| Porbeagle | |
|---|---|
| Conservation status | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Elasmobranchii |
| Order: | Lamniformes |
| Family: | Lamnidae |
| Genus: | Lamna |
| Species: | L. nasus |
| Binomial name | |
| Lamna nasus (Bonnaterre, 1788) |
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| Confirmed (dark blue) and suspected (light blue) range of the porbeagle | |
| Synonyms | |
|
Lamna philippii Perez Canto, 1886 |
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The porbeagle (Lamna nasus) is a species of mackerel shark in the family Lamnidae, distributed widely in the cold and temperate marine waters of the North Atlantic and Southern Hemisphere. In the North Pacific, its ecological equivalent is the closely related salmon shark (L. ditropis). The porbeagle typically reaches 2.5 m (8.2 ft) in length and a weight of 135 kg (300 lb); North Atlantic sharks grow larger than Southern Hemisphere sharks and differ in coloration and aspects of life history. Gray above and white below, the porbeagle has a very stout midsection that tapers towards the long, pointed snout and the narrow base of the tail. It has large pectoral and first dorsal fins, tiny pelvic, second dorsal, and anal fins, and a crescent-shaped caudal fin. The most distinctive features of this species are its three-cusped teeth, the white blotch on the back of its first dorsal fin, and the two pairs of lateral keels on its tail.
Preying mainly on bony fishes and cephalopods, the porbeagle is an opportunistic hunter that regularly moves up and down in the water column, catching prey in midwater as well as off the bottom. It is most commonly found over food-rich banks on the outer continental shelf, but does make occasional forays close to shore or into the open ocean, down to a depth of 1,360 m (4,460 ft). It also conducts long-distance seasonal migrations, generally shifting between shallower and deeper water. The porbeagle is fast and highly active, with physiological adaptations that enable it to maintain a higher body temperature than the surrounding water. It can be solitary or gregarious, and has been known to perform seemingly playful behavior. This shark is aplacental viviparous with oophagy, meaning that the developing embryos are retained within the mother's uterus and subsist on non-viable eggs. Females typically bear four pups every year.
Only a few shark attacks of uncertain provenance have been attributed to the porbeagle. It is well regarded as a game fish by recreational anglers. The meat and fins of the porbeagle are highly valued, which has led to a long history of intense human exploitation. However, this species cannot sustain heavy fishing pressure due to its low reproductive capacity. Direct commercial fishing for the porbeagle, principally by Norwegian longliners, led to stock collapses in the eastern North Atlantic in the 1950s, and the western North Atlantic in the 1960s. The porbeagle continues to be caught throughout its range, both intentionally and as bycatch, with varying degrees of monitoring and management. The International Union for Conservation of Nature (IUCN) has assessed the porbeagle as Vulnerable worldwide, and as either Endangered or Critically Endangered in different parts of its northern range.
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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.[2] Another is that it is derived from the Cornish porth, meaning "harbor", and bugel, meaning "shepherd".[3] The Oxford English Dictionary states that the word was either borrowed from Cornish or formed from a Cornish first element with the English "beagle"; however, none of the proposed Cornish root words are fully satisfactory. The Dictionary also notes that there is no evidence for a connection to the French porc, meaning "swine", or to porpoise, as has been proposed.[4] Other common names for the porbeagle include Atlantic mackerel shark, Beaumaris shark, bottle-nosed shark, and blue dog.[2]
The first scientific description of the porbeagle was authored by French naturalist Pierre Joseph Bonnaterre in the 1788 Tableau encyclopédique et methodique des trois règnes de la nature, and based on an earlier 1769 account by Welsh naturalist Thomas Pennant. Bonnaterre named the shark Squalus nasus, the specific epithet nasus being Latin for "nose".[2][5] In 1816, French naturalist Georges Cuvier placed the porbeagle into its own subgenus, Lamna, which later authors elevated to the rank of full genus.[6]
Several phylogenetic studies, based on morphological characters and mitochondrial DNA sequences, have established the sister species relationship between the porbeagle and the salmon shark (L. ditropis),[7][8] which replaces it in the North Pacific.[9] The genus Lamna evolved 65–45 Ma. When its two extant species diverged from each other is uncertain, though the precipitating event was likely the formation of the ice cap over the Arctic Ocean, which would have isolated sharks in the North Pacific from those in the North Atlantic.[10][11]
Fossilized porbeagle remains are known from Late Miocene epoch (c. 7.2 Ma) deposits in Belgium and the Netherlands, Pliocene epoch (5.3–2.6 Ma) deposits in Belgium, Spain, and Chile, and Pleistocene epoch (2.6 Ma to 12,000 BP) deposits in the Netherlands.[12][13][14] However, Lamna teeth that closely resemble those of the porbeagle have been found in the La Meseta Formation on Seymour Island off the Antarctic Peninsula, which date to the middle to late Eocene epoch (50–34 Ma). There is much taxonomic confusion regarding Lamna in the fossil record due to the high degree of variability in adult tooth morphology within species.[13][15]
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.[9] 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,[6] 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.[16] 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.[6] It is speculated that the porbeagle colonized the Southern Hemisphere during the Quaternary glaciation (beginning c. 2.6 Ma), when the tropical climate zone was much narrower than it is today.[11]
Offshore fishing banks are the favored habitat of the porbeagle, though it can be found anywhere from a depth of 1,360 m (4,460 ft) in oceanic basins to littoral (close to shore) waters less than 1 m (3.3 ft) deep, over the entire water column.[6][17][18] There is a single, anomalous record from brackish water, of a juvenile in Mar Chiquita in Argentina.[19] A tracking study off the British Isles has found substantial variation in the short-term movements of this species, both between and within individuals. Vertical movements tended to increase with water depth and corresponding temperature stratification: in shallow, unstratified waters, sharks either showed no pattern in changing depth or made reverse diel movements, spending the day in shallow water and descending at night. In deeper, stratified waters, the sharks performed a regular diel migration, spending the day below the thermocline and rising towards the surface at night.[20] The porbeagle favors water temperatures of 5 to 10 °C (41 to 50 °F), though it has been reported across a temperature range of 1 to 23 °C (34 to 73 °F).[6][21]
Porbeagle populations in the Northern and Southern Hemispheres appear to be completely separate. There are two stocks 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.[9]
Seasonal migrations have been observed in porbeagles from both hemispheres. In the western North Atlantic, much of the population spends the spring in the deep waters of the Nova Scotia continental shelf, and migrates north a distance of 500–1,000 km (310–620 mi) to spend late summer and fall in the shallow waters of the Newfoundland Grand Banks and the Gulf of St. Lawrence.[9][16][22] In December, large mature females migrate south over 2,000 km (1,200 mi) into the Sargasso Sea for pupping, keeping deeper than 600 m (2,000 ft) during the day and 200 m (660 ft) at night so as to stay in the cooler waters beneath the Gulf Stream.[18] In the eastern North Atlantic, porbeagles are believed to spend spring and summer in shallow continental shelf waters, and disperse northwards to overwinter in deeper waters offshore.[20] Migrating sharks may travel upwards of 2,300 km (1,400 mi), though once they reach their destination they tend to remain within a relatively localized area.[9][18][20] In the South Pacific, the population shifts north past 30°S latitude into subtropical waters in winter and spring, and retreats south past 35°S latitude in summer, when sharks are frequently sighted off subantarctic islands.[9]
The porbeagle is a very stout-bodied shark with a spindle-like (fusiform) shape. The long, conical snout tapers to a sharp point, and is supported by enlarged, highly calcified rostral cartilages. The eyes are large and black, without nictitating membranes (protective third eyelids). The small, "S"-shaped nostrils are positioned in front of and below the level of the eyes. The mouth is large and strongly curved, with moderately protrusible jaws.[6] North Atlantic sharks have 28–29 upper tooth rows and 26–27 lower tooth rows, while Southern Hemisphere sharks have 30–31 upper tooth rows and 27–29 lower tooth rows.[23] Each tooth has a strongly arched base and a nearly straight, awl-like central cusp, which is flanked by a pair of smaller cusplets in all but the smallest individuals. The five pairs of gill slits are long and precede the pectoral fin bases.[6]
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 pelvic fins are much smaller than the first dorsal fin. The second dorsal and anal fins are smaller still, and placed about even with each other on narrow bases that allow pivoting from side to side. The sides of the caudal peduncle are expanded into prominent lateral keels. A second, shorter pair of keels are present below the main keels. The caudal fin is large and crescent-shaped, with the lower lobe almost as long as the upper; there are both dorsal and ventral depressions (precaudal pits) at the caudal fin base, and a deep ventral notch near the tip of the upper caudal fin lobe.[6] The skin is soft and covered by tiny, flattened dermal denticles (scales), lending a velvety texture. Each denticle has three horizontal ridges that lead to teeth on the posterior margin.[2]
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).[6][16][17] Female sharks grow larger than males in the North Atlantic, with maximum confirmed fork lengths (snout tip to caudal fin fork) of 2.5 m (8.2 ft) for males and 3.0 m (9.8 ft) for females. Southern Hemisphere sharks are smaller and the two sexes are similar in size, with males and females attaining fork lengths of 2.0 m (6.6 ft) and 2.1 m (6.9 ft) respectively.[9] Most porbeagles weigh no more than 135 kg (300 lb), with the record being a 230 kg (510 lb) individual caught off Caithness, Scotland in 1993.[6][19]
Fast and energetic, the porbeagle can be found singly or in groups.[6] Its fusiform body, narrow caudal peduncle with lateral keels, and crescent-shaped tail are adaptations for efficiently sustaining speed, which have also been independently evolved by tunas, billfishes, and several other groups of active fishes. It and the salmon shark are the thickest-bodied members of their family (length-depth ratio approaching 4.5), and consequently have the stiffest swimming style: they oscillate their tails while holding their bodies mostly rigid, which confers propulsive power with high energy efficiency, but at the cost of maneuverability. The large gill surface area of the porbeagle allows more oxygen to be delivered to its tissues. It also has a short band of aerobic "red muscle" along each side, which can contract independently of the regular "white muscle" at a lower energy cost, enhancing the shark's stamina.[24][25]
Porbeagles are among the few fishes that exhibit apparent play behavior.[19] There are reports, principally off the Cornish coast, of this species rolling and repeatedly wrapping itself in long kelp fronds near the surface; this activity may have an exploratory or self-stimulatory purpose, though alternately the shark may be attempting to feed on small kelp organisms or scrape off parasites.[19][26] In addition, porbeagles within a group have been seen chasing each other, and they will reportedly "play with anything floating on the water": individuals have been observed prodding, tossing, or biting natural and artificial objects, including pieces of driftwood and balloon floats used by anglers.[19][26][27]
Great white sharks (Carcharodon carcharias) and killer whales (Orcinus orca) are plausible, albeit undocumented, predators of the porbeagle. There is a record of a small individual caught off Argentina that bore bite marks from a copper shark (Carcharhinus brachyurus) or similar species, but it is uncertain whether the porbeagle was the target of attempted predation or if the two were simply involved in interspecific aggression.[6] Known parasites of this species include the tapeworms Dinobothrium septaria and Hepatoxylon trichiuri,[28][29] and the copepods Dinemoura producta,[30] Laminifera doello-juradoi,[31] and Pandarus floridanus.[32] Natural annual mortality is low, estimated to be 10% for juveniles, 15% for adult males, and 20% for adult females in the western North Atlantic.[9]
The porbeagle is an active predator that predominantly ingests small to medium-sized bony fishes. It chases down pelagic fishes such as lancet fish, mackerel, pilchards, herring, and sauries and forages near the bottom for groundfishes such as cod, hake, icefish, dories, sand lances, lumpsuckers, and flatfish. Cephalopods, particularly squid, also form an important component of its diet, while smaller sharks such as spiny dogfish (Squalus acanthias) and tope sharks (Galeorhinus galeus) are rarely taken. Examinations of porbeagle stomach contents have also found small shelled molluscs, crustaceans, echinoderms, and other invertebrates, which were likely ingested incidentally, as well as inedible debris such as small stones, feathers, and garbage fragments.[2][19][22]
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.[22] During spring and summer in the Celtic Sea and on the outer Nova Scotian shelf, porbeagles congregate at tidally induced thermal fronts to feed on fish that have been drawn by high concentrations of zooplankton.[20][21] Hunting porbeagles regularly dive from the surface all the way to the bottom, cycling back every few hours; this vertical movement may aid in the detection of olfactory cues.[20] There is a report of a one-year-old porbeagle 1 m (3.3 ft) long, which had fed on krill and polychaete worms.[30]
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.[33] Mating takes place mainly between September and November, though females with fresh mating scars have been reported as late as January off the Shetland Islands. The male bites at the female's pectoral fins, gill region, and flanks while courting and to hold on for copulation.[34] Two mating grounds are known for western North Atlantic porbeagles, one off Newfoundland and the other on Georges Bank in the Gulf of Maine.[34][35] Adult females have a single functional ovary, on the right, and two functional uteruses. They probably reproduce every year. The litter size is typically four, with two embryos oriented in opposing directions sharing each uterus; on rare occasions a litter may contain as few as one or as many as five pups.[33] The gestation period is 8–9 months.[6][34]
Like other members of its family, the porbeagle is aplacental viviparous with oophagy, i.e. the main source of embryonic nutrition are unfertilized eggs. During the first half of pregnancy, the mother ovulates enormous numbers of tiny ova, packed into capsules up to 7.5 cm (3.0 in) long, into her uteruses. A newly conceived embryo is sustained by a yolk sac and emerges from its egg capsule at 3.2–4.2 cm (1.3–1.7 in) long. At this time, the embryo has well-developed external gills and a spiral valve intestine. When the embryo is 4.2–9.2 cm (1.7–3.6 in) long, it has resorbed its external gills and most of its yolk sac, but cannot yet feed as it lacks the means to open egg capsules. At a length of 10–12 cm (3.9–4.7 in), the embryo grows two massive, recurved "fangs" in the lower jaw for tearing open capsules, as well as two much smaller teeth in the upper jaw. It begins to feed voraciously on yolk, acquiring an enormously distended stomach; to accommodate this, the muscles on the belly split down the middle and the skin on the abdomen stretches greatly.[33][34]
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 region 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 non-functional until birth.[33][34]
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.[9][33] The overall embryonic growth rate is 7–8 cm (2.8–3.1 in) per month.[33][34] 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.[33] 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 of around 500 m (1,600 ft).[18]
Both sexes grow at similar rates until the onset of maturation, with females maturing later and at a larger size than males.[36] In the first four years of life, the annual growth rate is 16–20 cm (6.3–7.9 in) and similar in both hemispheres; thereafter, sharks from the western South Pacific begin to grow slower than those from the North Atlantic.[37] In the North Atlantic, males mature at a fork length of 1.6–1.8 m (5.2–5.9 ft) and an age of 6–11 years, and females at a fork length of 2.0–2.2 m (6.6–7.2 ft) and an age of 12–18 years.[34][36] In the Southwest Pacific, males mature at a fork length of 1.4–1.5 m (4.6–4.9 ft) and an age of 8–11 years, and females at a fork length of 1.7–1.8 m (5.6–5.9 ft) and an age of 15–18 years.[33][37][38] The oldest porbeagle on record was 26 years of age and measured 2.5 m (8.2 ft) long.[39] The maximum lifespan of this species appears to be 30–40 years in the Atlantic,[39] but could be as much as 65 years in the South Pacific.[37]
Like other members of its family, the porbeagle is endothermic: metabolic heat generated by its red muscles is conserved within the body by specialized systems of blood vessels called retia mirabilia (Latin for "wonderful nets"; singular rete mirabile), that act as highly efficient countercurrent heat exchangers. The porbeagle has several rete mirabile systems: the orbital retia accessing its brain and eyes, the lateral cutaneous retia accessing its swimming muscles, the suprahepatic rete accessing its viscera, and the kidney rete.[25]
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 spine, and its lateral rete is composed of over 4,000 small arteries arranged in bands.[40] It has one of the highest core temperatures within its family, 8–10 °C (14–18 °F) warmer than that of the surrounding water.[41] Being warm-bodied may allow this shark to maintain higher cruising speeds, hunt in deep water for extended periods of time, and/or enter higher latitudes during winter to exploit food resources not available to other sharks.[25][30] The orbital retia of the porbeagle can raise the temperature of its brain and eyes by 3–6 °C (5–11 °F), and likely serve to buffer those sensitive organs against the large temperature shifts that accompany changes in depth; potential benefits of this include increased visual acuity and response times.[42]
Although physically capable of doing so, the porbeagle has very rarely, if ever, attacked swimmers or boats.[6] As of 2009, the International Shark Attack File attributes three attacks on humans to this species, one provoked and none fatal, and two on boats.[43] One older anecdote tells of a fisherman who provoked a porbeagle into leaping from the water and tearing his clothes. In another account of a swimmer bitten by a "mackerel shark", the species responsible could easily have been a misidentified shortfin mako or great white shark. Recently in the North Sea, adult porbeagles have been filmed charging at divers working on oil platforms, sometimes even brushing lightly against them without doing harm. These rushes do not appear to have predatory intent and may instead be motivated by curiosity or defense.[6]
At one time, porbeagles were regarded as nuisances by some commercial fishers because they damaged lighter fishing gear intended for smaller species and stole hooked fish from lines.[6] 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. Novice anglers often mistake this shark for the mako, which has earned it the affectionate moniker "fako" in New England. The International Game Fish Association keeps records on the porbeagle.[6][19]
Prized for its meat and fins, the porbeagle has long been under heavy fishing pressure.[1] The meat is sold fresh, frozen, or dried and salted, and ranks among the most valuable of any shark: in 1997 and 1998 it had a wholesale price of EUR 5–7/kg, four times that of the blue shark (Prionace glauca). Most of the demand comes from Europe, though the United States and Japan also import this species. The fins are shipped to East Asia for use in shark fin soup. The remainder of the shark may also be utilized, for production of leather, liver oil, and fishmeal. International trade in the porbeagle appears to be significant, but remains unquantified as shark products tend not to be reported to the species level, and many consist of a mix of various species.[44][45] This shark is caught most readily on longlines, but is also susceptible to gillnets, driftnets, trawls, and handlines. It is valuable enough to be generally retained when caught as bycatch; if storage space cannot be spared it may be finned and the carcass discarded.[1]
Intensive fishing for the porbeagle dates back the 1930s, when Norway and to a lesser extent Denmark began operating longline vessels in the western North Atlantic. The Norwegian annual catch rose from 279 tons in 1926 to 3,884 tons in 1933, and peaked at around 6,000 tons in 1947, with the resumption of fishing after World War II. Soon after the stock collapsed: Norwegian annual catches declined steadily to 1,200–1,900 tons from 1953 to 1960, 160–300 tons in the early 1970s, and 10–40 tons in the late 1980s to early 1990s. Similarly, Danish annual catches fell from 1,500 tons in the early 1950s to under 100 tons in the 1990s.[6][46] Presently, many European countries continue to catch porbeagles in the eastern North Atlantic, with Norway, Denmark, France, and Spain foremost among them. France and Spain began to target porbeagles in the eastern North Atlantic in the 1970s. French fishers operate mainly in the Celtic Sea and the Bay of Biscay, and saw a decline from an annual catch of over 1,000 tons in 1979 to 300–400 tons in the late 1990s. Catches by Spanish fishers have been highly variable, ranging from negligible to over 4,000 tons per year, which may reflect shifts of fishing effort into historically less-exploited waters.[1]
As porbeagles had become scarce in the eastern North 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 Faroe Islands. Norwegian annual catches rose from 1,900 tons in 1961 to over 9,000 tons in 1965;[16] the catch was largely exported to Italy, where porbeagle (smerglio) is an extremely popular food fish.[19][47] Again, the stock collapsed, this time in only six years: by 1970 Norwegian catches had fallen to under 1,000 tons per year, and Faroese catches observed a similar trend. With the population decimated, most fishers moved on or switched to other species. Porbeagle numbers gradually recovered in the ensuing 25 years, to about 30% of pre-exploitation levels. In 1995, Canada established an Exclusive Economic Zone (EEZ) and became the primary fisher of porbeagles in the region. Between 1994 and 1998, Canadian fishing vessels landed 1,000–2,000 tons per year, which depleted the population to 11–17% of pre-exploitation levels by 2000.[16] Strict regulations and greatly reduced fishing quotas introduced in 2000 have since begun to reverse the stock decline, though recovery of the stock is projected to take decades due to the low productivity of the species.[48] There is evidence that incidental artificial selection caused by heavy fishing has led to a compensatory growth response, i.e. faster growth and earlier maturation.[49]
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 (Xiphius 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 (CPUE) for this fishery have shown a 90% decline in porbeagle landings from 1988 to 1998, though it is uncertain whether this reflects a real population decline or changing fishing habits. New Zealand has reported annual catches of 150–300 tons, mostly of immature individuals, from 1998 to 2003.[1]
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 overfishing.[50] The International Union for Conservation of Nature has assessed the porbeagle globally as Vulnerable,[1] Endangered in the western North Atlantic,[47] and Critically Endangered in the eastern North Atlantic[46] and Mediterranean Sea.[51]
The porbeagle is listed on Annex 1 (Highly Migratory Species) of the UN Convention on the Law of the Sea (UNCLOS), but this has not resulted in any management schemes. Although the Animals Committee of the UN Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) recommended additional conservation actions for the porbeagle, CITES rejected proposals to list the species in 2008 and again in 2010. This species benefits from bans on shark finning instituted by several nations and supranational entities, including Canada, the United States, Brazil, Australia, the European Union, and the International Commission for the Conservation of Atlantic Tunas (ICCAT).[1]
The only regulation of porbeagle catches in the Southern Hemisphere is New Zealand's total allowable catch (TAC) of 249 tons per year, instituted in 2004.[1] In the eastern North Atlantic, porbeagle fisheries have never been restricted despite well-documented historical declines. Since 1985, Norway and the Faroe Islands are allowed annual quotas of 200 tons and 125 tons respectively from European Community waters. Although these quotas are lower than the originals set in 1982 (500 tons for Norway and 300 tons for the Faroe Islands), they are still consistently higher than the yearly total porbeagle catch in the region and thus have no practical effect.[46]
In the Mediterranean Sea, the porbeagle is on the verge of extinction, with a population decline of over 99.99% since the mid-20th century. Its range has contracted to the waters around the Italian Peninsula, where there may be a nursery area. Only a few dozen specimens have been recorded in the past few decades, from scientific surveys, swordfish fishery bycatch, and sport fishers.[51][52] 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.[51]
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 (DFO) 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 ground off of Newfoundland was also closed to shark fishing. In 2004, the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) 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.[48] In US waters, the 1993 Fishery Management Plan for Sharks of the Atlantic Ocean sets an annual quota of 92 tons (post-processing) for the porbeagle.[47] In 2006, this species was listed as a "Species of Concern" by the National Marine Fisheries Service (NMFS), meaning that it merits conservation concern but there is insufficient data for inclusion on the US Endangered Species Act.[53]
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