COSEWIC Assessment and Status Report on the Barndoor Skate Dipturus laevis in Canada NOT AT RISK 2010 COSEWIC status reports are working documents used in assigning the status of wildlife species suspected of being at risk. This report may be cited as follows: COSEWIC. 2010. COSEWIC assessment and status report on the Barndoor Skate Dipturus laevis in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. xiii + 71 pp. (www.sararegistry.gc.ca/status/status_e.cfm). Production note: COSEWIC would like to acknowledge James Simon and Sherrylynn Rowe for writing the status report on the Barndoor Skate (Dipturus laevis) in Canada, prepared under contract with Environment Canada. This report was overseen and edited by Paul Bentzen, Co-chair of the COSEWIC Marine Fishes Specialist Subcommittee with the support of Dave Kulka, member of the Marine Fishes Specialist Subcommittee. For additional copies contact: COSEWIC Secretariat c/o Canadian Wildlife Service Environment Canada Ottawa, ON K1A 0H3 Tel.: 819-953-3215 Fax: 819-994-3684 E-mail: COSEWIC/[email protected] http://www.cosewic.gc.ca Également disponible en français sous le titre Ếvaluation et Rapport de situation du COSEPAC sur la grande raie (Dipturus laevis) au Canada. Cover illustration/photo: Barndoor Skate — Drawn by P.D. MacWhirter and extracted from Sulak et al. (2009). Her Majesty the Queen in Right of Canada, 2011. Catalogue No. CW69-14/620-2011E-PDF ISBN 978-1-100-18597-2 Recycled paper COSEWIC Assessment Summary Assessment Summary – November 2010 Common name Barndoor Skate Scientific name Dipturus laevis Status Not at Risk Reason for designation This species, one of the largest skates in the western Atlantic Ocean, and with an estimated generation time of 13 years, ranges on continental shelf habitats from Cape Hatteras to the Grand Banks. In Canadian waters, it is most common on Georges Bank and the western Scotian Shelf. Numbers declined in the 1960s, likely due to bycatch in fisheries directed at other species. Indices of abundance are made less precise by fluctuations in distributions and the ability of large mature fish to evade survey gear, but indicate that the abundance of mature individuals has not declined over the last three generations, and has increased during the last 1-2 generations. Survey catch rate data indicate an ongoing increase in the abundance of mature and immature individuals on Georges Bank and western Scotian Shelf. Data from American surveys on Georges Bank suggest that the species has increased to a level that is approximately half the abundance estimated for this species in this area in the early 1960s. There are no directed fisheries for the species, and regulations are in place to reduce mortality from bycatch. Occurrence Atlantic Ocean Status history Designated Not at Risk in November 2010. iii COSEWIC Executive Summary Barndoor Skate Dipturus laevis Wildlife species information Barndoor Skate is one of the largest skates in the western Atlantic, reaching a total length of 163 cm. Another common name is the sharpnose skate and, in French, it is called grande raie. It is distinguished from other skates by (collectively) having an acute snout, a relative absence of thorns, an upper surface that is generally brown with numerous dark spots, and an oval spot or eye in the centre of each pectoral fin (wing). The lower surface is white to grey with dark pigmented ampullar pores in larger individuals. Distribution Barndoor Skate (Dipturus laevis Mitchill 1818) have been reported as far south as northeastern Florida but it has been suggested that the most southerly records may have been a misidentification. Recent US reports indicate that the species occurs north of Cape Hatteras but is most common north of Cape May (39ºN), on Georges Bank and in the Gulf of Maine. In Canadian waters, the species is most common on Georges Bank and the western Scotian Shelf to Sable Island. It is less common on the southern edges of the Scotian Shelf and the Grand Banks. This deepwater extension continues outside of Canadian waters to the tail of the Grand Banks as far east as the Flemish Cap. There are scattered reports of Barndoor Skate on the northern portion of the Grand Banks (identified by the Northwest Atlantic Fishery Organization (NAFO) as Div. 3L) and the Laurentian Channel in the Gulf of St. Lawrence. There is no evidence to support more than one designatable unit for Barndoor Skate. Historically, they were generally distributed continuously from the southern half of the Grand Banks to south of Georges Bank. As the population declined, their distribution contracted to Georges Bank. As the population rebounded, the species expanded from relatively small areas on Georges Bank to the whole of Georges Bank, then to NAFO Div. 4X(southwest Scotian Shelf), and recently, back into NAFO Div. 4W(central Scotian Shelf) to the northeast. Low catchability by research vessel (RV) surveys may give the impression of apparent gaps in their distribution, but industry surveys in the same areas indicate that they are iv Habitat Barndoor Skate have been found on soft mud, sand, and gravel bottoms. They are widespread from 38-351 m (the deepest part of the survey) in Div. 5Z (Georges Bank). On the Scotian Shelf, Barndoor Skate are primarily caught in 50-150 m but have been reported from 24 to 375 m. Length frequency data from the halibut industry/science survey suggests that adults are common at depths greater than those surveyed by the DFO RV surveys. Juveniles and adults on both the Canadian and US sides of Georges Bank appear to occupy the same geographic areas. Juveniles were widespread on the western Scotian Shelf to Sable Island but adults were concentrated in the Fundian Channel. Off Newfoundland, Barndoor Skate have been recorded as deep as 1174 m although most fishery observer records, including some of the deepest records have not been confirmed with specimens or photos. Preferred temperature range from US RV surveys is 3-18ºC for the Gulf of Maine to Cape Hatteras with a reported peak of 20ºC off Cape Hatteras. Salinity preference range is 32-36 ppt in the same areas. Based on Canadian surveys on Georges Bank, preferred temperatures are 3-13ºC, while on the Scotian Shelf, the temperature range is 2-11 ºC with a preference for 3-9ºC. In the Gulf of St. Lawrence, they have been recorded at temperatures as low as 1-2ºC. Information on skate purse distribution is limited to reports of females containing fully developed eggs in December and January, both near Sable Island and in Kennebecasis Bay, N.B. As well, a single female with almost complete purses was sampled from the western half of Div. 4W, in September. Biology A comprehensive study on the biology of Barndoor Skate on the US side of Georges Bank revealed that length at 50% maturity (Lmat) for females was 116 cm while males matured at 108 cm. A preliminary aging analysis revealed faster growth (k = 0.14- 0.18) and younger age of female maturation (6.5-7.2 years) than had been previously assumed. A preliminary study of Barndoor Skate in on the Scotian Shelf (Div. 4W) estimated Lmat at 114 cm. Annual estimates of fecundity from a single captive female from NAFO Div. 5Z were 33, 69, 85, and 115 purses from 2003 to 2007. Mean incubation time was 421 days. In the study of Barndoor Skate from Div. 5Z, generation time was calculated to be 13 years but given the uncertainty in natural mortality (M) and the likelihood that growth is slower in Canadian waters, it is possible that the true generation time may be greater than 13 years. v Population sizes and trends Because catchability in survey trawls is < 1, values of abundance and biomass reported here are considered to be minimum estimates. Mean population estimates were calculated from Canadian and US RV surveys for NAFO Div. 4VWX and 5Z based on one generation time or natural breaks in the data. Recent trends in population abundance were estimated using linear regression after log transformation for all sizes combined given the very low adult catch rates. These data are not provided for the other regions given very low catch rates. Minimum abundance from the US Fall RV survey (Div. 4X5ZY6) during 1963-1975 averaged 1.1 million juveniles and 62,000 adults annually. During 1976-1994, mean abundance was 54,000 juveniles and 2,800 adults. During 1995-2007, there were 79,700 juveniles and 112,000 adults. The average of the first four years of this survey was 3.2 million fish while the average of the last four years was 1.7 million fish or 53% of the earlier period. Abundance estimates from the Canadian RV survey for Div. 5Z were calculated for the whole of Georges Bank as well as the Canadian portion only. Abundance of all size classes on Georges Bank averaged 52,000 fish during 1987- 1995. During 1996-2008, abundance averaged 622,000 juveniles and 11,000 adults. On the Canadian side of the bank, total abundance of all sizes averaged 15,000 individuals during 1987-1995. Since 1996, there was an average of 43,000 juveniles and 2,300 adults. On the Scotian Shelf, the data were grouped into 3 generations: 1970-1982, 1983-1995, and 1996-2008. The average number of juveniles was 152,000, 15,000, and 118,000 for each of these three respective time periods. Number of adults for the same time periods was 11,000, 0, and 39,000. The annual rate of increase from 1995 to 2007 for the US Fall and Spring RV surveys was 11% and 14%, respectively.