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Research Document - 2011/067

Estimation of Pelagic Shark Bycatch and Associated Mortality in Canadian Atlantic Fisheries

By S.E. Campana, J. Brading, and W. Joyce

Abstract

The estimation of pelagic shark bycatch, discards and discard mortality was an objective of both the Workplan to Address Incidental Catch in Canadian Large Pelagic Fisheries and the associated Regional Advisory Process meeting (held 11-12 July, 2011). In addition to quantifying all sources of bycatch for porbeagle (Lamna nasus), shortfin mako (Isurus oxyrinchus), and blue shark (Prionace glauca), we used documented or inferred capture and post-release mortality rates to estimate total discard mortality, and tested the key assumptions underlying the use of observer data to estimate fishery-scale discards. Annual estimates of shark discards by fishery indicated that the swordfish/tuna fishery accounted for 58% of 57 mt of porbeagle discards, 70% of 23 mt of mako discards, and 99% of 1414 mt of blue shark discards in 2010. Aggregated across all fisheries, an estimated 29 mt of non-retained porbeagle died from fishing-related causes in 2010, which is equivalent to 35% of reported landings. A total of 11 mt of non-retained mako did not survive fishing in 2010, which is equivalent to 29% of the reported landings. Discarded blue sharks which did not survive fishing totalled 495 mt in 2010, which far exceeded landings. Based on tests of accuracy of the bycatch estimation method, the bycatch and discard amounts for these shark species are expected to be reasonably close to reality. Porbeagle bycatch is largely limited to Emerald Basin and the edge of the Scotian Shelf, but bycatches of mako and blue shark are more broadly representative of the distribution of the pelagic longline fishery in the northwest Atlantic.

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