|Scientific Name:||Dipturus batis|
|Species Authority:||(Linnaeus, 1758)|
Dipturus cf. flossada (Risso, 1827)
Raja batis Linnaeus, 1758
|Taxonomic Notes:||There are some doubts about the validity of historical identification, this species could potentially be confused with Dipturus oxyrhinchus, despite morphological and colour differences (Ragonese et al. in press).|
|Red List Category & Criteria:||Critically Endangered A2bcd+4bcd ver 3.1|
|Assessor(s):||Dulvy, N.K., Notarbartolo di Sciara, G., Serena, F., Tinti, F. & Ungaro, N., Mancusi, C. & Ellis, J.|
|Reviewer(s):||Cavanagh, R.D., Valenti, S.V., Heenan, A. & participants of the Shark Specialist Group Northeast Atlantic workshop (Shark Red List Authority)|
This skate, the largest European rajid, was once an abundant constituent of the demersal fish community of north-western Europe. It formerly occupied the shelf and slope areas of the Mediterranean excluding North Africa west of Morocco but now appears to be virtually absent from much of this range. Caught as bycatch of multispecies trawl fisheries, which cover much of its shelf and slope habitat. Fisheries data indicate that populations of D. batis have undergone an extremely high level of depletion in the central part of its range around the British Isles since the early 20th century (the three generation period). It has been extirpated from most inshore areas, but is still caught in Scottish waters, especially around the Shetlands and off North-west Scotland, and also along the shelf edge and in the Celtic Sea. Fishing capacity and effort in the Mediterranean have also increased substantially over the later half of the 20th century. Accurate international species-specific landings data are lacking, although Icelandic landings have declined. French landings appear stable, though this is likely to be attributed to a re-direction of fishing effort from shelf seas (where common skate are now very rare) into deeper water. The life history and demography of this species allow little capacity to withstand exploitation by fisheries, its large body size renders it catchable by fishing gears even from birth. As fishing pressure on this species is unlikely to be reduced in the future, it is assessed as Critically Endangered throughout its range.
|Range Description:||The historical geographical range of D. batis covered much of the continental shelf of the North-east Atlantic, from Madeira and the coast of northern Morocco in the south, to Iceland and northern Norway in the north, including the Mediterranean Sea. In several parts of its range, including the Western Baltic, western Mediterranean and southern North Sea, it is considered scarce (Stehmann and Bürkel 1984). It is absent from the Black Sea and Levantine Mediterranean basin (Serena 2005). At the start of the twentieth century it was considered to have a wide distribution over the shallower waters of the continental shelf surrounding the British Isles, albeit more common in the northern and western regions (Walker and Heesen 1996). Though individual specimens are reported very occasionally from the Irish Sea, Bristol Channel and central North Sea, the current range tends to occupy the deeper waters off northwestern Scotland and in Celtic Sea, and along the edge of the continental shelf.|
Native:Albania; Algeria; Belgium; Croatia; Denmark; France; Germany; Greece; Iceland; Ireland; Italy; Montenegro; Morocco; Netherlands; Norway; Portugal (Madeira); Slovenia; Spain; Turkey; United Kingdom
|FAO Marine Fishing Areas:||
Atlantic – eastern central; Atlantic – northeast; Mediterranean and Black Sea
|Range Map:||Click here to open the map viewer and explore range.|
North-east Atlantic: Common skate, as the name implies, was historically one of the most abundant skates and rays in the North-east Atlantic. It was widely distributed in the seas surrounding the British Isles, though catch rates of this species in this area declined during the 20th century. By the 1970s common skate was considered extirpated from the Irish Sea (Brander 1981), and they have also disappeared from the English Channel and the southern and central North Sea (Walker 1999, Rogers and Ellis 2000). Though individual specimens are reported occasionally from these areas (e.g., Ellis et al. 2002, Ellis et al. 2005), common skate are now only regularly observed off northern and north-western Scotland, Celtic Sea and along the edge of the continental shelf (>150 m depth).
A time series of comparative trawl surveys in the Gulf of Lions between 1957-1960 indicate that D. batis was historically present in both shelf and slope trawl surveys. It was captured in 10% of hauls (n=27) in shelf surveys (coast-150 m depth) and in approximately 17% of hauls (n=37) in slope surveys (150-800 m) (Aldebert 1997). In contrast, comparable surveys carried out from 1966-1995 in the Gulf of Lions (totalling 1,295 hauls) did not record this species (Aldebert 1997). In the Adriatic Sea, the "Hvar" 1948 trawl surveys (based on 138 valid hauls taken in the spring-summer of 1948) revealed D. batis present in 3.2% of hauls. In a comparable survey conducted in the spring-summer of 1998 (127 valid hauls) (MEDITS) it was not recaptured, suggesting D. batis may now be absent from this area (Jukic-Peladic 2001). The MEDITS survey began in 1994 while another study of the Adriatic Sea had begun in 1985 (GRUND), with each project carrying out one survey per year. A single individual was captured in the first GRUND survey of 1985 and since then no specimens have appeared in the Adriatic in either of these surveys (Marano et al. in press). It is now regarded as locally extinct in the Adriatic Sea (Tinti et al. 2003). Although these surveys are exhaustive, it should be noted that the MEDITS net is thought to have a low sampling efficiency of truly benthic species (Jukic-Peladic 2001). Along the Algerian coast from 1996-1997, regular systematic surveys of elasmobranchs present in markets have been undertaken. Although eight species of skate have been recorded, D. batis has not been reported (Hemida 1998). Tunisian fisheries use prawn trawl and larger "French" trawls, which frequently capture skates and demersal sharks. While this species has been documented from Tunisian waters in the early part of the 20th century, it has not been recorded since 1971 and it is now presumed absent from this area (Bradaï 2000).
The former range of this skate included much of the west, north and eastern shelf and slope habitat of the Mediterranean. These results suggest that this species may only be found in the western area of the Mediterranean (Morocco, Spain and France) representing a substantial reduction in area of occurrence of this species.
|Habitat and Ecology:||
This demersal species is found from shallow coastal waters down to depths of 600 m, although it is primarily within the 200 m depth range (Stehmann and Burkel 1984, Bouchot 1987, Serena 2005). This is the largest species of skate attaining a length of more than 250 cm. The age and growth of D. batis has been reported by Du Buit (1972 1976) and more recently by Fahy (1991) who examined the vertebrae of 75 individuals landed in Ireland. Du Buit (1976) gave the following growth parameters: Maximum length 253.73 cm; k = 0.057; t0 = -1.629. Males are thought to mature at a length of 125 cm (Du Buit 1972) and although the size at maturity has not been accurately determined for females, an estimate of 150 cm was presented in the 2000 Red List assessment (Ellis et al. 2000). Du Buit (1976) determined that maturity is reached at 11 years of age and that individuals may live for 50 years. The overall sex ratio has been reported to be approximately 1:1, although this may differ geographically and seasonally (Fulton 1903, Steven 1933). The fecundity has not been accurately determined but was estimated at 40 eggs/year over the spawning season (Brander 1981) and the rate of reproduction has been calculated at 0.38. Oviparous, with large egg-cases covered with close-felted fibres (150 to 250 mm long and 80 to 150 mm wide reported in the Mediterranean (Serena 2005) and 145 to 245 mm long in the UK (Clark 1922, Wheeler 1969). The egg-cases are deposited in spring and summer in both the Mediterranean and Northeast Atlantic (Clark 1922, Wheeler 1969, Serena 2005). The young hatch at a lengths of up to 21.2 to 22.3 cm (Clark 1926). There is no detailed information on the developmental time.
Dipturus batis preys mostly on crustaceans and teleost fish, although Steven (1947) reported several species of elasmobranch, including other species of rajid, in the stomach contents of fish landed in Devon and Cornwall. The skate hunts actively and envelops its prey prior to capture and ingestion. The dark ventral surface may facilitate hunting in the pelagic phase.
While there is almost no life history and ecological information specific to the Mediterranean it is likely that it is similar to that exhibited in more northerly waters.
Rajids are an important component of the demersal fisheries of northwest Europe (Holden 1977) and Dipturus batis has traditionally been landed due to its large size (see Ellis et al. 2000 for details). It has taken in targeted fisheries where/when abundant, and as a bycatch elsewhere within its range. Accurate species-specific landings data are not available, though species-specific data from Iceland indicate declining landings of this species. Though French data appear more stable, these landings include landings from fisheries operating towards the edge of the continental shelf, where the population now appears to be concentrated. Given that this species is likely to be taken in trawl and gillnet fisheries that target high-value telesosts (e.g., megrim, anglerfish and hake), it is unlikely that fishing effort will decrease. Common skate are targeted by recreational fisheries (e.g., off Scotland), most anglers release captured individuals.
The situation appears similar in the Mediterranean, where D. batis is probably captured as part of the bycatch of multispecies trawl fisheries. Benthic trawl effort has increased both numerically and in technological terms in the shelf and slope area of the Mediterranean over the last 50 years. For example, the Gulf of Lions area was initially exploited by small-scale benthic trawl fisheries comprising 27 small low powered boats (total nominal horse power of 2,700 hp), more recently effort has increased to a total nominal horse power of 19,940 hp (1974 to 1987). Since then half of the fishing effort has been displaced to targeting small pelagic fish (Aldebert 1997). The Adriatic Sea is subject to trawling mainly by Italian, Croation, Slovenian, and Albanian fleets, however, no landings data are available (Jukic-Peladic et al. 2001). The large body size, slow growth, low fecundity and large size of juveniles of this species makes it especially vulnerable to fishing exploitation when compared to other rajids (Brander 1981, Walker and Hislop 1998, Dulvy et al. 2000, Dulvy and Reynolds 2002). Moreover, although only large individuals may be landed for consumption, most size classes are taken in fishing nets, including the eggs (which are often found in the trawl cod-end, Ragonese et al. 2003), as the legal mesh size used in much of the Mediterranean is approximately 20 mm. Considering the large size at maturity (around 130 cm) this means that the exploitation of juveniles is likely to be high. Additionally D. batis has been found to have undergone declines and disappearance from other shelf seas, notably the Irish and North Sea (Brander 1981, Walker and Hislop 1998, Dulvy et al. 2000).
It is recommended that suitable non-trawling areas are defined to protect both adults and deposited eggs as these are both susceptible to capture by trawling gear.
Though there are no species-specific management measures for this species, there is a TAC for skates and rays in the North Sea and adjacent waters, and they may benefit from more generic management measures for demersal fisheries (e.g. mesh size regulations, effort reduction). Diptutus batis has been identified by the International Council for the Exploration of the Sea (ICES) Study Group on Elasmobranch Fish as one which "requires information on either fisheries statistics, biology or status of exploitation" or in the case of D. batis, all three (Anonymous 1995). Demographic information indicates that population recovery might be achieved by allowing increasing juvenile survival (Walker and Hislop 1998). Dipturus batis was proposed for strict protection under the UK Wildlife and Countryside Act in 2001; a decision is still (2006) awaited from government. D. batis is also a Biodiversity Action Plan (BAP) species in the UK, and was listed on the OSPAR Priority List of Threatened and Endangered Species.
For the conservation of chondrichthyans within the Mediterranean region, the FAO SAC Subcommittee on the Environment and Ecosystem recommended that all fishing states implement a Mediterranean Action Plan for the Management and Conservation of chondrichthyans, in line with IPOA sharks (International Plan of Action on the management and conservation of chondrichthyans).
The MEDLEM project was adopted at the last SAC meeting (FAO, Rome 2005) for data collection within the Mediterranean basin on large elasmobranches. The Bern Convention encourages research programs aimed at the assessment of the conservation status of chondrichthyans in the Mediterranean Sea (Serena et al. 2002). In Italy a national action plan (PAN-SHARKS) was co-ordinated by ICCRAM (Central Institute for Marine Research) within the guidelines of the Bern Convention and FAO IPOA-Sharks (Serena et al. 2002, Vacchi and Notarbartolo 2000). The Barcelona Convention proposed this species for urgent legal protection under its Mediterranean Action Plan.
A new European requirement is that common skates, when caught, should be returned to the sea - applies only to the North Sea.
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|Citation:||Dulvy, N.K., Notarbartolo di Sciara, G., Serena, F., Tinti, F. & Ungaro, N., Mancusi, C. & Ellis, J. 2006. Dipturus batis. The IUCN Red List of Threatened Species. Version 2014.3. <www.iucnredlist.org>. Downloaded on 31 March 2015.|