Research Document - 2004/094

Biology, Fishery and Stock Status of Shortfin Mako Sharks (Isurus oxyrinchus) in Atlantic Canadian Waters

By S. E. Campana, Marks, L., Joyce, W.

Abstract

Shortfin makos are a high-value bycatch of pelagic longline fisheries off the eastern coast of Canada. Annual catches in Canadian waters average 60-80t per year. Both Canadian and U.S. tagging studies indicate that makos are highly migratory, seasonal residents of Canadian waters, representing the northern extension of a North Atlantic-wide population centred at more southerly latitudes. Therefore, Canadian catches represent but a small part of that estimated for the population as a whole.

The two indices of population abundance examined in this analysis did not provide a consistent 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.

It is widely recognized that elasmobranchs are unproductive compared with teleosts, largely as a result of their low fecundity and late age at sexual maturation. Published results suggest that makos are somewhat more productive than many other sharks. However, this conclusion was based in part on growth studies which have recently been discredited; the results presented here and elsewhere indicate that makos grow more slowly than was previously reported. Nevertheless, the more rapid growth and greater fecundity of makos compared to porbeagles implies that makos should be somewhat more resilient to exploitation than are porbeagles (which are severely overexploited in Canadian waters).

In summary, shortfin makos in Atlantic Canadian waters represent the margins of the distribution of the population, and are fished most heavily outside of the Canadian EEZ. Given the bycatch nature of the fishery in Canada, it appears unlikely that current exploitation rates in Canada are having an appreciable impact on the population. Nevertheless, there are some signs that the population is declining in abundance, implying that continued monitoring is warranted.

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