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Effects of recreational and commercial fishing on blue sharks (Prionace glauca) in Atlantic Canada, with inferences on the North Atlantic population

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Abstract:

The nominal catch of blue sharks (Prionace glauca) reported for the Canadian Atlantic grossly underestimates the annual catch mortality of about 1000 tonnes (t), making blue sharks the most frequently caught large shark in Canadian waters. Although blue sharks accounted for 99% of all sharks landed at recreational shark fishing tournaments, tournament catches accounted for only 3% of total fishing mortality. Standardized catch rate indices suggested a decline in blue shark abundance of about 5%–6%·year–1 since 1995. An increased mortality rate in recent years was suggested by a decline in the median size of blue sharks in the commercial catch. Two independent calculations suggest that North Atlantic catches exceeded 100 000 t, with catch mortalities ranging between 26 000 and 37 000 t. Because tagging studies indicated that blue sharks are highly migratory with a single population in the North Atlantic, the Canadian contribution to overall population mortality accounts for only 2% of the total. The fact that blue shark populations are relatively productive and resilient may help explain their persistence in the face of high international catch mortality and a decline in relative abundance.

Les captures nominales de requins bleus (Prionace glauca) signalées dans l'Atlantique canadien sous-estiment considérablement la mortalité annuelle due à la pêche d'environ 1000 tonnes (t); il s'agit donc du grand requin le plus couramment capturé dans les eaux canadiennes. Bien que les requins bleus représentent 99 % de tous les requins débarqués lors des tournois récréatifs de pêche au requin, les captures lors des tournois ne représentent que 3 % de la mortalité totale due à la pêche. Les indices standardisés du taux de capture indiquent un déclin de l'abondance des requins bleus d'environ 5–6 %·an–1 depuis 1995. La diminution de la taille médiane des requins bleus dans les captures commerciales laisse croire à une augmentation du taux de mortalité au cours des années récentes. Deux calculs indépendants indiquent que les captures en Amérique du Nord dépassent 100 000 t et que les mortalités dues à la capture sont de l'ordre de 26 000 à 37 000 t. Comme des études de marquage indiquent que les requins bleus migrent beaucoup et qu'ils forment une seule population dans l'Atlantique Nord, la contribution canadienne à la mortalité globale de la population représente seulement 2 % du total. La persistance des requins bleus malgré une capture internationale importante et un déclin de leur abondance relative s'explique peut-être par la productivité relativement élevée et la résilience des populations.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: 2006-03-01

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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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