Leucoraja ocellata

Winter Skate

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Winter Skate Winter Skate Winter Skate

 

Description

The winter skate is a fascinating species known to deter predators and stun prey with a quick jolt of electricity. Most are found in the northwest Atlantic Ocean, from the Gulf of St. Lawrence in Canada to North Carolina in the United States. Once thought to be a "trash fish," the winter skate is now harvested and processed into fishmeal and lobster bait, and is even marketed for human consumption. Increased trawling for the species has resulted in the accidental capture of juveniles, which are easily mistaken for smaller, more abundant species. This has led to a staggering population decline among winter skate, which are slow to reach sexual maturity and have few offspring.

Geography

Winter Skate has a narrow latitudinal ranges and high degree of endemicity. It is endemic to the shelf waters of the northwest Atlantic, from the Grand Banks, Gulf of St. Lawrence and occasionally to the Northeast Newfoundland Shelf in Canada (Compagno et al. 1989) and to Cape Hatteras, North Carolina, in the USA (Robins and Ray 1986, Collette and Klein-MacPhee 2002, COSEWIC 2005). The population(s) (structure unknown) appear to be concentrated in four geographic locations: 1) southern Gulf of St. Lawrence; 2) Scotian Shelf; 3) Southern Gulf of Maine/Georges Bank; and 4) the Mid Atlantic Bight.

Ecosystem

Winter Skate is a benthic species. Habitat ranges from shoreline to 317 m, but they are most abundant at depths <150 m (Bigelow and Schroeder 1953, McEachran 2002). The temperature range for this species is -1.2?19°C (Bigelow and Schroeder 1953, McEachran 2002). This species prefers sandy and gravel substrate (Bigelow and Schroeder 1953, Scott 1982). Substrate type rather than depth appears more important in determining distribution (Scott 1982).

Size, reproduction and age data in life history table below. Winter Skates are slow-growing, produce few eggs each year. Size at maturity increases with latitude (McEachran and Martin 1977). On Georges Bank and Gulf of Maine, individuals mature between 70 and 109 cm total length (TL). The Gulf of St Lawrence population however matures at a smaller size and does not reach as large a size as other populations (McEachran and Martin 1977). Length at 50% maturity for female winter skate on the eastern Scotian Shelf is thought to be around 75 cm (Simon and Frank 2000).

Eggs of Winter Skate are deposited throughout the year off southern New England and from summer to autumn off Nova Scotia (Vladykov 1936, Collette and Klein-MacPhee 2002). However, a peak in the reproductive season was observed during the summer months in the Gulf of Maine (Sulikowski et al. 2004). Industry has previously noted females extruding complete purses only in the late summer/early autumn west of Sable Island and suggestions were made that this may be a spawning area (Simon and Frank 2000). Frisk et al. (2002), suggest winter skate fecundity to be between 18?35 eggs per year.

Winter Skate migrate to deeper colder waters during summer months in some areas and the species is sometimes termed a winter periodic (Scott and Scott 1988). Research vessel survey data for the Scotian Shelf, however, show that winter skate appear to be concentrated in deeper warmer waters in the winter and move into shallower waters during spring and summer.

Most important prey items appear to be fish, decapods, amphipods and molluscs. Primary food sources shift from invertebrates to fish as skates increase in size. Moreover, the prey size also increases as the skates become larger and older. Studies of food habits of winter skate and little skate by McEachran et al. (1976) have shown that although the two species occur together over most of their range they avoid serious competition by eating different proportions of the same food resources. Winter Skate tended to eat infauna and Little Skate eat epifauna.

Little is known about predation on Winter Skate, but they are eaten by many predators including sharks, other rays (such as L. erinacea), and Grey Seals (Scott and Scott 1988). Winter Skate are also prone to several parasites, including protozoans, myxosporidian, haematazoa, trematodes and nematodes (Scott and Scott 1988).

Conservation

The Winter Skate (Leucoraja ocellata) is a common shelf-water species, found in the northwest Atlantic from Labrador to the Carolinas. The Winter Skate has a narrow latitudinal range and a high degree of endemicity. Like many other elasmobranchs, it possesses life history characteristics (including delayed age at maturity, long generation time, low fecundity, and consequently slow population growth) that may increase its vulnerability to exploitation, reduce its rate of recovery, and increase its risk of extinction. Presently there is no directed fishery for the species. Population trends vary in different areas of the Winter Skate's range, as described below.

Northern Gulf-Newfoundland population, Canada: At the northern extent of its distribution, the Winter Skate exists in low concentrations in the northern Gulf of St. Lawrence, in the coastal waters off the southern coast of Newfoundland, and on the southern portion of the Grand Bank. A quantitative analysis of spatial and temporal variation in population size is not possible because of the infrequency with which the species is caught. It is subject to bycatch.

Southern Gulf of St Lawrence, Canada: Here the species appears to have a restricted distribution and individuals mature at a significantly smaller size than those found elsewhere. Abundance of mature individuals is estimated to have declined by 98% since the early 1970s, and is now at a historically low level. The probable cause is an unsustainable rate at which they were captured as bycatch in fisheries directed at other demersal species.

Eastern Scotian Shelf, Canada: Individuals here mature at a significantly larger size than those in the Southern Gulf and mature at a significantly different age than those inhabiting waters further south. Abundance of mature individuals is estimated to have declined by more than 90% since the early 1970s and is now at a historically low level. The area occupied by the population appears to have declined significantly since the mid 1980s. Larger, older individuals have been severely depleted from this population. The probable cause of the decline is an unsustainable rate at which they were captured as bycatch in fisheries directed at other groundfish species, although current reported catches are low.

Western Scotian Shelf-Bay of Fundy, Georges Bank, Canada: The area of occupancy of Winter Skate here has been stable. Estimates of population status show no discernible trend over time. There is a high probability that the population receives immigrants from the population inhabiting the USA portion of Georges Bank. The population is subjected to bycatch in fisheries for other groundfish and shellfish species.

Georges Bank to Carolinas, USA: Abundance currently is slightly above the low levels recorded in the early 1970s. Presently, it is about 25% of the peak observed in the mid-1980s which followed the establishment of the 200 mile EEZ. The trajectory declined in the late 1980s?early 1990s to a level similar to the 1970s then increased slightly and has been relatively stable since. Here it continues to be taken as bycatch but is no longer commercially targeted, It has declined by almost 50% since the inception of a fisheries management plan (FMP) in 2003 (NEFMC 2007). However, since the 2004?2006 survey index was only 9% lower than the 2003?2005 NEFSC survey index, overfishing is not occurring for this species, but is in an overfished state (NEFMC 2007). Since abundance is low in all areas (particularly in the southern Gulf and the eastern Scotian Shelf), despite a reduction in groundfish effort and management measures (no directed fishing) in place, a regional Vulnerable listing is recommended for the USA.

Substantial declines (>90%) have occurred in two of the major areas of this species? range and declines have also occurred off the USA. Although the causes of these declines are mixed and uncertain, given the level of the declines and that these are ongoing in some areas, a precautionary assessment of Endangered globally is warranted. The situation should continue to be monitored.

 

Threats

Major Threats
Like many other elasmobranchs, the winter skate possesses life history characteristics that may increase vulnerability to exploitation, reduce rate of recovery and increase the risk of extinction: including delayed age at maturity, long generation time, low fecundity, and consequently slow population growth.

Otter trawling is the principal commercial fishing method used to target skates in the USA but are also taken as bycatch in gillnets and longlines off Canada. They are also taken as bycatch (and often discarded) in groundfish and shrimp trawls and scallop dredges. Eastern European factory trawlers fished west Atlantic shelf waters during the 1960s (at the beginning of the current three generation period) and into the early 1970s, causing population depletion. The stock possibly rebounded in parts of its range with the adoption of 200 mile EEZs and a reduction in fishing effort, with catch rates of juveniles increasing. The directed fishery in the USA began in the early 1960s, and the targeted Canadian fishery began in 1994.

The status of Winter Skate appears to differ throughout its Canadian range. Skate in the southern Gulf (where there is no directed fishery for Winter Skate) and on the eastern Scotian Shelf (where it is targeted) have shown a 98% and 90% reduction in abundance respectively since the early 1970s when surveys began. The probable cause is an unsustainable rate at which they were captured as bycatch in fisheries directed at other demersal species. In contrast, skate in the most southerly part of its Canadian range (western Scotian Shelf-Bay of Fundy, Georges Bank) appear to have been relatively stable in abundance over the time period for which data are available (COSEWIC 2005). The COSEWIC Assessment and Status Report (COSEWIC 2005) and DFO Recovery Potential Assessments for Winter Skate (DFO 2005 a&b) provide detailed accounts of decline trends in the different population units for this species, which are only outlined in summary here.

DFO (2005a) reports that catch rates of adult winter skates on the eastern Scotian Shelf have declined 90% since 1970. The decline appears to be ongoing and is projected to continue in future at recent fishery removal levels. The area occupied by the population appears to have declined significantly since the mid 1980s. Larger, older individuals have been severely depleted from this population. Juvenile abundance, which was low in 1970s, increased during the 1970s, was fairly stable in the 1980s, but has been declining since the 1990s. At the current high rate of adult natural mortality, no recovery is expected even if target and by catches are held to zero. Recovery of this population is uncertain.

The quota for the directed fishery on the Scotian Shelf was reduced to 200 t in 2003 from 2,000 t at the start of the fishery in 1994, in response to the population decline identified by DFO surveys and is estimated as less than 300 t annually over the past decade, with a low bycatch.

For the Southern Gulf, where there has been no target fishery, DFO (2005b) reports a 98% decline in abundance of mature individuals in surveys since 1971 to the lowest level on record. Catch rates of juveniles increased in surveys from the early 1970s to the mid 1980s, but have since also declined to low levels. Catch rates in the DFO Northumberland Strait survey (conducted since 2000) declined by over 50% between 2001 and 2004. Bycatch is now very low in groundfish and shrimp trawls but is unknown in the scallop fishery. Adult mortality appeared to increase during the 1980s and 1990s. Abundance is projected to decline even with no bycatch.

Elsewhere in Canada, winter skate are also taken in low numbers incidentally in fisheries targeting other species. The area of occupancy in the Georges Bank-Western Scotian Shelf-Bay of Fundy, in the centre of this species? range, has been stable and there has been no discernable trend over time (this population is shared with the US area of the Georges Bank).

In USA waters skate landings are not reported by species, with over 99% of the landings reported as ?unclassified skates?. ?Skate? landings in the USA reached 9,500 mt in 1969, but declined quickly during the 1970s, falling to 800 mt in 1981. Landings for all skates increased to 12,900 mt in 1993 and then declined somewhat to 7,200 mt in 1995. Landings have increased again since 1995, and the 1998 reported commercial landings of 17,000 mt were the highest on record. Winter Skate abundance is currently about the same as in the early 1970s (within the three generation period for this species), but is about 25% of the peak observed in the mid-1980s (from Packer et al. 2003). The U.S. population of winter skate has also declined by almost 50% since the inception of a fisheries management plan (FMP) in 2003 (NEFMC 2007) and a reduction in groundfish effort. However, since the 2004-2006 NEFSC survey index was only 9% lower than the 2003?2005 NEFSC survey index, overfishing is not occurring for this species, but it is in an overfished state (NEFMC 2007). Recreational and foreign landings are currently insignificant, at <1% of the total USA fishery landings (Packer et al. 2003).

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