|Scientific Name:||Dipturus laevis|
|Species Authority:||(Mitchill, 1818)|
|Taxonomic Notes:||Synonym = Raja laevis.|
|Red List Category & Criteria:||Endangered A1bcd ver 3.1|
|Reviewer/s:||Fowler, S., Kulka, D. & Cavanagh, R.D. (Shark Red List Authority)|
Dipturus laevis was previously assessed as Vulnerable under the 1994 Categories and Criteria (VU A1bcd). With better information available on this species, it is now reassessed as Endangered.
The barndoor skate is highly vulnerable to exploitation because of its slow growth rate, late maturity, low fecundity and large body size. The slow life history exhibited by the barndoor skate would render it particularly vulnerable to decline under exploitation and would be associated with an elevated risk of extinction. Although never directly targeted, it was a bycatch of multispecies trawl fisheries on the Georges Bank, Scotian Shelf, Grand Banks and Labrador Shelf and is also taken on longlines. Catch rates of barndoor skates in USA waters < 400 m within the centre of its latitudinal range on the southern shelf ( 450 m depth), where less fishing occurs. There is no evidence for a decline in the newly discovered, northern and deepwater parts of the species' range, but fisheries have only recently moved into these areas and it is assumed that the population here is much smaller. The shallow-water section of the population is presumed to have, in the past, represented most of the global population and also the most productive section of the population. However, it should be noted that increases in trawl fishing effort and/or opening of no-take areas could lead to the decline of the barndoor skate in these areas.
|Range Description:||Restricted to the NW Atlantic continental shelf and slope of Canada and the USA. Its range was thought to extend from Cape Hatteras, North Carolina, to southwestern Grand Bank, Canada (Kulka et al. 2002). Recent fisheries observer data, however, indicate the barndoor skate extends further north along the Labrador shelf edge and slope as far as 63°N in deep slope waters (Kulka et al. 2002). The barndoor skate is found on most bottom types and is typically caught in cool water, with a preferred temperature range between 0.4 to 10.9 ºC and up to 20 ºC (Bigelow and Schroeder 1953, Kulka et al. 2002, McEachran and Musick 1975, Packer et al. 2003). Historically it was found as shallow as the tideline (Bigelow and Schroeder 1953, Anonymous 2000) and the lower depth distribution was thought to be around 715 m (Bigelow and Schroeder 1953, McEachran and Musick 1975, Scott and Scott 1988) but recent observer data indicate a depth distribution down to 1,400 m (Kulka et al. 2002).|
Native:Canada; United States
|FAO Marine Fishing Areas:||
Atlantic – northwest; Atlantic – western central
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||Historically, it is not exactly known how abundant the barndoor skate was on the continental shelf, but it is now generally uncommon throughout its range. A comprehensive examination of all available data indicates they were captured in only 1,015 of a total of 80,427 gear sets (1.26%) (Simon et al. 2002). These data came from nine surveys, including both non-standard and standardised research vessel surveys, and covered virtually all of the Canadian continental shelf waters. Surveys off Newfoundland have been carried out since 1950, the Scotian Shelf and Gulf of St. Lawrence surveys commenced in 1970 and 1971 respectively. The species is currently more abundant in the southern part of its range, including the Gulf of Maine, Georges Bank, Scotian Shelf and southern New England (Anonymous 2000). But it appears to be very rare on the shallower continental shelf and the main part of the population is now found in shelf channels and along the continental shelf edge in waters > 450 m deep (Anonymous 2000, Kulka 1999). The shelf locations where barndoor skate can be captured with any regularity are along the southern and eastern edge of the Georges Bank and inside areas closed to trawling on the Georges Bank and along the Scotian Shelf (J.A. Musick, T. Gedamke and S. Murawski, pers. comms., Simon et al. 2002).|
|Habitat and Ecology:||
The barndoor skate attains a maximum length of 153 cm and a maximum weight of 20 kg; the largest skate species in the Northwest Atlantic and eleventh largest in the world (Bigelow and Schroeder 1953). Age at maturity was assumed to be similar to that of the common skate D. batis (the sister species from the Northeast Atlantic): an estimated 11 years (Casey and Myers 1998), but this is likely to be an overestimate. Age at maturity has been estimated as eight years by extrapolating from skate allometric relationships (Frisk et al. 2001). The average age of mature individuals in the population (generation length) therefore can be estimated to be 10 to 14 years. It is likely that the lower value for age at maturity is closer to reality. Longevity has been estimated as between 13 and 18 years, assuming that the age of maturity is 60% of the lifespan, then age at maturity can be assumed to be 8 to 11 years (Frisk et al. 2001). Length frequency data indicate few adult individuals remaining in the population, with these generally less than 130 cm long. However, these data were using a sample gear (scallop dredge) that may underestimate the abundance of larger size classes.
Length at maturity has been estimated to be 115 cm for females and 112 cm for males (T. Gedamke, pers. comm.), but these may have changed due to variations in population density, food availability and exploitation rates. Egg production is estimated as 47 per year, based on the inverse relationship between fecundity and weight of hatchlings and common skate life history parameters (Casey and Myers 1998). Body size is a good general predictor of demography and vulnerability to exploitation in skates; the large size of the barndoor skate suggests that it is one of the most vulnerable of all the skates (Dulvy et al. 2000, Dulvy and Reynolds 2002, Frisk et al. 2002, Frisk et al. 2001, Musick et al. 1999, Walker and Hislop 1998).
The diet includes bivalves, squid, rock crabs, lobsters, shrimps, worms and fishes (Packer et al. 2003, Robbins and Ray 1986).
The barndoor is too rare to be specifically targeted, but is captured as part of the skate complex in USA waters (Anonymous 2000). Skate landings are not recorded by species and total skate catch in the USA never exceeded several hundred metric tonnes until the advent of distant water fleets during the 1960s (Anonymous 2000). Skate landings reached 9,500 mt in 1969 then rapidly declined during the 1970s, falling to 800 mt in 1981 (this excludes landings in Canada, including the Grand Banks and Scotian Shelf.)
Since 1981, USA 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 (Anonymous 2000). It is thought that the winter, thorny and little skates comprised most of this catch. Total skate landings increased to 12,900 mt in 1993 and declined to 7,200 mt in 1995, since then landings have increased to 17,000 mt, the highest on record (Anonymous 2000). It must be noted that there is little fishing activity in waters greater than 200 m deep, where the remainder of the barndoor skate population exists. Landings data were aggregated across species. However, the large size and distinctive colouration of barndoor skate may mean some have been identified correctly, indicating commercial fishery discards of a few hundred metric tonnes per year (Anonymous 2000).
In Canada, a directed fishery for all skates began in 1994 in northerly areas of the barndoor skates distribution, on the Scotian Shelf and Grand Banks. However, the major threat to this species? continued existence is bycatch in commercial fisheries (Casey and Myers 1998), particularly the benthic trawl fisheries for cod and redfish (Sebastes spp.) and dredge fisheries for scallops. It should be noted that fishing effort on demersal fish in Canadian waters has declined substantially in the past decade as a result of several moratoria (cod, plaice), closed fisheries (grenadier) and reduced quotas (redfish, witch) (D. Kulka, pers. comm.).
A number of fishery-independent trawl survey data collected in both spring and autumn consistently indicate declines in the abundance and biomass of the barndoor skate and a truncation of the population size structure, indicating the loss of the largest individuals in USA waters (Anonymous 2000). Historical abundance is unknown but must have been higher given the North Atlantic-wide depletion of large high trophic level fishes (Christensen et al. 2003); consequently the patterns of decline observed could be regarded as conservative. Surveys in most areas off Canada began in the early 1960s, but some areas have been surveyed since the 1950s (S. Grand Bank, St Pierre Bank, Sydney Bight). In northerly parts of the barndoor skate distribution, the major abundance decline, according to survey data, was from the 1950s. In more southerly areas, namely USA waters, the major decline in abundance occurred between the 1960s-1970s. On average the mean catch rate was 1.922 kg per hour in 1963?5, which declined to an average of 0.0786 kg per hour in 1996?8. This represents a 96% decline in abundance (catch rate) over approximately three generations (30?42 years) (Table B17, Anonymous 2000). According to Casey and Myers (1999) the barndoor range has also contracted over this time. They described it as being currently found in three of the nine NAFO statistical areas in which it was formerly abundant. This is not strictly correct, it continues to be found in all NAFO areas, but at reduced numbers (Kulka 1999, Kulka et al. 2002, Simon et al. 2002). The trends in abundance and biomass of barndoor skate in deepwater (> 200 fathoms) off the US coast are unknown. One highly regulated fishery (for monkfish) currently operating in waters > 200 fathoms could catch barndoor skate (D. Kulka, pers. comm.).
According to these survey data there appears to have been a steady increase in the barndoor skate abundance in the centre of their range on the SE Georges Bank and the SW of Browns Bank since 1992. An alternative interpretation of these data has been suggested. It has been argued that the census biomass was not statistically significant from zero (i.e., the species was ?statistically extinct?) in 17 of the 18 years from 1980 to 1998 inclusive. Thus, the trawl surveys from which the population trends are derived may have very low power to detect either further increases or decreases in abundance (Dulvy et al. 2003). However, it has been argued that such patterns are common even in more abundant fishes due to the large extent to which they are aggregated or clumped (contagion) (D. Kulka, pers. comm.). Such patterns will produce highly skewed sampling distributions that would invalidate the use of statistics based on the assumption of normality.
Trends in survey abundance and biomass indices for barndoor skate in shallow waters (< 200 fathoms) are well documented for Canada (Kulka 1999, Kulka et al. 2002). A decline in the survey indices occurred in the mid-1960s to early 1970s, likely caused by the high fishing effort of the distant water fleet on Georges Bank, followed by a period of low to zero catches. In 1985, consistent catches of barndoor skate began occurring and increases in survey indices were observed that have continued through 2001.
The current overall population trend should be considered uncertain at the worst or increasing at best. Some areas of the Georges Bank and southern New England shelf have been closed to fishing since 1993, primarily to protect scallop stocks, and there is good evidence for an increase in the abundance of barndoor skate within these no-take areas (S. Murawski, pers. comm.). Also, fishing effort surrounding these areas has declined with the closure of the cod fishery. It is possible that opening the no-take areas and increasing fishing effort will again lead to the decline of the barndoor skate in these areas. However, this evidence for increase should be considered in light of the sampling issues touched upon above. In addition to this problem it is worth considering other sampling issues that would provide cause for a more optimistic view of the barndoor skate abundance. The historical trawl data comprising the early parts of the time series compiled by Casey and Myers (1999) have been criticised (Kenchington 1999). It is argued that the survey gears and design were not optimised for barndoor skate also that historical data are not comparable with more recent surveys (Kenchington 1999). It should be noted that survey estimates are not absolute abundance, but they are estimates of relative abundance, this results in two reasons for a more cautious and more optimistic view of the reported declines can be taken. A large part of the barndoor skate is rarely or never sampled, particularly the northern > 520 and in deep water > 400 m, thus abundance indices may not fully reflect population trends (D. Kulka, pers. comm.). The survey abundance estimates should be considered as minimum estimates of the population because it is believed that the catchability of barndoor skate by the survey gear is low (D. Kulka, pers. comm.). For example, using underwater cameras attached to the head ropes of standard trawl gear, it was noted that the barndoor skate were, "extraordinarily adept at avoiding capture" (Edwards 1968). Also the escapement of all sizes of skates from ?Engels? survey gear was high (Walsh 1992). Consequently this makes any assessment of the conservation status of the barndoor skate quite uncertain because catchability may well change with skate density and changing gears or technological creep. Therefore accounting for and understanding the influence of this potential source of error upon the estimate of population trends is difficult.
The status of the barndoor skate has been the subject of considerable debate since Casey and Myers (1998) reported on its decline, which they described as a ?near extinction?, and a number of petitions were made to list the species under the US Endangered Species Act (ESA).
As a result, fisheries scientists used US Northeastern Fisheries Science Centre (NEFSC) research survey data from the southern part of the barndoor skate?s range (<45°S from the Gulf of Maine to southern New England) to assess its status. These data indicate declines in biomass and abundance indices of 96% and 99% respectively over approximately three generations (18 to 33 years), with mean catch rates in 1963 to 1965 of 1.922 kg/tow and average 1.82 individuals/tow, declining to 0.0786 kg and 0.025 individuals in 1996 to 1998 (Table B17, Anonymous 2000). However, there have been apparent recent increase in abundance and biomass in surveys in US and Canadian waters and the species has been discovered in waters deeper than previously covered by these surveys (Kulka et al. 2000), where fishing effort is very low. It was concluded that there was no evidence that 'they were in danger of extinction or likely to become endangered within the foreseeable future' (Anonymous 2000). The ESA petitions were not accepted.
To date no management or regulatory measures are in place apart from ad hoc protection in closed areas of the Georges Bank, although the increased numbers of juveniles reported from this area indicates that recovery is possible.
|Citation:||Dulvy, N.K. 2003. Dipturus laevis. In: IUCN 2013. IUCN Red List of Threatened Species. Version 2013.2. <www.iucnredlist.org>. Downloaded on 04 December 2013.|
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