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Marine Ecology Progress Series

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MEPS 387:241-253 (2009)  -  DOI:

Bycatch and discard mortality in commercially caught blue sharks Prionace glauca assessed using archival satellite pop-up tags

Steven E. Campana1,*, Warren Joyce1, Michael J. Manning2,3

1Bedford Institute of Oceanography, PO Box 1006, Dartmouth, Nova Scotia, Canada B2Y 4A2
2National Institute of Water and Atmospheric Research Ltd (NIWA), Private Bag 14901, Kilbirnie, Wellington, New Zealand
3Oceanic Fisheries Programme, Secretariat of the Pacific Community (SPC), BP D5, 98848 Noumea Cedex, New Caledonia

ABSTRACT: Blue sharks Prionace glauca are the most frequently discarded fish species during commercial pelagic longline fishing operations worldwide, yet their post-release mortality rate has never been measured. A generalized linear model of 12404 blue sharks observed during the Canadian Atlantic pelagic longline swordfishery suggested a hooking mortality of 12 to 13%, yet scientific examination of 902 of these sharks indicated that hooking mortality was actually higher. A random sample of 40 of these blue sharks were tagged with satellite pop-up archival transmission (PAT) tags, then monitored for periods of up to 6 mo after release. All of the surviving sharks exhibited a depth-holding recovery behaviour for a period of 2 to 7 d after release. All healthy sharks survived, while 33% of those that were badly injured or gut hooked subsequently died. Overall blue shark bycatch mortality in the pelagic longline fishery was estimated at 35%, while the estimated discard mortality for sharks that were released alive was 19%. Survival time models indicated that 95% of the mortality occurred within 11 d of release, indicative of death by trauma rather than starvation. The annual blue shark catch in the North Atlantic was estimated at about 84000 t, of which 57000 t is discarded. A preliminary estimate of 20000 t of annual dead discards for North Atlantic blue sharks is similar to that of the reported nominal catch, and could substantially change the perception of population health if incorporated into a population-level stock assessment.

KEY WORDS: Discard mortality · Hooking mortality · Tagging · Bycatch · Satellite tags

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Cite this article as: Campana SE, Joyce W, Manning MJ (2009) Bycatch and discard mortality in commercially caught blue sharks Prionace glauca assessed using archival satellite pop-up tags. Mar Ecol Prog Ser 387:241-253.

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