|Scientific Name:||Amblyraja radiata|
|Species Authority:||(Donovan, 1808)|
Raja radiata Donovan, 1808
|Taxonomic Notes:||McEachran and Dunne (1998) moved this species from genus Raja to Amblyraja.|
|Red List Category & Criteria:||Vulnerable A2b ver 3.1|
|Assessor/s:||Kulka, D.W., Sulikowski, J., Gedamke, J., Pasolini, P. & Endicott, M.|
|Reviewer/s:||Dulvy, N.K., Valenti, S.V. & Musick, J.A. (Shark Red List Authority)|
The Thorny Skate (Amblyraja radiata) is found in the northeast and northwest Atlantic at depths of 18–1,400 m, but is most common in 27–439 m. There is evidence which might support population segregation and sub-division into subpopulations, but at this time it is unknown if genetic mixing of subpopulations takes place in either the northeast or northwest Atlantic stocks. Its geographic range includes contrasting population trends: relatively stable in recent years in Canada and the Northeast Atlantic, yet declining in the USA. Moreover, the potential occurrence of subpopulations with different age and growth rates and the potential lack of protection under a continuing USA wing fishery, warranted a precautionary approach to the evaluation. On the other hand, the overall abundance (whether divided among subpopulations or not) still constitutes several hundred millions of individuals. Skates are under fisheries management measures in both Canada and the USA, but the causes of observed declines are not well-understood across its range. Overall, the extent of the decline is considered to warrant a global assessment of Vulnerable. Regional population trends are summarised below.
The Thorny Skate is generally considered a deeper water species off the continental USA where it is not commercially targeted, but is landed as bycatch. The biomass of large size skates (>100 cm TL max. length: Barndoor Skate, Winter Skate and Thorny Skate) has steadily declined since the mid-1980s. Moreover, the abundance of Thorny Skate has declined to historic lows, and is currently 10–15% of the peak abundance observed in the late 1960s to early 1970s. In addition, the most recent NEFSC autumn average biomass index of 0.55 kg/tow is well below the biomass threshold level of 2.2 kg/tow. Thus the Thorny Skate is considered to be overfished (population level is below threshold levels) in the USA. A fishery management plan for the seven species skate complex of the USA Northwest Atlantic was implemented in September 2003. The plan prohibits the possession of Thorny Skate, Barndoor Skate and Smooth Skate in the Gulf of Maine. However, landings are not reported by species (even though this is now mandated under the fishery management plan), with over 99% of the landings reported as “unclassified skates. The low relative abundance, below the fisheries limit reference point compared to early survey abundance estimates, the long-term population decline, lack of population increase with strict management laws, and the inability to monitor species specific landings result in an assessment of this species as Critically Endangered in USA waters.
In Canada this species is found largely on the continental shelf. The population on the Grand Banks, the centre of its mass in Canadian waters underwent a ~68% decline in extent of occurrence between the 1970s and the early 1990s. Causes for this decline are unclear since the majority of the decline occurred in an area that is largely unfished. Recent work suggests that the decline coincided with a period of cold ambient temperatures. Spring surveys there indicate a minimum estimate of biomass about 100,000 t, which has been stable or increasing slightly over the past 15 years. This compares to about 350,000 t observed in the 1970s. Both exploitation levels and population abundance are relatively stable at the present time. It has been recently recommended that Thorny Skate catches in the Grand Banks not exceed current levels. Summer survey biomass on the Scotian Shelf (which lies between the Grand Banks and U.S. Waters) is less than that on the Grand Banks, but has declined by 80% since 1970, with no obvious environmental cause. Survey biomass has been relatively stable at a low level over the past 12 years, and is correlated with greatly reduced groundfish fishing effort. In Canadian waters as a whole, the low abundance relative to early survey abundance estimates and the long-term population decline result in an assessment of Vulnerable in Canadian waters.
The species is common in the Northeast Atlantic It is the most abundant skate in the North Sea, and has shown a marked increase between 1970 and 1983 in the Central North Sea and from 1982–1991 in English groundfish surveys. Although a survey of this species indicated a decline recently in the North Sea, this is believed to be a result of a change in survey gear. This species is occasionally landed as bycatch of demersal fisheries, but its distribution lies outside the main beam trawling areas in this region. It has a relatively low length at first maturity (44 cm) and demographic modelling suggests this species is less susceptible to fishing mortality in this region than other larger-bodied skate species. For these reasons in the Northeast Atlantic region this species is assessed as Least Concern.
|Range Description:||Eastern Atlantic: Svalbard, Greenland and Iceland to the English Channel, including the North Sea (except southern part) and the western part of the Baltic (Compagno et al. 1989).
Western Atlantic: Greenland (Davis Strait) and Hudson Bay, Labrador Shelf, Grand Banks, Gulf of St. Lawrence and Scotian Shelf (Canada), Gulf of Maine to mid-Atlantic (USA) (Robins and Ray 1986, Kulka and Miri 2003).
Native:Canada (Labrador, New Brunswick, Northwest Territories, Nova Scotia); United Kingdom (Great Britain, Northern Ireland); United States (Maine)
|FAO Marine Fishing Areas:||
Atlantic – northeast; Atlantic – northwest
|Range Map:||Click here to open the map viewer and explore range.|
Tagging studies (Templeman 1984b for the northwest Atlantic, Grand Banks (Canada); Walker et al. (1997) for the northeast Atlantic) suggest that Thorny Skates are rather sedentary, as most (85%) were recaptured less than 90 km from the initial capture point (some after twenty years at liberty). This information along with differences in size of egg capsules (Templeman 1984a), and the great latitudinal differences in size (length) at sexual maturity among the areas sampled, led Templeman (1987) to conclude that no large-scale migrations of thorny skates occurred between the sampled areas. However, recent distribution studies showed that thorny skate on the Grand Banks undergo a seasonal migration, shifting to deeper waters during the winter/spring and to shallower waters in the summer/fall (Kulka and Miri 2003). They may also undergo a limited migration on the Scotian Shelf.
The centre of concentration of Thorny Skate in the northwest Atlantic is on the Grand Banks where it is managed as a single stock. Spring surveys there indicate a minimum estimate of biomass about 100,000 t, which has been stable or increasing slightly over the last 15 years. This compares to about 350,000 t observed in the 1970s (Kulka et al. 2006). Since the mid-1980s, the range of Thorny Skate on the Grand Banks has been contracting (Kulka and Miri 2003). The centre of mass of this species on the Grand Banks underwent a ~68% decline in extent of occurrence (EOO) between the 1970s and early 1990s. Formerly covering the entire area in fairly dense concentrations, 80% of the biomass is now concentrated into 20% of the area along the southwest slope of the Grand Bank. Twenty-five percent of the area (to the north) is now devoid of thorny skate. This “hyper-aggregation” has progressed although the size of the population has been stable or slightly increasing since the early 1990s. The causes for this decline in EOO are unclear since the majority of the decline occurred in an area that is largely unfished. Recent work suggests that the decline coincided with a period of cold ambient temperatures.
Life history characteristics, such as size at sexual maturity, vary by area. Samples from the Scotian Shelf, (the southern most area sampled) indicated some mixing of small and large reproductive size groups, leading Templeman (1987) to suggest that more than one Thorny Skate stock may exist within this area. A more recent study by Simon and Frank (1996) in the Scotian Shelf found results comparable to those of Templeman (1987). Whether mixing of mismatched reproductive groups is occurring or if separate stocks exist, can not be determined based solely on Templeman’s (1984b, 1987) or Simon and Frank’s (1996) data. To some degree these lines of evidence would lend support for some degree of population segregation and sub-division into sub populations.
According to the 44th SAW report (NEFSC 2007), the NEFSC autumn survey biomass indices for Thorny Skate have declined continuously over the last 40 years and the 2005 index was a record low in the United States. The decline in Thorny Skate SSB is similar but more pronounced than for total biomass. In addition, Thorny Skate recruitment indices (number/tow between 25 and 35 cm) were relatively high although variable from 1963 to 1990, but have since declined to very low levels.
Landing statistics (1903–1993), International Bottom Trawl Surveys (1979–1993) and survey data collected in Dutch coastal waters between 1951 and 1994 reported that A. radiata is the most abundant species, representing 80% of the biomass and that showed a marked increase in the central North Sea between 1970 and 1993. (Walker and Heessen 1996).
Scottish trawl surveys in the central and northern North Sea for the period 1929–1956 and 1981–1995 shows that the starry ray was caught throughout the two periods and became more abundant between 1981 and 1995. The length-frequency pattern of the Thorny Skate was unchanged for the two periods (Greenstreet and Hall, 1996, Walker and Hislop 1998).
In and along the northeastern North Sea and the Skagerrak (Norwegian Deep) this species is very common and is the most abundant skate species (Bergstad 1990). In the research cruises carried out in the northeastern North Sea and Skagerrak from 1984–1987 and 1995–1996 the species was caught in 269 of total of 639 trawls (41%). In the central North Sea it has shown a marked increase between 1970 and 1983 (Heesen and Daan 1995) and from 1982–1991 in English groundfish surveys (Ellis et al. 2005). Catches of the Thorny Skate occurred in the entire area, but in the Skagerrak the catches were larger. Although a survey of this species indicated a decline recently in the North Sea, this is believed to be a result of a change in survey gear (Ellis et al. 2005).
The average individual weight was three to six times higher in the Skagerrak that in the North Sea. The larger individuals were most abundant in waters less than 300 m but there were indications of a movement towards deeper waters in winter. Estimates of total biomass of this species in the North Sea are of the order of 100,000 t (Sparholt and Vinther 1991) but the sampling effort was restricted by depth and area hence the results were not representative of the deeper northeastern part of the North Sea and the Skagerrak. The comparatively shallow areas off the west coast of Norway are probably used as nurseries (Skjæraasen and Bergstad 2000). The Barents Sea is considered a spawning ground (ICES 2006).
|Habitat and Ecology:||
The depth range of Thorny Skate is from approximately 18–1,400 m but is most common in 27–439 m (McEachran and Musick 1975, McEachran 2002, Kulka and Miri 2003). The temperature range for Thorny Skate appears to be from -1.3°C to 14°C. On the Grand Banks, they are concentrated in bottom temperatures between 2.5°C and 5°C. Thorny Skates are found over a wide variety of bottom types from sand, gravel, broken shell, pebbles, to soft mud (Bigelow and Schroeder 1953, McEachran 2002).
Along with this species’ broad geographic range, marked differences in size exist for specimens captured in different regions of the Atlantic. Moreover, a longitudinally increasing trend in size at sexual maturity in this species as one moves south from Labrador. For example, size at 50% maturity occurred within the ranges of 44–50 cm and 44–47 cm total length (TL), respectively, for males and females captured off West Greenland (Templeman 1984), but size at 50% maturity was about 55 cm for the Grand Banks (del Rio and Junquera 2001, del Rio 2002). In the Gulf of Maine, size at 50% maturity appears to be 87.5 cm TL for females and 86.5 cm TL for males (Sulikowski et al. 2006). In the North Sea size at maturity is reported at 44 cm TL for males and females (Skjæraasen and Bergstad 2000) and at 80 cm TL for females in the northern North Sea (Walker 1998). Maximum size in the Gulf of Maine is 111 cm TL (NEFSC 2000), whereas in the northern North Sea the species attains 90 cm TL and in the central North Sea, 60 cm TL (Walker 1998). This skate reproduces throughout the year in the Gulf of Maine and in autumn months on the Grand Banks (Sulikowski et al. 2005, del Rio 2001, 2002). Size at birth is 8–12 cm TL (Berestovskii 1994, Kulka and Miri 2003, Walker 1998). Litters of between 10–45 eggs are reported (Templeman 1987, Walker 1998).
Age estimates are available for the Gulf of Maine: Age at maturity is estimated at ~11 years and longevity at 16 years (Sulikowski et al. 2005). Generation period is estimated at 13 years (Sulikowski et al. 2005, 2006).
Prey of Thorny Skate in the western north Atlantic includes hydrozoans, aschelminths, gastropods, bivalves, squids, octopus, polychaetes, pycnogonids, copepods, stomatopods (larvae), cumaceans, isopods, amphipods, mysids, euphausids, shrimps, hermit crabs, crabs, holothuroideans, and fishes. However, the diet of Thorny Skate is size-dependent. Skates of less than 40 cm TL feed mostly on amphipods while fish >40 cm TL fed mostly on polychaetes and decapods. Mysids decreased in the diet while fishes increased with increase in size of the Thorny Skate. Fishes are a major component of the diet of skates >70 cm TL (McEachran et al. 1976). Scavenging behaviour has also been observed, Templeman (1982) recorded that nearly 14% of the stomach contents of A. Radiata >61 cm was composed of fish offal. In the northeast Atlantic the diet of juveniles and adolescents consisted mainly of polychaetes and crangonid shrimps. Upon reaching maturity, a major diet shift to fish, larger shrimps and squat lobsters occurred (Skjæraasen and Bergstad 2000).
Skates have been taken primarily as bycatch in groundfish trawl fisheries in New England waters for decades (Burgess et al. 2005). A. radiata is primarily landed for wings in the U.S. Since 1981, U.S. skate landings have increased substantially, primarily due to the increased export market for ‘skate wings’ and to a lesser extent because of the increased demand for lobster bait. It is thought that the winter Leucoraja ocellata, Thorny Skate (A. radiata) and Little Skates (Leucoraja erinacea) comprised most of this catch (Dulvy 2005). However, in the USA, landings are not reported by species, with nearly 100% of the landings reported as unclassified skates. “Skate landings 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 (from Packer et al. 2003)”.
The biomass of large size skates (>100 cm TL max. Length; including Barndoor Skate, Winter Skate and Thorny Skate) has steadily declined since the mid-1980s. The abundance of Thorny Skate has declined to historic lows and current abundance is about 10–15% of the peak observed in the late 1960s to early 1970s. In addition, the most recent NEFSC autumn average biomass index of 0.55kg/tow is well below the biomass threshold level of 2.2 kg/tow (NEFMC 2007). Thus the Thorny Skate is considered to be overfished (population level is below threshold levels) in the USA.
The principal commercial fishing method used to catch thorny skates is otter trawling. In USA waters these skates are frequently taken as bycatch during groundfish trawling and scallop dredge operations and discarded. Recreational and foreign landings are currently insignificant, at et al. 2003).
The main human induced threat in Canada is a directed fishery on the Grand Banks where an average of about 11,800 t have been removed during the past five years using trawl gears (Kulka et al. 2004). Thorny Skate are also taken as bycatch in much smaller amounts in other areas from the Scotian Shelf, north to the Davis Strait. Recent work also shows that the decline on the northern Grand Banks is correlated to the period of cooling in that area (Colbourne and Kulka 2004). The cause of the cooling and the likelihood of future similar occurrences are unknown. Summer survey biomass on the Scotian Shelf (which lies between the Grand Banks and U.S. Waters) is less than that on the Grand Banks, but has declined by 80% since 1970, with no obvious environmental cause. Survey biomass has been relatively stable at a low level over the past 12 years, and is correlated with greatly reduced groundfish fishing effort (McPhie 2007). About 3,000 of the 11,800 t of removals from the Grand Banks fishery from the Grand Banks and 100% of the landings are exported as wings to Europe. Most of the remainder of the landings are attributable to Spain, Portugal and Russia in the NRA (NAFO Regulatory area outside of Canada's 200 mile limit). All non-Canadian catches are landed in Europe.
This species is occasionally landed as bycatch of other demersal fisheries in the northeast Atlantic. Thorny Skates are known to be scavengers (Templeman 1982) and are less susceptible to fishing mortality in this region than other skates because of a low length at first maturity (~44 cm TL in the North Sea). Moreover, this species is distributed outside the major beam trawling areas in the southern and southeastern North Sea (Walker and Heessen 1996). They are bycatch in demersal trawl fisheries in the North Sea, Irish Sea and Barents Sea. They are not considered a desirable species and thus are not targeted.
Thorny Skate is the most abundant skate species in the North Sea and Skagerrak, and populations trends appear to be stable or increasing throughout this area. It is taken as bycatch in the Barents Sea and it is the dominant skate species in this area, comprising 96% by number of total number and about 92% by weight of skates caught in surveys or as bycatch (ICES 2006). Relative CPUE data for A. radiata in the Barents Sea from 1997–2003 indicate that biomass and abundance increased during this time period (ICES 2006). Obtained fishery-independent survey cruise data (from 1998–2001) on stocks of A. radiata in the Barents Sea remained almost unchanged during survey timeframe, possibly suggesting stable stocks in the examined area (Dolgov et al. 2004, ICES 2006).
Commercial and recreational fisheries for Thorny Skates are now prohibited in the continental U.S. as per the current fisheries management plan (NEFMC 2003, 2005, 2007). However, landings are not reported by species (even though this is now mandated under the fishery management plan), with over 99% of the landings reported as “unclassified skates.
Recommendations for future conservation action: Enforce U.S. species-specific landings reporting requirements and increase scientific research and government sea sampling attention to skates. Reduce bycatch by closing and/or reducing fishing effort (particularly otter trawling) in areas of high thorny skate concentration (USA and Canada). Reduce discard mortality by encouraging and/or mandating, gear modifications, more careful handling and discard techniques.
The current quota for skate (90%+ of the catch is thorny so we assess as thorny) on the Grand Banks is 13,500 t inside and outside Canada’s 200 mile limit, managed by NAFO. NAFO is presently setting the quota for the next three years. It has been recently recommended that thorny skate catches in the Grand Banks not exceed current levels. The rationale is that abundance as well as catch levels have remained relatively stable in years. Skates are recorded to species by fishery observers at sea but not reported in logs to species (Kulka et al. 2006).
Maintain current efforts to reduce the fishing effort of the demersal fleet.
|Citation:||Kulka, D.W., Sulikowski, J., Gedamke, J., Pasolini, P. & Endicott, M. 2009. Amblyraja radiata. In: IUCN 2012. IUCN Red List of Threatened Species. Version 2012.2. <www.iucnredlist.org>. Downloaded on 25 May 2013.|
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