|Scientific Name:||Bathyraja griseocauda (Norman, 1937)|
|Red List Category & Criteria:||Endangered A2bd+4bd ver 3.1|
|Assessor(s):||McCormack, C., Lamilla, J., San Martín, M.J. & Stehmann, M.F.W.|
|Reviewer(s):||Haywood, M., Kyne, P.M. & Valenti, S.V. (Shark Red List Authority)|
Bathyraja griseocauda occurs in the Southwest Atlantic off Argentina and the Falkland/Malvinas Islands on the slope from 37°S and the shelf from 41°S and in the Southeast Pacific off Chile from 41°S. It is a large skate (at least to 156 cm TL) reported at depths of 82 to 941 m in the Southwest Atlantic and 137 to 595 m off Chile. It is known to be slow growing and matures later than other species of Bathyraja. The vulnerability of large skates to overexploitation and subsequent population depletion is well documented and in the case of B. griseocauda, directed and bycatch fishing pressure are impacting on the species. Around the Falkland/Malvinas Islands, B. griseocauda is taken in the multispecies skate trawl fishery where it was formerly the dominant species in the catch, especially in a ray "hot spot" to the south of the Islands where it comprised around 70% of the catch. Between 1993 and 1995, the proportion of B. griseocauda in catches from this southern area fell to around 5%. The proportion in catches north of the Islands also fell and B. griseocauda was replaced as the dominant species in the catch by B. albomaculata in the north and B. brachyurops in the south. Total catches of the species fell from ~1,500 t to ~100 t between 1993 and 1995 in the south and from over 1,000 t to ~250 t in the northern areas between 1993 and 1997. The mean disc width of B. griseocauda also decreased from 52.18 cm to 38.08 cm between 1993 and 1997. In 1996, the southern fishing area (below 52°S) was closed to the ray fleet. A recent assessment of the northern ray population indicated that the CPUE of this species continued to decline from 1992 to 2001 from almost 100 kg/hr to <50 kg/hr. However, the quality of the data was relatively poor and this low level of precision should be taken into consideration. There have been no studies to determine the abundance of B. griseocauda in the southern area since the rajid fishery closure; however, it is also caught as bycatch by finfish trawlers that operate around the Falkland/Malvinas Islands and within the closure area. The species is also taken as bycatch in deeper water benthic trawl teleost target fisheries off Argentina. There are no species-specific catch estimates available for these fisheries, however, fishing pressure has increased substantially over the past decade and in 1999, there was a decrease in the capture of rays by the deep sea fishing fleet of around 15% with regard to 1998. There was a reported a decline in the biomass of B. griseocauda captured during fishery-independent investigations at 45° to 55°S off Argentina from 1998 to 1999, however the second phase of investigations employed gear which likely reduced the capture of rays. The species is also taken by longliners off Argentina and Chile targeting Dipturus chilensis. The catch in these fisheries has been reported to comprise up to around 18% and <5% in Argentina and Chile, respectively. Off Chile overall biomass of D. chilensis has declined by 51% since fishing began in 1979 and declines are thus also likely to have occurred for bycatch species. The species is also taken as bycatch in the Patagonian toothfish longline fishery off Chile. Globally then, with documented declines off the Falkland/Malvinas Islands and direct targeting and bycatch pressure continuing throughout its range, the species is assessed as Endangered on the basis of observed and inferred past and suspected future declines.
|Range Description:||This species occurs in the Southwest Atlantic south of 37°S on the slope and south of 41°S on the shelf off Argentina and the Falkland/Malvinas Islands in the range of Atlantic Antarctic convergence. The species also occurs in the Southeast Pacific off Chile south of 41°S.|
Native:Argentina (Buenos Aires, Chubut, Rio Negro, Santa Cruz, Tierra del Fuego); Chile; Falkland Islands (Malvinas)
|FAO Marine Fishing Areas:|
Atlantic – southwest; Pacific – southeast
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||The population size is unknown. Declines have been detected around the Falkland/Malvinas Islands (Agnew et al. 2000, Wakeford et al. 2004). Length frequency data around the Falkland/Malvinas Islands show B. griseocauda of all sizes to be present (Wakeford et al. 2004). Unlike some other species, there is no evidence for large spatial or temporal movements and the species may complete its entire life cycle within Falkland Island waters (Wakeford et al. 2004).|
|Current Population Trend:||Decreasing|
|Habitat and Ecology:||Marine, benthic species reported from depths of 82 to 941 m in the Southwest Atlantic in bottom temperatures ranging from 3.0°C to 8.0°C (Menni and Stehmann 2000) and at depths of 137 to 595 m off Chile (J. Lamilla pers. comm. 2006). This species shows a strict stenothermic-stenohaline behaviour, in relation to water masses (Figueroa et al. 1999). |
During research trawls around the Falkland/Malvinas Islands, B. griseocauda were more abundant in deeper trawls (200 and 350 m) and formed only a small part of the catch in shallow trawls (150 m) (Wakeford et al. 2004). Length frequency data for individuals captured around the Falkland/Malvinas Islands showed that all sizes of B. griseocauda were present, with smaller individuals found in deeper water (Wakeford et al. 2004). Unlike some other species, there is no evidence for large spatial or temporal movements and the species may complete its entire life cycle within Falkland Island waters (Wakeford et al. 2004).
Known to be a slow growing, long-lived species (Wakeford et al. 2004). Individuals mature at around 15 years (Agnew et al. 2000). Size at maturity has been estimated at around 120cm TL in male specimens (Stehmann et al. unpubl. data). Individuals have been reported from a minimum of 13 cm (Stehmann et al. unpubl. data.) to a maximum of 157 cm (Agnew et al. 2000).
Small individuals feed opportunistically on benthic isopods and larger specimens are predominantly piscivorous on Patagonotothen ramsayi. This species is considered to be an active predator (Brickle et al. 2003). Like other skates, this species is oviparous.
Further information on the life history of the species may become available in the near future as the ECORAYA project is finalised (M. Stehmann pers. obs. 2006).
Throughout this section the term rajid refers to skates of both the families Rajidae and Arhynchobatidae.
Skate landings have been increasing considerably in Argentina due to international demand. Prior to 1994, skate captures were less than 1,000 t, however, since that year skate landings increased considerably, reaching 15,000 t in 2001 and 17,465 t in 2003 (Massa et al. 2004).
Bathyraja griseocauda is a regular bycatch in bottom trawl fisheries for bony fishes. It has been captured during fishery-independent investigations for hake Merluccius hubbsi and other species (García de la Rosa et al. 2000). García de la Rosa et al. (2000) reported a 59% decline in the biomass of B. griseaocauda captured from 45° to 55°S during the summer investigations of 1999 compared to 1998, however, it was acknowledged that during the second phase of the investigations, new gear was employed which likely reduced the capture of rays. During 1999, there was a decrease in the captures of rays by the deep sea fishing fleet of around 15% with regard to 1998 (García de la Rosa et al. 2000).
The species is also taken in the Dipturus chilensis directed skate fishery off Argentina which currently comprises a single vessel. Onboard observation of the fishing operation in 2000 and 2001 indicated that the vessel fished from 37° to 44°S off Argentina in two regions; around 50 m of depth and along the 100 m isobath. At greater depths, the processed catch composition varied between trips and comprised up to around 18% of B. griseocauda (Colonello et al. 2002). Species-specific bycatch data are not generally collected for this fishery, however, and remain a priority for this and the trawl fishery.
This species is taken in the multispecies skate trawl fishery around the Falkland/Malvinas Islands which has been operating since 1989. The fishery initially operated over two main areas, one located on the shelf edge to the north of the Islands, and the other to the south of the Islands.
In 1993 (the first year where observer data is available), this species was the dominant species of skate caught by finfish and ray-licensed vessels, especially in a ray "hot spot" to the south of the Islands where it comprised around 70% of the catch (Agnew et al. 2000). By 1993, however, the proportion of the catch comprising B. griseocauda in this south had fallen to around 5% (Agnew et al. 2000). The proportion of this species in catches north of the islands also fell and B. griseocauda was replaced as the dominant species in the catch by B. albomaculata in the north and B. brachyurops in the south (Agnew et al. 2000). Total catches of the species fell from around 1,500 t to around 100 t in 1993 to 1995 in the south and from over 1,000 t to around 250 t in the northern areas between 1993 and 1997 (Agnew et al. 2000).
The mean disc width of B. griseocauda also decreased from 52.18 cm in 1993 to 38.08 cm in 1997. Following declines in the early 1990s, the southern fishing area (south of 52°S) was closed to the ray fleet in 1996 and the fishery. A recent assessment of the northern ray population indicated that the CPUE of this species continued to decline from 1992 to 2001 from almost 100 kg/hr to et al. 2004). However, the quality of the data was relatively poor and the data had to be grouped into discrete time periods rather than as a continuous variable. This low level of precision should be taken into consideration (D. Wakeford pers. comm. 2006).There have been no studies to determine the abundance of B. griseocauda in the southern area since the rajid fishery closure; however, it is also caught as bycatch by finfish trawlers that operate around the Falkland/Malvinas Islands and within the closure area. Observer data has indicated that fishing occurs throughout the depth range of the species (Wakeford et al. 2004). The smallest specimens of B. griseocauda are found in deeper water, beyond the normal fishing depths of the ray fleet, but present as occasional bycatch for these other fleets (Wakeford et al. 2004).
Although no studies have been conducted to determine the abundance of this species in the southern area since the skate fishery closure, this species is also caught as bycatch by finfish trawlers. These trawlers target Micromesistius australis, Macruronus magellanicus, Merluccius hubbsi, M. australis and Salilota australis around the Falkland/Malvinas Islands (Brickle et al. 2003) and within the southern rajid closure. Vessels fishing under general finfish licenses are prohibited from targeting rajids, however a small bycatch (below 10%) is allowed so rajids to the south of the Islands are continuing to face bycatch fishing pressure.
This species is also taken in the directed skate fishery off Chile which primarily targets Dipturus chilensis but also lands other skate species. Landings comprise 85% D. chilensis and 10% D. trachydermus with the remaining 5% made up of Bathyraja albomaculata, B. brachyurops, B. griseocauda and Rajella sadowskii (Lamilla et al. 2001, 2002). Overall biomass of the target species (D. chilensis and D. trachydermus) has declined by 51% since fishing began in 1979 (Quiroz 2005) and declines are thus also likely to have occurred for bycatch species.
Bathyraja griseocauda is also taken as bycatch in the artisanal Patagonian toothfish longline fishery which operates at depths of 300 to 2,500 m between Iquique (20°S) to Ladrillero Gulf (49°S) (Lamilla 2003).
There are theoretically TACs, minimum sizes and overall annual quotas for quite a number of elasmobranch species in Argentina, however, little attention is paid to these and there is no regular monitoring by authorities (M. Stehmann pers. obs. 2006). Species-specific assessments of direct and indirect catches are a priority.
The following information is taken from Agnew et al. (1999 and 2000) unless otherwise specified.
The Falkland/Malvinas Islands multispecies skate fishery is managed by limiting fishing effort. The effort that each vessel is likely to exert is calculated (based on size, duration of license and past fishing history) and since 1994 only a limited number of licenses are granted to ensure that the total allowable effort (determined from assessments of stock status) is not exceeded. Stock status assessments are not, however, species-specific and a sustainable total allowable effort for the entire stock may not translate to sustainable levels of effort for individual species.
Following declines in CPUE in the early 1990s, in 1996, the southern area (below 52°S) was closed to rajid fishing and the fishery is now resticted to the area north of the Islands. This closure is extended to 50°30'S (between 56°30W and 58°W) during the second season of each year to exclude the skate fishing fleet from Loligo gahi fishing grounds.
All licensed vessels are required to provide daily catch and effort details, including discards of commercial and non-commercial species to the Falkland Island Fisheries Department. There is, however, no requirement to report species-specific information. Scientific observers are deployed onboard vessels in order to quantify the catch composition by species and to obtain detailed biological data on individual species.
Vessels fishing under general finfish licenses are prohibited from targeting rajids, although a small bycatch (below 10%) is allowed.
In Chile, since 2005, there has been an annual quota for Dipturus spp. caught south of 41°28'S. Each year, there is also a seasonal fishery closure for the entire Chilean coast between December 1 and February 28 to protect the reproductive season. It is unknown whether this measure also protects the reproductive season of this B. griseocauda.
|Citation:||McCormack, C., Lamilla, J., San Martín, M.J. & Stehmann, M.F.W. 2007. Bathyraja griseocauda. The IUCN Red List of Threatened Species 2007: e.T63113A12609854.Downloaded on 18 December 2017.|