By Dr Ben Diggles
THE recent debate regarding listing of longfin mako, shortfin mako and porbeagle sharks as migratory species under international environmental laws has put them under the fisheries management spotlight. Let’s have a brief look at some of the biological facts surrounding these open ocean speedsters, all of which are members of the family Lamnidae of warm blooded, pelagic sharks (and are hence closely related to the great white shark).
The shortfin mako (Isurus oxyrinchus) is the largest and most commonly encountered of the three species, being found throughout the world in tropical, subtropical and temperate oceanic waters. This species grows to around four metres long and over 500 kilos and is reasonably long lived, with the age at maturity for males being 7–9 years old at around 1.8-2m long. Females are thought to mature much later, even as old as 19–21 years at lengths of 2.7 to 3m. Female shortfin makos are thought to birth their live young every three years or so, after a gestation period of 15–18 months. They give birth to between four and 30 pups.
The porbeagle (Lamna nasus) also has a circumglobal distribution, but is occasionally found close inshore. In Australian waters, the porbeagle is occasionally encountered by those fishing offshore from southern Queensland along the entire southern coastline to southern Western Australia. Porbeagles grow to a maximum size of around 3.6m and 230kg, and possibly live to as much as 65 years of age (in lightly fished populations). The length at first maturity in the southern hemisphere populations of porbeagles ranges between 140-150 cm fork length for males (around 8-11 years old) and 170-180cm for females (13 to 18 years old). After a gestation period of 8-9 months, female porbeagles give birth to 1-5 pups which are 65 to 80cm long when born.
The longfin mako (Isurus paucus) is widely distributed throughout tropical and subtropical oceans worldwide, but is a rarely encountered species. The longfin mako grows to around 4.3m long and is often confused with the shortfin mako. However, the longfin mako has relatively longer fins, a stouter body, and tends to live in deeper waters. The longfin mako is found in Australian waters with a distribution similar to that of the porbeagle shark.
All three species are highly active sharks which have counter current heat exchanger systems which keep their blood warmer than the surrounding water. There is evidence to suggest that stocks of these species have been overfished in the waters of the North Atlantic and North Pacific. However, there is currently no evidence of overfishing in Australian waters, and it seems likely that southern hemisphere populations of these species have little, if any, connectivity to the northern hemisphere populations that are in trouble.
