Research Document 2017/039

Current Status and Threats to the North Atlantic Shortfin Mako Shark (Isurus oxyrinchus) Population in Atlantic Canada

By Showell, M.A., Fowler, G.M., Joyce, W., McMahon, M., Miri, C.M., and Simpson, M.R.

Abstract

The Shortfin Mako Shark (Isurus oxyrinchus) is widely distributed pan-globally in temperate waters of the Atlantic, Pacific and Indian oceans.  Although there is a small degree of mixing between areas, genetic studies indicate Shortfin Mako in the North Atlantic to be a discrete population. Both conventional and satellite tagging studies show Shortfin Mako to be highly migratory and widely distributed in the North Atlantic.  Individuals tagged in Canadian waters travel long distances outside Canada’s Exclusive Economic Zone and there is no evidence of year-round residency in Canadian waters.

Shortfin Mako Shark exhibit life history characteristics typical of Elasmobranchs. Age at maturity is high, particularly for females, fecundity is low at 4 to 17 pups per litter and estimates of gestation period are around 20 months. Based on estimates of natural mortality, generation time is estimated to be 26 to 30 years. The low productivity resulting from these characteristics makes this species vulnerable to over-exploitation.

There are no fishery independent indices of abundance for Shortfin Mako sharks in Canadian waters. A standardized catch rate index based on at-sea observer data shows a modest decline in recent years, but has high variability and may represent local changes in distribution rather than population abundance. A number of catch rate series reported for fleets outside Canadian waters show no trend or modest increases in recent years.

Most Shortfin Mako landings in Canadian waters are bycatches reported by Maritimes Region fishing vessels, primarily by the pelagic longline fleet.  Landings data underestimate actual catches for this species, as about 30% of the total catch is discarded at sea. Of the sharks discarded, many will die but a proportion will survive.  Here at-sea observer data have been extrapolated to calculate estimates of both discards and post release mortality by fleet.  Combined with reported landings, this provides an estimate of total fishing related mortality. However, even when adjusted for these factors, the Shortfin Mako catch in Canadian waters is less that 2% of the total reported for the North Atlantic.

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