Barndoor skate

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Barndoor skate
Barndoor skate, Dipturus laevis
Conservation status
Endangered [1]
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Chondrichthyes Order: Rajiformes Family: Rajidae Genus: Dipturus Species: D. laevis
Binomial name
Dipturus laevis Mitchill, 1818

The barndoor skate, Dipturus laevis, is a species of marine cartilaginous fish in the skate family (family Rajidae) of the order Rajiformes. It is native to the northwestern Atlantic Ocean, and is found from the Grand Banks of Newfoundland and the southern side of the Gulf of St. Lawrence south to North Carolina.^[2] The fish is one of the largest skates found in the North Atlantic Ocean, reaching lengths of up to 1.5 meters (5 ft.). It is carnivorous, feeding on invertebrates and other fish found near the sea floor.

After peaking in the 1950s, the population of the barndoor skate dramatically declined in the 1970s as a result of overfishing and is now listed as endangered by the World Conservation Union. In one area, the fish population declined from over 600,000 to fewer than 500 in that period. In most cases, the barndoor skate is not intentionally harvested by the commercial fishing industry– it is usually considered by-catch in the trawling nets used to target other species of fish.

Contents 1 Physical description 2 Habitat and diet 3 Importance to humans 4 Conservation 5 Taxonomy and naming 6 See also 7 References

[edit] Physical description

The barndoor skate is a flat-bodied fish with a large, disk-like body with sharply angled corners and a pointed snout. Its pectoral fins have evolved into broad, flat, wing-like appendages used to propel the fish through the water. These fins have a concave front edge with rounded posterior corners. Like sharks, it has a boneless skeleton made of cartilage, a tough, elastic substance composed of collagenous and/or elastic fibers, cells, and a firm, gel-like substance called the matrix. It has slot-like body openings called gill slits on the underside of the body beneath the pectoral fins that lead from the gills. The dorsal fins are close together and far removed from the tail.^[3] It has two eyes on its dorsal surface, located approximately 5.5 millimeters (0.2 in.) apart.^[2]

The fish's upper surface is brown to reddish brown with many scattered darker spots, lighter streaks, and reticulations. The center of each pectoral fin is marked with an oval spot or blotch. The lower surface is light, white to grey, blotched irregularly with gray spots.^[4][3] The barndoor skate is unique from other species of skate in its having a straight line that begins at the snout and ends at the anterior margin of the outer corner of the disk, but stopping short short of the disk.^[3]

The barndoor skate is one of the largest skates found in the North Atlantic Ocean.^[2] It can reach lengths of up to 1.5 meters (5 ft.) and can weigh up to 18 kilograms (40 lb).^[5] There have been unconfirmed reports of individuals reaching lengths of 1.8 meters (71 in.).^[3] A 71–76 centimeter (28–30 in.) barndoor skate weighs an average of 2–3 kilograms (4–7 lb).^[3]

The tail is moderately short and does not have large thorn-like structures called dermal denticles that are normally found on skates. This lack of denticles distinguishes it from all but two species of skates that are found in the western Atlantic.^[2] Larger individuals do have three rows of smaller denticles on the tail, and mature females also posses denticles on the head and shoulders, and along the dorsal midbelt of the disk and tail. Denticles are completely absent on small individuals.^[3]

[edit] Habitat and diet

The barndoor skate occurs in a range extending from the banks of Newfoundland, the southern Gulf of St. Lawrence, and along the northeastern coast and offshore banks of Nova Scotia down to North Carolina.^[6] There were reports in the 19th century that the range of the fish extended as far south as northeastern Florida, but more recent research suggests that the Florida discoveries may have actually been a misidentification of R. floridana.^[7] It is found on various types of ocean bottom including soft muddy, sandy, and rocky bottoms. It can be found from the shoreline to depths of up to 750 meters (2,500 ft), although it is most commonly found at depths of less than 150 meters (500 ft). It inhabits waters in a broad range of temperatures, from just above freezing to 20°C (70°F).^[6] It appears to move closer to shore in the autumn and further out to sea in the warmer months. It tolerates brackish water where the salinity is as low as 21 to 24 parts per thousand, but it prefers salinity between 31 and 35 parts per thousand.^[5] It is believed to not exhibit any north-south migratory patterns.

The fish is carnivorous, with its prey consisting mainly of benthic invertebrates and fishes. Such food items include polychaetes, gastropods, bivalve mollusks, rock crabs, cancer crabs, spider crabs, lobsters, shrimps, squids, and fishes including spiny dogfish, alewife, Atlantic herring, menhaden, hakes, sculpins, cunner, tautog, sand lance, butterfish, and various flounders. Juveniles primarily subsist on benthic invertebrates such as polychaetes, copepods, amphipods, isopods, crangon shrimp, and euphausiids.^[8] Individuals have been found with the denticles on the snout worn smooth, indicating that the snout is used to dig in the mud or sand to obtain bivalve mollusks.^[3]

[edit] Importance to humans

The barndoor skate is one of five skates in the Gulf of Maine that has commercial value, but of those, the species that are most frequently targeted are the winter skate (Leucoraja ocellata) and the thorny skate (Amblyraja radiata).^[8] The barndoor skate is most commonly considered by-catch by trawlers operating in the northwestern Atlantic that target other commercially valuable species of fish using bottom trawling.^[5] When harvested, the flesh of the barndoor skate is used as bait, fish meal, pet food, and the meat from its wings is sold for human consumption.^[3] Since 1981, landings of skates have increased substantially, partly in response to increased demand for lobster bait, and more significantly, to the increased export market for skate wings.^[9]

Skates have been reported in New England fishery landings since the late 1800s. However, commercial fishery landings, primarily from off Rhode Island, never exceeded several hundred tons until the advent of distant-water fleets during the 1960s. The exact number of barndoor skates that were caught by the fishing industry is not known, since fisheries reports recorded over 99% of all skates caught as "unclassified skates". Landings of all skate species reached 9,500 metric tons (10,400 short tons) in 1969, but declined quickly during the 1970s, falling to 800 metric tons (900 short tons) in 1981. Landings increased to 12,900 metric tons (14,200 short tons) in 1993 and then declined somewhat to 7,200 metric tons (7,900 short tons) in 1995. Landings have increased since then, and the 1998 reported commercial landings of 17,000 metric tons (18,700 short tons) were the highest on record.^[9] The barndoor skate was once frequently caught by anglers in the Gulf of Maine in the 1950s, but it is now rarely found.^[2]

[edit] Conservation

Intensive trawling has threatened the species with extinction, primarily through by-catch in multi-species fisheries. Elliot A. Norse, president of the Marine Conservation Biology Institute in Redmond, Washington, said, "We're not talking about [the population] being down by half or even three-quarters. It looks like it's down by 99.99 percent, raising the very real possibility of extinction."^[10] An independent study by the National Marine Fisheries Service indicated that the abundance of the species peaked in the early 1960s, then declined to a low in the 1990s. In that period, the species declined 96-99% in the center of its geographic range on the southern shelf.^[11]

Some ecologists, such as Peter J. Auster, science director of the National Undersea Research Center at the University of Connecticut in Groton, see the creation of marine reserves, where fishing is prohibited, as imperative to saving the skate. "It's the most precautionary approach we can have," he said in a 1999 interview.^[10]

In 1999, two conservation groups, GreenWorld, based in Cambridge, Massachusetts, and The Center for Marine Conservation, based in Washington, D.C., petitioned the National Marine Fisheries Service (NMFS) to have the barndoor skate listed under the Endangered Species Act. The petition relied heavily upon a 1998 study by Casey & Meyers showing that the population of the species was decreasing at a rate corresponding to an exponential decay function. The study found that since the 1970s, the species had been caught in only two of the nine fisheries in the region, even though in earlier studies, the species had been caught in every one of those same fisheries. The analysis of data for the St. Pierre bank, south of Newfoundland, showed that the population of barndoor skates in the area had fallen from about 600,000 in the 1950s to fewer than 500 in the 1970s.^[12] After a 12-month study, the NMFS announced in 2002 that listing the species as endangered was not warranted, although it would retain the species on its candidate species list. It cited recent increases in abundance and biomass of barndoor skate observed during surveys taken after the Casey & Meyers study, evidence indicating that the fish is found in a greater range than what had been earlier assumed, and increases in the number of juvenile individuals collected in "no-take" zones on Georges Bank and the Southern New England shelf as well as adjacent areas to the north and south.^[11] Listing as a candidate species means federal officials will monitor the numbers of barndoor skates on an ongoing basis.

In 1994, the World Conservation Union had listed the barndoor skate as "vulnerable" under the 1994 Categories and Criteria, but in 2003, it reassessed the species as "endangered" on the IUCN Red List. In changing its assessment, the IUCN cited findings that the increases in catch data that were cited by the NMFS were likely the result of new fishing activities in previously unexploited deeper waters, not the result in an increase in overall population. It estimated that populations in these new areas were likely much smaller than the populations in the previously overfished waters, which remained depleted.^[1]

Each year the National Oceanic and Atmospheric Administration (NOAA) estimates current population levels for a variety of aquatic species of special interest, and releases an annual report showing the progress being made to reduce harvesting of overfished species. When a species has been determined to be either overfished or subject to overfishing, the regional fishery management councils are required to develop a plan to correct the problem. In 2006, NOAA published a press release stating that as a result of conservation efforts, between 2004 and 2005, monitored stocks of the barndoor skate had grown to a level that the NOAA no longer considers "overfished".^[13]

[edit] Taxonomy and naming

The fish was originally described as Raja laevis by Samuel Latham Mitchill in 1818. The scientific name was later changed to the currently valid name Dipturus laevis. It has also been misidentified as Raja granulata by Theodore Gill, an American ichthyologist, in 1879.^[14] The genus name, Dipturus, is derived from the Greek words di, meaning two, and pteryx, meaning wing. Raja, the original genus which was coined by Carolus Linnaeus in 1758, is still recognized as a valid subgenus.^[3]

[edit] See also

• Bottom trawling
• Fisheries management

[edit] References

Dipturus laevis (TSN 564139). Integrated Taxonomic Information System. Accessed on 14 November 2006.