Shortfin mako (Isurus oxyrinchus) COSEWIC assessment and status report: chapter 3

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Species Information

Name and classification

Shortfin mako (Isurus oxyrinchus) is one of two species in the genus Isurus (the other being the longfin mako, Isurus paucus) and one of five species in the family Lamnidae or mackerel sharks. Other lamnid sharks found in Canada include the white shark (Carcharodon carcharias), salmon shark (Lamna ditropsis), and the porbeagle shark (Lamna nasus). There are no recognized sub-populations of shortfin mako, although there is some genetic variability within the species. Based on colouration it has been suggested that a population around the Azores may represent a distinct local population worthy of ‘population variant’ status (Compagno 2001). Molecular work is required to confirm this possibility. In French this species is known as Requin-taupe bleu.

Morphological description

Description modified from Compagno (2001). Shortfin makos are identified by a pointed snout, relatively small eyes, and a U-shaped mouth (Figure 1). The lower anterior teeth protrude horizontally on jaws even when mouth is closed. The body is moderately slender but more fusiform than the closely related Isurus paucus. Pectoral fins are slightly curved with tips relatively narrow, anterior margins about 16 to 22% of total length and shorter than head length (the pectoral fins of the longfin makos are equal to or longer than the head length). Origin of first dorsal fin over or just behind the pectoral free rear tip; first dorsal-fin apex broadly rounded in young but more angular and narrowly rounded in large juveniles and adults; first dorsal-fin height greater than base length in large individuals but equal or smaller in young below 185 cm. The dorsolateral colouration is brilliant blue or purplish, with white below the underside of snout in young and adults. The head is dark in colour and partially covering the gill septa. The dark colour of their flanks does not extend ventrally onto the abdomen; the pelvic fins are dark on anterior halves, white on posterior halves, with the undersides white.

In the field, shortfin makos are identified by a spindle-shaped body, a long, acutely conical snout, large blade-like teeth, narrow-tipped pectoral fins, a large first dorsal fin and minute, pivoting second dorsal, a crescentric caudal fin, and ventral surface of body that is usually white.

Misidentifications have occurred in warmer waters where the two mako species ranges commonly overlap; however, in Canada where longfin makos are extremely rare, misidentification between the two species is not believed to be a problem. In Atlantic Canada, shortfin makos have been misidentified as porbeagle shark (Campana et al. 2004).

Genetic description

Heist et al. (1996) used mitochondrial DNA (mtDNA) to analyze population structure in shortfin mako from both the Atlantic and Pacific. They found that the north Atlantic samples differed substantially from samples from the south Atlantic and north and south Pacific (overall F_ST = 0.15), and the authors concluded that the north Atlantic samples experience very restricted gene flow from other areas and therefore may warrant separate management consideration. Schrey and Heist (2003) investigated microsatellite (nuclear) DNA of shortfin mako at four loci using 433 samples from the North Atlantic, South Atlantic, North Pacific, South Pacific, and the Atlantic and Pacific coasts of South Africa. This latter study found very low levels of differentiation even among the major ocean basins (global F_ST < 0.003) and only a weak basis for rejecting the hypothesis that shortfin mako comprise a single global population; under one mutation model the P value was slightly less than 0.05, whereas under another mutation model the P value was slightly above 0.05. Power analysis indicated very high power to detect population structure at the level indicated by the mtDNA study. Schrey and Heist (2003) suggested that one way to explain both datasets is that females are strongly philopatric (hence the strong differences at the maternally inherited mtDNA) but males are good dispersers (hence at best weak differentiation at nuclear DNA makers). This pattern has been reported in other shark species.

Tagging studies indicate that individuals are capable of transoceanic migrations. A small Canadian tagging program applied a total of 110 tags to shortfin makos between 1961-1980 (Burnett et al. 1987). There were only 5 recaptures from this study, which showed that some of the sharks migrate between continental shelf and offshore (non-shelf) waters, and between Canadian and U.S. waters (Figure 2). The U.S. National Marine Fisheries Service (NMFS) between 1962-1993 applied 3,457 tags to shortfin makos in U.S. and international waters, of which 320 tags were recovered (Figure 3, Kohler et al. 1998). Most of the recaptures were made in U.S. waters, where fishing effort is highest, but it was also shown that many of the shortfin makos migrated over long distances (>500 km). For the purposes of this report the Atlantic Canada population is considered to be part of the entire North Atlantic population.

Designatable units

Based on biogeographical separation and genetic differences, Shortfin mako populations in Canada’s Atlantic and Pacific should not be considered part of the same designatable unit. From a biogeographical perspective, the continental landmass of North and South America combined with the shortfin mako’s preference for subtropical and tropical waters limits the amount of exchange between ocean basins. Based on a single genetic study by Schrey and Heist (2003) (described above), there is some level of differentiation between the North Atlantic and North Pacific.

Although Canadian waters are near the periphery of the species’ range in the North Atlantic, shortfin mako are commonly encountered in the Canadian Atlantic and represent a designatable unit. In the Pacific, the species is represented by a single observation near the periphery of Canadian waters, which suggests that the population should not be considered eligible for assessment by COSEWIC. Since most records of this species are associated with pelagic longline fisheries, which are currently absent in waters of the Canadian Pacific, it is possible that the species is more abundant in Canadian waters than the record indicates; however, there are regular records of other large pelagic sharks (e.g. blue shark) in Canada’s Pacific waters.