Shortfin mako (Isurus oxyrinchus) COSEWIC assessment and status report: chapter 6

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Biology

Unless otherwise noted, biological information is from Compagno (2001).

Life cycle and reproduction

The life cycle of shortfin mako is incompletely known. Shortfin makos are ovoviviparous. Developing embryos feed on unfertilized eggs during the 15-18 month gestation period. After parturition it is thought that females may rest for 18 months and therefore the breeding cycle may be three years (Mollet et al. 2000). Birth can occur from late winter to mid-summer with typically 4-25 free-swimming young born at about 70 cm in length.  Females mature at total lengths of 2.7-3.0 m and males at 2.0-2.2 m corresponding to a minimum age of 7-8 for both sexes. Size is a better indicator of reproductive maturity than age. In the northwest Atlantic it was found that females up to 272 cm fork length were immature despite being between 7-18 years old (Campana et al. 2004). Maximum size is about 4.2 m.

Based on length frequency data collected in Canadian waters by the International Observer Program (IOP), it is clear that mature animals are either rare in Canada or do not encounter commercial fishing gear. A coarse approximation of the percentage of mature males and females encountered by the commercial fishing fleet is estimated at 10-23% (n=1054) and 0.2-0.6% (n=850) respectively based on length data collected by the IOP from 1980-2003. Of the 1904 observations, only five were mature females. The range of percentages was calculated assuming a length of maturity (total length) of 200-220 cm for males and 270-300 cm females. Size frequency distributions of commercially caught males and females from waters around Newfoundland and Nova Scotia (1979-2002) are shown in Figure 6 (Campana et al. 2004).

Longevity of shortfin makos in Northwest Atlantic based on vertebral cross-sections has been estimated by Campana et al. (2002) to be at least 24 years, with a theoretical maximum age using von Bertalanffy’s growth equation of 28 years (Smith et al. 1998), and 45 years (Cailliet and Bedford 1983).  The instantaneous rate of natural mortality has been estimated at 0.16 (Smith et al. 1998). Using the combination of natural mortality rate and age of first reproduction (8 years), a conservative (minimum) generation length of 14 years is estimated (GL=8+1/0.16=14).

Predation

Shortfin mako have few natural predators due to their relatively large size and ability for high speed. Evidence of predation by white sharks has been observed off California, in the Mediterranean Sea and off South Africa. Makos have been caught with visible scars and injuries from interactions with sailfish and swordfish. The primary predator of shortfin makos are humans in the form of directed and incidental fishing mortality.

Physiology

Shortfin makos are endothermic, with body temperatures 1-10°C above ambient water temperature. This adaptation is advantageous in maintaining elevated swimming, visual, central nervous system and digestive functions when the makos are in cooler waters in higher latitudes, as is the case with those in Canadian waters, or when diving below the thermocline in warm waters.

Dispersal/migration

Shortfin makos have been shown to be highly migratory as determined from conventional tagging studies (Kohler et al. 1998) and more recently from genetic evidence (Schrey and Heist 2003). Based on both thermal preference and highly mobile behaviour, it is unlikely that shortfin makos have extended residency in Canadian waters. Shortfin makos (N_tagged=3457, N_returned=320) tagged and recaptured between 1962-1993 in the northwestern Atlantic coast demonstrated a wide range of movement patterns (Figure 3).  Individuals show potential for considerable movement at the scale of ocean basins but most tag recoveries were within 500 km of the tagging location.

Interspecific interactions

Shortfin makos prey upon a wide variety of species, primarily fish including tunas, mackerels, bonitos, and swordfish. Squid are also an important component of their diet. Prey items are typically 10-35% of the size of the predator. There is some suggestion that larger makos shift towards consuming larger prey including other sharks, small cetaceans and turtles. In the western North Atlantic, bluefish are considered to be the most important prey, comprising 78% of their diet (Stillwell and Kohler 1982).

Adaptability

This species is likely well adapted to withstand natural changes in its environment (i.e., change in prey type or temperature), as it can readily move long distances and prey upon a wide variety of species. Furthermore, the species is distributed over a large area, thereby reducing its susceptibility to small stochastic events. Their endothermic physiology allows the animal to remain highly active in cooler waters.