Porbeagle

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. Another is that it is derived from the Cornish porth, meaning "harbor", and bugel, meaning "shepherd". 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. Other common names for the porbeagle include Atlantic mackerel shark, Beaumaris shark, bottle-nosed shark, and blue dog. 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". In 1816, French naturalist Georges Cuvier placed the porbeagle into its own subgenus, Lamna, which later authors elevated to the rank of full genus. ==Phylogeny and evolution==

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), which occurs in place of it in the North Pacific. The genus Lamna evolved 65–45 Mya. 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. Fossilized porbeagle remains are known from Late Miocene epoch (about 7.2 Mya) deposits in Belgium and the Netherlands, Pliocene epoch (5.3–2.6 Mya) deposits in Belgium, Spain, and Chile, and Pleistocene epoch (2.6 Mya to 12,000 BP) deposits in the Netherlands. 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 Mya). Much taxonomic confusion remains regarding Lamna in the fossil record due to the high degree of variability in adult tooth morphology within species. ==Distribution and habitat==

The porbeagle has an almost global amphitemperate distribution, i.e., it is absent from the tropics; in the North Pacific, where its ecological niche is assumed by the salmon shark. It is found mostly within 30–70°N and 30–50°S latitudes. 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. Offshore fishing banks are the favored habitat of the porbeagle, though it can be found from a depth of in oceanic basins to littoral (close to shore) waters less than deep, over the entire water column. A single, anomalous record was made of a juvenile in brackish water in Mar Chiquita in Argentina. 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. The porbeagle has been reported across a temperature range of , with most records between . In a study that included 420 porbeagles caught in the northwest Atlantic off Canada, all were in water below and the majority between . 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 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. 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 to spend late summer and fall in the shallow waters of the Newfoundland Grand Banks and the Gulf of St. Lawrence. In December, large, mature females migrate south over into the Sargasso Sea for pupping, keeping deeper than during the day and at night so as to stay in the cooler waters beneath the Gulf Stream. 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. Migrating sharks may travel upwards of , though once they reach their destination they tend to remain within a relatively localized area. 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. ==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 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. 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. 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. 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; both dorsal and ventral depressions (precaudal pits) are at the caudal fin base, and a deep ventral notch is near the tip of the upper caudal fin lobe. 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. 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 , though this is uncertain and may have resulted from confusion with other mackerel shark species. A more typical length is . Female sharks grow larger than males in the North Atlantic, with maximum confirmed fork lengths (snout tip to caudal fin fork) of for males and for females. Southern Hemisphere sharks are smaller and the two sexes are similar in size, with males and females attaining fork lengths of respectively. Most porbeagles weigh no more than , with the record being a individual caught off Caithness, Scotland, in 1993. ==Biology and ecology==

Fast and energetic, the porbeagle can be found singly or in groups. 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. 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. 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. 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. A one-year-old porbeagle long, was reported to have had fed on krill and polychaete worms. 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. 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. Two mating grounds are known for western North Atlantic porbeagles, one off Newfoundland and the other on Georges Bank in the Gulf of Maine. Adult females have a single functional ovary, on the right, and two functional uteri. 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. The gestation period is 8–9 months. 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 long, into her uteri. A newly conceived embryo is sustained by a yolk sac and emerges from its egg capsule at long. At this time, the embryo has well-developed external gills and a spiral valve intestine. When the embryo is 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 , 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. At 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 long. The embryo gains pigment and sheds its fangs at a length of . 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 , and has assumed its newborn appearance by a length of . 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. Newborn porbeagles measure long and do not exceed . 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. The overall embryonic growth rate is per month. 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. 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 . Both sexes grow at similar rates until the onset of maturation, with females maturing later and at a larger size than males. In the first four years of life, the annual growth rate is and similar in both hemispheres; thereafter, sharks from the western South Pacific begin to grow slower than those from the North Atlantic. In the North Atlantic, males mature at a fork length of and an age of 6–11 years, and females at a fork length of and an age of 12–18 years. In the Southwest Pacific, males mature at a fork length of and an age of 8–11 years, and females at a fork length of and an age of 15–18 years. The oldest porbeagle on record was 26 years of age and measured long. The maximum lifespan of this species appears to be 30–40 years in the Atlantic, but could be as much as 65 years in the South Pacific. Thermoregulation 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. 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. It has one of the highest core temperatures within its family, warmer than that of the surrounding water. 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. The orbital retia of the porbeagle can raise the temperature of its brain and eyes by , 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 reduced response times. ==Human interactions==

The porbeagle has very rarely, if ever, bitten swimmers or boats. 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. In 2012, similar restriction came into effect in Norway. 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 Faroe Islands. Norwegian annual catches rose from 1,900 tons in 1961 to over 9,000 tons in 1965; the catch was largely exported to Italy, where porbeagle (smeriglio) is an extremely popular food fish. Again, the stock collapsed, this time in only six years; by 1970, Norwegian catches had fallen 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 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. 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. Some evidence shows that incidental artificial selection caused by heavy fishing has led to a compensatory growth response, i.e. faster growth and earlier maturation. 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. ==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 overfishing. and critically endangered in the eastern North Atlantic 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. 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. 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. 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. 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. ==References==