The biology and fishery of shortfin mako sharks (Isurus oxyrinchus) inAtlantic Canadian waters

摘要

Shortfin mako sharks are a high-value bycatch of pelagic longline fisheries off the eastern coast of Canada. Tagging studies indicate that they are highly migratory, seasonal residents of Canadian waters, representing the northern extension of a North Atlantic-wide population centred at more southerly latitudes. Annual catches in Canadian waters average 60-80 mt per year, which represents but a small part of that estimated for the population as a whole. New ageing results indicate that the species grows more slowly than was reported previously, thus making the population less productive and more susceptible to overexploitation than has been reported. Two indices of population abundance did not provide a definitive view of mako shark population status. A standardized catch rate index from the commercial large pelagic fishery suggested stable abundance since 1988. However, the analysis did not have the statistical power to detect anything less than a severe decline. In contrast, the median size of mako sharks in the commercial catch has declined since 1998, suggesting a loss of larger sharks. These results are broadly consistent with a previous report of population decline, although it appears unlikely that current exploitation rates in Canada are having an appreciable impact on the population.