, nonreporting, and after release mortality were to reduce the overall estimate of juvenile and adult abundance by approximately 50%. The adjusted estimate of population size for both juvenile and adult sharks over the last decade appears to have remained constant (P > 0.05).

Nous avons mis au point un modèle Cormack–Jolly–Seber pour estimer l’abondance, la survie et la probabilité de capture de requins taureaux (Carcharias taurus) jeunes (<1,8 m de longueur totale, TL) et adultes (>1,8 m TL) au large de la côte est de l’Afrique du Sud. Les estimations du modèle ont été ajustées pour tenir compte des pertes d’étiquettes, des captures non signalées et de la mortalité après la remise à l’eau. Le modèle est basé sur des données de marquage–recapture provenant des programmes coopératifs de marquage de l’Oceanographic Research Institute et du Port Elizabeth Museum de 1984 à 2004. Après ajustements, la survie estimée des jeunes requins est de 0,56 et celle des requins adultes de 0,89 et la probabilité estimée de capture des jeunes varie de 0,06 à 0,17 et celle des adultes de 0,02 à 0,04. L’abondance annuelle moyenne des jeunes requins est de 6800 (coefficient de variation, CV = 13 %) et celle des adultes 16 700 (CV = 9 %). L’effet cumulatif des pertes d’étiquettes, des captures non signalées et de la mortalité après la remise à l’eau réduit l’estimation globale de l’abondance des jeunes et des adultes d’environ 50 %. La taille estimée après ajustements de la population de requins jeunes et adultes semble être demeurée constante (P > 0,05) au cours de la dernière décennie.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: April 1, 2008

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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