Porbeagle (Lamna nasus)
|Also known as:||Atlantic mackerel shark, Beaumaris shark, blue dog, bottle-nosed shark, mackerel shark|
|Spanish:||Marrajo Sardinero, Tiburón Sardinero, Tintorera|
|Size||Length: 300 cm (2)|
Classified as Vulnerable (VU) on the IUCN Red List (1). The Northeast Atlantic and Mediterranean subpopulations are classified as Critically Endangered (CR) and the Northwest Atlantic subpopulation is classified as Endangered (EN) on the IUCN Red List (1).
The porbeagle is a stout, heavy shark with a pointed, conical snout and a crescent-shaped caudal, or tail, fin. Its name is thought to arise from a combination of ‘porpoise’ referring to its shape, and ‘beagle’ referring to its hunting ability (3). It is dark bluish-grey to bluish-black on top, and white underneath (4). The first dorsal fin is large and triangular and has a white patch on the back of it, whilst the second dorsal fin is very small. The large pectoral fins, used for balancing and breaking, are situated behind long gill slits (2) (4). The porbeagle, which has moderately large blade-like teeth (2), is related to the much-feared shortfin mako and white shark, but seldom, if ever, attacks humans (3).
The porbeagle occurs in the Atlantic Ocean in the northern hemisphere, and has a circumglobal distribution in the southern hemisphere; through the southern Atlantic, southern Indian Ocean, southern Pacific and Antarctic Ocean (2) (3).
The porbeagle generally inhabits waters over continental shelves, but is also found far from land in ocean basins and occasionally close inshore, at depths less than one meter down to at least 700 metres. It prefers waters colder than 18°C but greater than 1°C (2).
This active shark wanders about the ocean, often aggregating in small schools (2). It is an opportunistic hunter that swims strongly in pursuit of prey, and on calm days their triangular dorsal fin followed by the tip of the caudal fin can be seen cutting through the waters’ surface (4). It feeds primarily on a wide range of bony fish such as mackerel, hake, herring and haddock, but cephalopods, such as squid and cuttlefish, are also consumed (2) (3). The porbeagle has been known to annoy fishermen by scavenging cod and other fish from longlines (4).
When porbeagles mate, the male bites the female to hold her in place while they copulate (3). They are ovoviviparous sharks, and thus embryos develop within the female’s uterus without forming a placental connection. Instead, the foetuses obtain nutrition by feeding on fertilised eggs within the uterus (2) (4). They possess fang-like teeth to tear open the egg capsules (2), and their stomachs become greatly swollen by feeding on the masses of yolk (2) (4). The gestation period is thought last for about eight or nine months, with litters of one to five pups born in the spring and summer in the northern hemisphere, or between April and September in the southern hemisphere (2).
The greatest threat to porbeagles comes from the fisheries industry. It has been targeted by commercial fisheries in the North Atlantic and Mediterranean and is frequently caught as by-catch in the southern hemisphere. The porbeagle is utilised for human consumption, for oil and fishmeal and for fins for sharkfin soup (2). This exploitation has had a significant impact on porbeagle populations; the porbeagle has virtually disappeared from Mediterranean records (1), and abundance of the northwest Atlantic population in 2004 was at a record low (5). It is estimated that the northwest Atlantic population may take a minimum of several decades to recover from its current low abundance (5). At present there is no evidence to indicate that the decline in porbeagle abundance has ceased (5), and yet regulated Norwegian, Canadian and New Zealand fisheries for porbeagles still continue (2).
Catches of porbeagle are regulated by the European Community, which permits only a small regulated catch for Norway and New Zealand (3). In Canada, a porbeagle shark management plan was developed in the 1990’s (5), which includes measures such as limited fishing licenses, gear, fishing areas and seasons for the porbeagle (3). However, it is uncertain whether these measures to reduce exploitation are sufficient to allow for recovery of the northwest Atlantic population (5). In the United States the porbeagle is currently included in the Highly Migratory Species Fishery Management Plan, and there is an annual quota in place. Except for the New Zealand fishery, there is no regulation on catches of the porbeagle in the southern hemisphere (3).
At present, there is also no regulation of the international trade demand for meat that is driving many fisheries (1). In June 2007, at the meeting of the Conference of CITES Parties in June 2007, a proposal was raised to list the porbeagle under Appendix II, which would mean that trade in this species would be controlled in order to ensure it was compatible with their survival. Unfortunately, the proposal fell short of the votes needed to adopt this listing, leaving this heavily traded species vulnerable to further overexploitation (6).
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- By-catch: in the fishing industry, the part of the catch made up of non-target species.
- Caudal fin: the tail fin of a fish.
- Cephalopods: from the Greek for ‘head-foot’, a class of molluscs that occur only in marine habitats. All species have grasping tentacles, and either an internal or external shell. Includes nautiloids, cuttlefish, squids, octopuses, and extinct ammonites and belemnites.
- CITES: Convention on International Trade in Endangered Species.
- Dorsal fin: the unpaired fin found on the back of the body of fish, or the raised structure on the back of most cetaceans.
- Ovoviviparous: a method of reproduction whereby the egg shell is weakly formed and young hatch inside the female; they are nourished by their yolk sac and then ‘born’ live.
- Pectoral fins: the pair of fins that are found one on each side of the body just behind the gills. They are generally used for balancing and braking.
IUCN Red List (December, 2009)