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

Abstract

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.

Introduction

The shortfin mako shark (Isurus oxyrinchus) is a large temperate and tropical pelagic shark species of the family Lamnidae that occurs in the Atlantic, Pacific and southern oceans (Compagno, 2001). In Canadian waters the shortfin mako shark is most closely associated with the warm waters of the Gulf Stream. It has been recorded from Georges and Browns Bank, along the continental shelf of Nova Scotia, the Grand Banks and even into the Gulf of St. Lawrence (Templeman, 1963). The species is highly migratory, with tagging results suggesting that there is a single well-mixed population in the North Atlantic (Casey and Kohler, 1992). Atlantic Canada represents the northern extension of the species range, and most of the population is believed to reside in more southerly waters. Although these sharks are not abundant in Canadian waters, neither are they uncommon.

Little is known of the biology or fishery for this shark anywhere in the world, in part reflecting the difficulty of capturing or studying a large, fast-swimming marine predator. Dietary studies confirm that this is an apex predator, feeding mainly upon large teleosts such as tunas and swordfish, as well as other sharks, marine mammals and sea turtles (Stillwell and Kohler, 1982, Compagno, 2001). Females become sexually mature at a length of 2.7–3.0 m total length (TL), while males mature at 2.0–2.2 m TL (Pratt and Casey, 1983, Mollet et al., 2000). Developing embryos feed on unfertilized eggs in the uterus during the gestation period of 15–18 months. The 4–25 surviving young are born as free-swimming sharks in the late winter and early spring at a length of about 70 cm TL, and have no placental connection during development. Females may rest for 18 months after birth before the next batch of eggs is fertilized (Mollet et al., 2000). The combination of low fecundity and late age at sexual maturation results in the shortfin mako having low productivity compared to teleosts. This low productivity is typical of elasmobranch species and has prompted many authors to question the sustainability of intensive shark fisheries (Cortés, 1998, Walker, 1998, Musick, 1999, Stevens et al., 2000).

Recent years have seen an increasing number of countries discuss legislation to protect endangered elasmobranch stocks, highlighted by the Food and Agriculture Organization (FAO) (1998) recently-released International Plan of Action for the Conservation and Management of Sharks. The latter is based on the conclusion that many of the world's shark species are severely depleted (FAO, 1998). However, the health of shortfin mako populations has never been properly assessed. Based on an analysis of U.S. pelagic longline logbook information from vessels fishing in the NW Atlantic, Baum et al. (2003) suggested that the North Atlantic population had declined since 1986. An initial attempt to prepare a North Atlantic-wide stock assessment of shortfin makos also suggested that the population may have declined, but the assessment was hampered by poor data quality, and the conclusion was considered to be very provisional (ICCAT, 2004). The status of the mako shark population and fishery in Canadian waters has never been evaluated (O’Boyle et al., 1996).

Since 1995, fisheries management plans in Atlantic Canada have maintained non-restrictive catch guidelines of 250 mt annually for mako sharks in the directed shark fishery. The non-restrictive catch guidelines were not based upon estimates of stock abundance. Fishing gears to be used in the directed fishery were limited to longline, handline or rod and reel gear for commercial licenses and to rod and reel only for recreational licenses. The recreational fishery was restricted to hook and release only. No catch restrictions were put on sharks caught as bycatch in large pelagic fisheries. A ban on “finning” sharks (the removal of the fins and at-sea disposal of the finless carcass) was implemented in June 1994. Full details of the Canadian shark management plan are presented in Campana et al. (2002a).

The objective of the current analysis was to provide a detailed view of shortfin mako distribution, biology, fisheries and size composition in Atlantic Canadian waters. Also included is an index of abundance based on a standardized catch rate model, and a perspective on the relationship of the Canadian mako fishery to that of the North Atlantic population.