Research Document - 2004/069
Influence of Recreational and Commercial Fishing on the Blue Shark (Prionace Glauca) Population in Atlantic Canadian Waters
By Campana, S.E., Marks, L., Joyce, W.,
Kohler, N.
Abstract
The nominal catch of blue sharks in the Canadian Atlantic grossly underestimates the actual catch mortality. Based on new estimates of bycatch mortality, the sum of landed catch and bycatch mortality in the Canadian Atlantic has averaged about 1000 mt annually since 1986.
Most of the tagged blue sharks recaptured in our waters were tagged in U.S. waters. In contrast, most of the tags applied in Canadian waters were later recaptured in the central and eastern Atlantic, as far away as Africa. Overall, the tagging studies were consistent with the view that blue sharks are highly migratory, with no evidence of extended residency in Canadian waters.
The weight of sharks landed at recreational shark tournaments has increased from around 4 mt in 1993 to around 20 mt in recent years. Blue sharks accounted for 99% of all sharks landed. The recreational catch and release fishery catches another 13 mt annually. Mature females were not present in the catches due to their absence from Canadian waters. However, the size composition at the derbies was not representative of the population: small sharks were poorly represented (due to derby catch restrictions) and large males were over-represented (due to their being targeted by derby participants).
Some members of the public have expressed concern that recreational shark fishing tournaments are having an adverse effect on the blue shark population. The results of this analysis indicate that shark tournaments only account for about 3% of the fishing mortality in Canadian waters, and thus are having a negligible effect on population abundance and overall mortality. The ethical questions surrounding shark tournaments were not addressed by this analysis.
Several indices of population health suggest that blue shark abundance has declined, and mortality has increased, in the past decade. Median size in the catch has declined, as have standardized catch rates from both commercial longline fisheries and recreational shark tournaments. These declines are consistent with an unpublished analysis of U.S. logbooks, but of smaller magnitude than that reported in a widely-reported article in Science.
Catch curve analysis suggests a very high fishing mortality on the population as a whole. However, Petersen analysis of tag recaptures indicates that the exploitation rate in Canadian waters was < 1%.
Two independent approximations of total North Atlantic blue shark catch mortality, based on bycatch ratios and mortality estimates, suggest North Atlantic catches of more than 100,000 mt and catch mortalities of between 26,000 and 37,000 mt. As first approximations, they probably provide conservative estimates of actual blue shark catch mortality in the North Atlantic. As such, the Canadian contribution to overall population mortality is very low.
Blue sharks have low commercial value and are discarded in great numbers by commercial pelagic fisheries. Life table analysis indicates that blue shark populations are both productive and resilient compared to other shark species, a fact which may help explain their persistence in the face of a high overall catch mortality and a decline in relative abundance. Nevertheless, careful monitoring of population status appears to be warranted.
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