Porbeagle
Porbeagle | |
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Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Chondrichthyes |
Subclass: | Elasmobranchii |
Subdivision: | Selachimorpha |
Order: | Lamniformes |
Family: | Lamnidae |
Genus: | Lamna |
Species: | L. nasus
|
Binomial name | |
Lamna nasus (Bonnaterre, 1788)
| |
Confirmed range Suspected range
| |
Synonyms | |
Lamna philippii Perez Canto, 1886 |
The porbeagle or porbeagle shark (Lamna nasus) is a
The porbeagle is an opportunistic hunter that preys mainly on
Only a few
Taxonomy
The etymology of the word "porbeagle" is obscure. A common suggestion is that it combines "porpoise" and "beagle", referencing this shark's shape and tenacious hunting habits.[3] Another is that it is derived from the Cornish porth, meaning "harbor", and bugel, meaning "shepherd".[4] The Oxford English Dictionary states that the word was either borrowed from Cornish or formed from a Cornish first element with the English "beagle". The Dictionary also notes that no evidence exists for a connection to the French porc, meaning "swine", or to porpoise, as has been proposed.[5] Other common names for the porbeagle include Atlantic mackerel shark, Beaumaris shark, bottle-nosed shark, and blue dog.[3]
The first
Phylogeny and evolution
Several
Distribution and habitat
The porbeagle has an almost global amphitemperate distribution, i.e. it is absent from the tropics; in the North Pacific, its niche is assumed by the salmon shark. It is found mostly within 30–70°N and 30–50°S latitudes.[10] In the North Atlantic, the northern limit of its range extends from the Newfoundland Grand Banks off Canada, through southern Greenland, to Scandinavia and Russia; the southern limit of its range extends from New Jersey and Bermuda, through the Azores and Madeira, to Morocco. It is found in the Mediterranean Sea, but not the Black Sea. Normally, North Atlantic sharks only stray as far south as South Carolina and the Gulf of Guinea,[7] but pregnant females from the western North Atlantic population are known to range into the Sargasso Sea, almost as far as Hispaniola, to give birth.[17] In the Southern Hemisphere, the porbeagle apparently occupies a continuous band bound in the south by the Antarctic Convergence, and extending as far north as Chile and Brazil, the Western Cape province of South Africa, Australia to southern Western Australia and southern Queensland, and New Zealand.[7] In New Zealand it is most common south of the Cook Strait.[18] The porbeagle is thought to have colonized the Southern Hemisphere during the Quaternary glaciation (beginning around 2.6 Mya), when the tropical climate zone was much narrower than it is today.[12]
Offshore
Porbeagle populations in the Northern and Southern Hemispheres appear to be completely separate. Two stocks are in the North Atlantic, east and west, that seldom mix; only one individual is known to have crossed the Atlantic, covering 4,260 km (2,650 mi) from Ireland to Canada. Several discrete stocks are likely present in the Southern Hemisphere, as well. This species segregates by size and sex in the North Atlantic, and at least by size in the South Pacific. For example, males outnumber females 2:1 off Spain, females are 30% more numerous than males off Scotland, and immature males are predominant in the Bristol Channel. Older, larger sharks may frequent higher latitudes than younger individuals.[10]
Seasonal
Description
The porbeagle is a very stout-bodied shark with a fusiform (spindle-like) shape. The long, conical snout tapers to a sharp point, and is supported by enlarged, highly
The pectoral fins are long and narrow. The first dorsal fin is large and high, with a rounded apex, and originating just behind the pectoral fin bases. The
The dorsal coloration is a medium to dark gray or slate, extending to the bases of the pectoral fins. The underside is white; adults in the Southern Hemisphere often have dark coloring under the head and dusky blotches scattered over the belly. The free rear tip of the first dorsal fin is abruptly light gray or white, a feature unique to this species. The porbeagle may attain a length of 3.7 m (12 ft), though this is uncertain and may have resulted from confusion with other mackerel shark species. A more typical length is 2.5 m (8.2 ft).
Biology and ecology
Fast and energetic, the porbeagle can be found singly or in groups.
Porbeagles are among the few fishes that exhibit apparent
Feeding
The porbeagle is an active
In the western North Atlantic, porbeagles feed mainly on pelagic fishes and squid in spring, and on groundfishes in the fall; this pattern corresponds to the spring-fall migration of these sharks from deeper to shallower waters, and the most available prey types in those respective habitats. Therefore, the porbeagle seems to be an opportunistic predator without strong diet specificity.
Life history
The timing of the porbeagle's reproductive cycle is unusual in that it is largely similar in both hemispheres, rather than being offset by six months. This suggests that its reproduction is not significantly affected by temperature or day length, perhaps owing to its endothermic physiology.
Like other members of its family, the porbeagle is
At 20–21 cm (7.9–8.3 in) long, the embryo appears pink because it lacks pigment except in its eyes, and its head and gill regions are laterally enlarged and gelatinous. The yolk stomach can comprise up to 81% of the embryo's total weight when it is 30–42 cm (12–17 in) long. The embryo gains pigment and sheds its fangs at a length of 34–38 cm (13–15 in). Around this time, the mother stops producing ova. From then on, the embryo relies mainly on the yolk stored in its stomach, though it may continue to feed on remaining eggs by squishing the capsules between its jaws or swallowing them whole. It begins to transfer its energy stores from its stomach to its liver, causing the former to shrink and the latter to grow exponentially. The embryo is essentially fully pigmented by a length of 40 cm (16 in), and has assumed its newborn appearance by a length of 58 cm (23 in). By then, its stomach has shrunk enough for the abdominal muscles to close, leaving what has been termed an "umbilical scar" or "yolk sac scar" (neither is accurate). Several series of single-cusped teeth grow in both jaws, though they lie flat and remain nonfunctional until birth.[36][37]
Newborn porbeagles measure 58–67 cm (23–26 in) long and do not exceed 5 kg (11 lb). Up to a tenth of the weight is made up of the liver, though some yolk also remains in its stomach and continues to sustain the pup until it learns to feed.[10][36] The overall embryonic growth rate is 7–8 cm (2.8–3.1 in) per month.[36][37] Sometimes, one pup in a uterus is much smaller than the other, but otherwise normal. These "runts" may result from a dominant, forward-facing embryo eating most of the eggs as they arrive, and/or the mother being unable to provide an adequate egg supply for all her offspring.[36] Birthing occurs from April to September, peaking in April and May (spring-summer) for North Atlantic sharks and June and July (winter) for Southern Hemisphere sharks. In the western North Atlantic, birth occurs well offshore in the Sargasso Sea at depths around 500 m (1,600 ft).[20]
Both sexes grow at similar rates until the onset of maturation, with females maturing later and at a larger size than males.
Thermoregulation
Like other members of its family, the porbeagle is
Among sharks, the porbeagle's capacity for elevating body temperature is second only to the salmon shark's. Its red muscles are located deep within the body, adjacent to the
Human interactions
The porbeagle has very rarely, if ever, bitten swimmers or boats.
On 14 May 2018, a porbeagle shark was reported to have bitten a fisherman off the coast of Cornwall, United Kingdom, as it was being returned to the sea.[1]
At one time, porbeagles were regarded as nuisances by some commercial fishers because they damaged lighter fishing gear intended for smaller species and took hooked fish from lines.[7] This shark is highly valued as a game fish by sport fishers in Ireland, the United Kingdom, and the United States. It fights strongly on hook-and-line, but does not usually jump into the air like the related shortfin mako shark. Novice anglers often mistake this shark for the mako, which has earned it the affectionate moniker "fako" in New England.[21] The International Game Fish Association keeps records on the porbeagle.[7][21]
Commercial fishing
Prized for its
Intensive fishing for the porbeagle dates back the 1930s, when
As porbeagles had become scarce in the Northeast Atlantic, in the 1960s, the Norwegian fishing fleet moved west to the waters off New England and Newfoundland. A few years later, they were joined by longline vessels from the
In the Southern Hemisphere, commercial fishing for the porbeagle is mostly undocumented. Substantial numbers are caught incidentally by pelagic longline fisheries targeting more valuable species such as southern bluefin tuna (Thunnus maccoyii), swordfish (Xiphias gladius), and Patagonian toothfish (Dissostichus eleginoides), including vessels operated by Japan, Uruguay, Argentina, South Africa, and New Zealand. Porbeagle catches by the Uruguayan tuna pelagic longline fishery peaked in 1984 with 150 tons landed. Records of catch per unit effort for this fishery have shown a 90% decline in porbeagle landings from 1988 to 1998, though whether this reflects a real population decline or changing fishing habits is uncertain. New Zealand has reported annual catches of 150–300 tons, mostly of immature individuals, from 1998 to 2003.[1]
Conservation
The rapid collapse of porbeagle stocks on both sides of the North Atlantic is often cited as archetypal of the "boom and bust" pattern of most shark fisheries. Factors including a small litter size, long maturation time, and the capture of multiple age classes all contribute to this shark's susceptibility to
The porbeagle is listed on Annex 1 (Highly Migratory Species) of the
In March 2015, The
Southern Hemisphere
The only regulation of porbeagle catches in the Southern Hemisphere is New Zealand's total allowable catch of 249 tons per year, instituted in 2004.[1] In June 2018 the New Zealand Department of Conservation classified the porbeagle as "Not Threatened" with the qualifier "Threatened Overseas" under the New Zealand Threat Classification System.[60]
Eastern North Atlantic and Mediterranean Sea
In the eastern North Atlantic, porbeagle fishing has been illegal in waters of the European Union since 2011.
In the Mediterranean Sea, the porbeagle is on the verge of extinction, with a population decline over 99.99% since the mid-20th century. Its range has contracted to the waters around the Italian Peninsula, where a nursery area may exist. Only a few dozen specimens have been recorded in the past few decades, from scientific surveys, swordfish fishery bycatch, and sport fishers.[57][62] In 1995, it was included in Annex III ("species whose exploitation is regulated") of the Barcelona Convention Protocol on protected areas and biodiversity in the Mediterranean, which has not been ratified. In 1997, it was listed on Appendix III of the Bern Convention (the Convention on the Conservation of European Wildlife and Natural Habitats). However, these listings have yet to result in the implementation of new management plans, despite the recognized need for urgent action.[57]
The European Union prohibits EU vessels from fishing for, retaining, boarding, transhipping, or landing porbeagle sharks in all waters since January 2011.[50] It is subject to a zero total allowable catch in EU waters by any vessel.[63]
Western North Atlantic
The western North Atlantic porbeagle population has a more positive prospect than in the eastern North Atlantic. Fishing in Canadian waters was originally regulated by the 1995 Fisheries Management Plan for pelagic sharks in Atlantic Canada, which established an annual quota of 1,500 tons, restricted the time, place, and gear types allowed for commercial fishing, and set limits for bycatch and recreational fishing. In 2000–2001, Fisheries and Oceans Canada prepared a detailed population model and concluded that a quota of 200–250 tons would allow for population growth, resulting in a quota of 250 tons being adopted for the period of 2002–2007. The mating grounds off Newfoundland were also closed to shark fishing. In 2004, the Committee on the Status of Endangered Wildlife in Canada listed the porbeagle as endangered, largely on the basis of the low population abundance (<25% of original numbers). Canada decided not to list the species under its Species At Risk Act, but further reduced the total fishing quota to 185 tons.[53] In US waters, the 1993 Fishery Management Plan for Sharks of the Atlantic Ocean sets an annual quota of 92 tons (after processing) for the porbeagle.[52] In 2006, this species was listed as a species of concern by the NMFS, meaning that it merits conservation concern, but data are insufficient for inclusion on the US Endangered Species Act.[64]
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