Biology of the Porbeagle (original) (raw)
(Lamna nasus)
| Field Marks | Body heavy, spindle-shaped; snout conical and moderately long (distance from tip of snout to eye is about 50% distance from eye to first gill slit); teeth moderately large, blade smooth-edged with basal cusplets; gill slits long, extending partially onto top of head; first dorsal fin originating over axil ('armpit') of pectoral fins and with light free rear tip; second dorsal and anal fins minute, with pivoting bases; strong keels on caudal peduncle (tail stalk), with a short secondary keel on each side of the caudal fin base; caudal fin lunate, with lower lobe almost as long as the upper. Color: dark bluish grey to brown above, white below in adults, although juveniles often have dusky patches beneath the pectoral fins and around the undersurface of the gill slits. |
|---|---|
| Size | Pups are 24-30 in (60-75 cm) long and about 20 lbs (9 kg) in weight at birth; most individuals are 5-6 ft (1.5-1.8 m) long and about 300 lb (135 kg) in weight; maximum recorded length is 9.4 ft (2.9 m), but reported to reach 12 ft (3.7 m) in length and 500 lb (230 kg) in weight; specimens longer than 8 ft (2.5 m) are rare. |
| Range | Amphitemperate in coastal and oceanic regions; apparently does not occur in the North Pacific, where it is replaced by the Salmon Shark (Lamna ditropis). |
| Habitat | Common in littoral and epipelagic zones of cold to warm temperate waters; most abundant in cool temperate waters on offshore fishing banks over continental shelves, but also found far from land in ocean basins and occasionally close inshore; a recent record of this species in brackish waters of Mar Chiquita, Argentina. Known depth range from the surface down to at least 2 345 ft (715 m), although most catches occur at depths 180-295 ft (55-90 m); seems to prefer water colder than 64�F (18�C) and has been caught at high latitudes in water as cold as 35�F (2�C); partially endothermic, maintaining its body temperature 12.5-18�F (7-10�C) warmer than ambient. May come inshore and to the surface during summer months, but tends to stay at depth in offshore waters during winter; fisheries catches in Europe indicate segregation by size and sex. |
| Feeding | Preys on squids and a wide variety of bony and cartilaginous fishes, including pelagic schoolers such as mackerels, pilchards and herrings, various gadoids (cod, hakes, haddock, cusk, and whiting), john dories, flounders, and other sharks such as the Spiny Dogfish (Squalus acanthias) and Tope (Galeorhinus galeus); reported to be active and strong-swimming in pursuit of fish and squid prey, but also takes whelks and sea urchins, suggesting it may sometimes forage at or near the bottom (stomach contents have included stones). |
| Reproduction | Ovoviviparous, featuring oophagy at late stage of development; late-term pups develop grotesquely distended abdomens and branchial (gill) regions. Litter size 1-5, typically 4 (2 per uterus); gestation period in European waters has been estimated to be about 8 months. Breeds on both sides of the North Atlantic: off Europe and the British Isles pregnant individuals are caught during most of the year except July-September; off North America from Massachusetts to Maine, pregnant individuals are caught year-round. Pups are apparently born in the spring off Europe and late summer off North America; mating in European waters occurs in late summer (September-October), and breeding there probably occurs every year; mating behavior apparently includes the male biting the pectoral fins of females, fresh mating scars occur December-January off Shetland. Breeding populations almost certainly exist elsewhere in this species' range, but details are lacking. |
| Age & Growth | Males mature at a length of about 5-6.6 ft (1.5-2.0 m), females at 6.6-8.2 ft (2.0-2.5 m), although there is a record of an apparently mature female that was only 5 ft (1.52 m) long; both sexes grow at a similar average rate; age at maturity for both sexes in the North Atlantic has been estimated from vertebral growth rings at 8 yrs. Longevity for both sexes estimated to be at least 20 yrs. |
| Danger to Humans | Regarded as potentially dangerous due to its large size and relationship to known dangerous species, but few people willingly swim in the cold offshore waters it usually inhabits; the SAF currently lists 3 attacks attributed to this species, but these may be due to confusion with the White Shark (Carcharodon carcharias). Has been photographed underwater while hooked on a surface line and made no aggressive overtures toward the diver (apparently more concerned with escape than attack), but little is known of its disposition toward divers. |
| Utilization | Heavily fished and utilized in the North Atlantic and Mediterranean by many countries, including Norway, Denmark, the Faroe Islands, the United Kingdom, France, and previously Spain; a considerable fishery by Japanese longliners exists in the central Indian Ocean and it is often taken as a bycatch by Japanese longliners fishing for Southern Bluefin Tuna (Thunnus maccoyii) off Tasmania. Its flesh is used fresh and dried salted for human consumption, also used for liver oil, fishmeal, and the fins for shark-fin soup. Primarily caught with pelagic longlines, but also taken on pelagic and bottom trawls, handlines and gillnets. Classed as a gamefish because it puts up a determined fight, though it does not leap like the Shortfin Mako (Isurus oxyrinchus); IGFA all-tackle record is 507 lb (230 kg), caught in Pentland Firth, off Caithmess, Scotland, in March 1993. Stocks in the North Atlantic show signs of serious over-fishing in the form of greatly declining catches (total weight of catches in 1981 were about 47% those of 1978); IUCN Red List status LT/nt. |
| Remarks | During the 1960's, this species was actively fished off Newfoundland, Canada, by Norwegian fishermen, who took about 9,000,000 lbs (4,100,000 kg) annually, destined for the European food market; this fishery has all but collapsed due to overfishing. The species is once again fairly common in Canadian Atlantic waters. |
We humans have a highly developed sense of play. We devote an astonishing amount of our time, resources, and ingenuity toward amusing ourselves. Although we realize that play serves some very serious purposes in our intellectual and social development, mostly we love this class of activity purely and simply because it's fun. Due to play's undeniable importance to us, we tend to regard with particular favor � and even kinship � those relatively few animals that seem to share our frolicsome predilection. We regard play as a hallmark of joie de vivre, of high intelligence, of being like us. Dolphins, monkeys, puppies, kittens, and such, hold a special place in our hearts, in part, because they seem to enjoy play as much as we do.
The Porbeagle is among the very few fishes that seem to exhibit play behavior. There have been a few, sporadic accounts � principally from off the Cornish coast � of Porbeagles playing with floating objects, both man-made and natural. Porbeagles have been reported rolling while swimming along the surface, repeatedly wrapping and unwrapping their snouts and forward portion of their bodies in kelp fronds, which often trail behind the shark like rubbery streamers. Sometimes a Porbeagle thus engaged was observed being chased by other Porbeagles. One could argue, I suppose, that the kelp-rolling sharks were attempting to feed on bryozoans, gastropods, or other tiny organisms living on the kelp, or that they were using the resilient algae to help remove copepod skin parasites (either of which behaviors would be remarkable in its own right). But such 'pseudo-explanations' seem less-than-compelling. To my mind, such kelp-rolling behavior seems more likely to be exploratory object manipulation or tactile self-stimulation than simple feeding or scratching. The repetitive nature of this behavior is highly characteristic of what in other mammals would unhesitatingly be termed play. But because the Porbeagle is a 'mere' fish, ethologists are reluctant to use that term. The Porbeagle, of course, doesn't care about such semantic contention, and does what it does for its own reasons.
Such carefree behavior belies the fact that Porbeagles have been subject to much human pursuit and exploitation. This species has long been targeted commercial fishing in European waters and is much sought-after as a sportfish off New England � where they are affectionately known as "Fakos", as they are often mistaken by neophytes for the closely related Shortfin Mako (Isurus oxyrinchus) � and the United Kingdom. Many English sport anglers are well aware of the Porbeagle's curiosity. These sharks are apparently fascinated by anglers' balloon floats (from which bait is suspended), prodding them repeatedly with the snout and sometimes even trying to bite the soft, spheroid wonders; when a balloon that a Porbeagle was exploring 'pops', the shark typically shakes its head then pauses momentarily, as though it simply cannot believe its eyes (fish just don't do that sort of thing!).
On the other side of the North Atlantic, centered off Nova Scotia, Canada, this same species was nearly wiped out by heavy commercial fishing during the 1960's. At the height of the fishery, Norwegian longliners hauled in some 9 million pounds (4 million kilograms) of Porbeagle annually. If the average shark caught in this fishery weighed about 300 pounds (135 kilograms) � surely a generous estimate � that translates to at least 30 000 individual sharks per year. Most of this catch was shipped to Italy, where the meat � known as _smerglio_� is extremely popular: the carcasses are displayed at markets with their sandpapery skins on, so that buyers can be sure that they are not being sold some other, less desirable fish. Soon the Norwegian Porbeagle catch figures off Nova Scotia began to diminish, demonstrating that a shark population could easily be "fished out" in an area where the target species is nonmigratory and has a low reproductive rate. Porbeagles may take five or more years to reach sexual maturity and produce small litters of one to five pups (typically four, two per uterus) per year. These sharks thus cannot suffer vast depredations on their adult populations without drastic effects.
Despite its goofy-sounding name (from the Cornish porbugel, supposedly a combination of porpoise and beagle, referring to its chubby body and determined hunting behavior), the Porbeagle is a first-class predator. Also known as the Mackerel Shark, it feeds primarily on small pelagic schooling fishes, including silvery clouds of mackerels, pilchards, and herrings, as well as drably-garbed bottom-associated fishes such as cods, hakes, cusk, whitings, John Dories, dogfishes, and Tope Sharks (Galeorhinus galeus). It also feeds on squids and � at least occasionally � on the bottom, as whelks and sea urchins have been found as stomach contents. Because of its largish size (to 12 feet or 3.7 metres), the Porbeagle is regarded as potentially dangerous to humans; two attacks on people have been attributed to this species, but may have resulted from confusion with the Shortfin Mako or White Shark (Carcharodon carcharias). In any case, most people would not willingly enter the frigid waters inhabited by this shark. The Porbeagle inhabits inshore and offshore cold temperate seas, less than 66 degrees Fahrenheit (19 degrees Celsius), from the surface down to a depth of at least 1,200 feet (366 metres). Like all extant lamnids, the Porbeagle is warm-bodied, maintaining its internal temperature 12.5 to 18 degrees Fahrenheit (7 to 10 degrees Celsius) higher than the water temperature. It has been known to leap out of the water in pursuit of prey. With such a high-energy lifestyle, it's a wonder that a Porbeagle would spare any time to investigate or play with novel objects.
Even facing the serious business of capturing food for itself and extensive human depredations, the Porbeagle remains as curious and carefree as ever. Ernest Hemmingway often rhapsodized about the ideal of grace under pressure. Given our long and deep appreciation of play as a pursuit worthwhile for its own sake and our warm admiration (or is that envy?) of the ability of children to play happily no matter what societal horrors and adult stresses are going on about them, it is remarkable that we do not extend the Porbeagle especially high accord. The world may well be a happier place if we could only learn to play under pressure.