Shortfin Mako Shark
(Isurus oxyrinchus)

Isurus oxyrinchus (Rafinesque, 1810)

Diagnosis

A supremely streamlined, spindle-shaped shark with a long, pointed snout, large eyes without nictitating eyelids and highly visible dentition (even when the jaws are virtually closed); teeth long, reverse-curved and dagger-like without serrations or basal cusplets.

Crescent-shaped caudal fin; strong caudal keels but without secondary keels on the ventral lobe. First dorsal fin rather tall; apex somewhat rounded; origin behind the pectoral fin free-tips. Pectoral fins shorter than the head (i.e., distance from snout to 5th gill slit). Anal fin minute, its origin about midway beneath the base of the equally small 2nd dorsal fin. Colour cobalt blue on the upper dorsum, meeting metallic steel-blue at a strong line of demarcation on the upper flanks; undersides white with a black blotch in the axil of the pectoral fin; the anterior and posterior pectoral fin margins in newborns is frequently whitish.

At all size-classes the junction of latero-dorsal steel-blue and ventral white pigmentation is not as clearly defined as in the demarcation line of Carcharodon carcharias;  however, after death the striking upper-body pigment quickly fades to grey and may appear superficially similar to that of white sharks. A notably dark form of mako, apparently of this species, is endemic to Azorean waters and has large areas of dorsal pigment intruding latero-ventrally at the snout and posterior body, rather similar in guise to the Longfin mako, Isurus paucus.


Size

To ca. 390 cm; generally to 340 cm and commonly to 280cm; size at birth 60 to 70 cm.


Status and Distribution

N.E. Atlantic: Abundant, common or occasional; cosmopolitan but with a marked seasonal bias favouring high summer occurrences in northern parts of range (where these sharks are sympatric with superficially-similar Porbeagles, Lamna nasus). In these circumstances, shortfin makos may occur sporadically, or with localised, short-term abundance, in north European waters off Ireland (mainly southern coasts), England (Cornwall, Devon and Scilly Isles; occasionally straying to Bristol Channel and Wales; very rarely penetrating eastwards of the Isle of Wight and extremely scarce nomads within the North Sea); infrequent, nominal outliers reported as far north as Scottish waters and Norway although the majority of annual occurrences are essentially limited south of Latitude 50N in summer and Lat. 45N at colder times of the year. Frequent or even abundant from this latitude southwards; Bay of Biscay, Portugal, Spain and southwards to the tropics, including all oceanic islands. In all parts of range, abundance of this oceanic shark generally increases as a function of greater distance offshore.

Mediterranean: Common and cosmopolitan within waters above 16C; ranges throughout the Mediterranean basins, principally in offshore waters. Regularly encountered in Southern Spanish waters, particularly from Valencia southwards; occasionally along the C�te d�Azur and more often in the Ligurian Sea and environs; frequent in southern-central Mediterranean (Sicily, Isole Pelagie and Malta) and Ionian Sea; less common in the Adriatic and rarely encountered in that region from Ancona northwards. Occurs within the entire Aegean Sea; Crete, Cyprus and Turkey, but scarcer in extreme south-eastern waters.


Biology

A highly active, usually offshore pelagic species, occurring from the surface down to at least 150 m. Found in blue waters far from land but occasionally closer inshore, particularly near islands or islets (e.g., Filfla Islet in Malta) lying adjacent to deepwater drop-off�s and near coastal reefs rich in prey-fish (e.g., Manacles, Cornwall). The swiftest of all sharks, shortfin makos are major predators of a wide range of pelagic fish, including mackerel, scad, tunas, bullet tunas, bonito and swordfish; occasionally taking demersal fish, other sharks, rays, cephalopods and apparently marine turtles.

It has been suggested that adult makos sporadically attack free-swimming dolphins and this thesis appears validated on the basis of an amateur video, taken in Pacific waters, that shows a moribund spotted dolphin (tail-stock almost severed) just after an attack by a shark. The video captures a few frames of a very large shortfin mako circling the bleeding dolphin. These sharks will also scavenge upon longlined and netted fish; also cetaceans (dolphins); large specimens have been observed to attack harpooned adult swordfish in the Straits of Messina.

Their predatory relationship to swordfish is poorly known, but it is clear from regional data that these sharks can find their prey a dangerous adversary, as demonstrated by recent cases (at Ganzirri and Isola Lipari, Sicily) of makos stranding dead or moribund with amputated swordfish bills impaled in their gills and heads. These cases have coincided with the late spring - early summer spawning cycle of swordfish, perhaps suggesting that shortfin makos target these fish at this time whilst the prey is vulnerable at the surface but are nevertheless prone to aggressive defensive attack by their quarry. Interestingly, a form of mako endemic to Azorean waters has unusually dark ventral pigmentation (see diagnosis, above) but in all other respects appears identical to I. oxyrinchus.

Moreno and Moron (1992) give a lengthier analysis of this interesting variation. Further genetic analysis of these so-called "Marrajo criole" may yield interesting results. Data compiled by the National Marine Fisheries Service (NMFS) at Rhode Island, USA, has given information on the movements and possible population-structure of Northern Atlantic shortfin mako stocks. Trans-Atlantic recaptures are nominal, inferring limited interchange between what appear to be two distinct western and eastern populaces. This inference has been further supported by genetic sampling of animals from both sides of the Atlantic.

Ovoviviparous; Litter size ranging 4 to 16. Females mature at ca. 280cmTL and males at ca. 195cm.