|Scientific Name:||Rostroraja alba|
|Species Authority:||(Lacepède, 1803)|
|Red List Category & Criteria:||Endangered A2cd+4cd ver 3.1|
|Assessor(s):||Dulvy, N.K., Pasolini, P., Notarbartolo di Sciara, G. Serena, F., Tinti, F., Ungaro, N., Mancusi, C. & Ellis, J.E.|
|Reviewer(s):||Cavanagh, R.D., Heenan, A., Valenti, S.V. & participants of Shark Specialist Group Northeast Atlantic workshop (Shark Red List Authority)|
The size of this large benthic skate renders it particularly susceptible to capture by fishing gears, which in combination with its life history parameters and population demography allow little capacity for it to withstand exploitation by fisheries. This species is likely to be caught as bycatch to multispecies trawl fisheries which operate on much of the continental shelf and slope, coinciding with this species habitat. Based on anecdotal and trawl survey data, this species has undergone dramatic declines in abundance and substantial reductions in geographic range within the Mediterranean and the Northeast Atlantic, and it is listed in the Barcelona and Bern conventions.
The data available indicate that Rostroraja alba was formerly captured frequently in the northwestern Mediterranean during the 1960s and off Tunisia and Morocco in the early to mid 1970s. It is now considered rare and is believed to have undergone a significant but currently unquantifiable decline in abundance and extent. This, in combination with the continued and potentially increasing threat from fisheries call for this species to be assessed as Endangered in the Mediterranean. Similar declines in geographical range have occurred in the Northeast Atlantic and anecdotal evidence suggests this species, including localized populations have declined severely e.g., in the Irish Sea. There is a high potential for population decline in the Bay of Biscay, the Iberian coast, and in the Celtic Sea. The collapse of a directed long-line targeted fishery in Brittany highlights the incapacity for this species to withstand fisheries exploitation. The data presented here for the Northeast Atlantic indicate that this species is assessed as Critically Endangered.
On this basis, this species is globally assessed as Endangered, however this will need to be revisited once information from West and Sub-Equatorial African range of its distribution is available (specifically with data from South African trawl fisheries).
|Range Description:||The overall geographical range of Rostroraja alba covers the Eastern Atlantic coasts from the southern British Isles south to South Africa, including the Mediterranean Sea, and extending into the southwestern parts of Indian Ocean.
This species is listed as occurring in the Northwestern European seas, however no valid records exist in northern areas of the Northeast Atlantic.
The geographic range of this species is further discussed in the following section, with reference to its decrease in abundance coupled with a decrease in geographic range.
Native:Albania; Algeria; Angola (Angola); Benin; Cameroon; Cape Verde; Congo; Côte d'Ivoire; Croatia; Equatorial Guinea; France; Gabon; Gambia; Ghana; Greece; Guinea; Guinea-Bissau; Italy; Liberia; Mauritania; Montenegro; Morocco; Namibia; Nigeria; Portugal; Sao Tomé and Principe; Senegal; Sierra Leone; Slovenia; South Africa; Spain; Togo; United Kingdom (Great Britain, Northern Ireland); Western Sahara
|FAO Marine Fishing Areas:|
Atlantic – northeast; Atlantic – southeast; Atlantic – eastern central; Indian Ocean – western; Mediterranean and Black Sea
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||Little information is available on the population size of the white skate. The data does indicate that this species has undergone a reduction in abundance and is now considered rare in the Northeast Atlantic and the Mediterranean. |
Historically, within the Mediterranean this species was captured frequently off northern coasts of Tunisia in the mid-1970s and the coast of Morocco in the early 1970s (Capape 1976). It was described as being more or less frequent in the Northwestern Mediterranean (1965) and the French coast (1950 to 1960s) and Italian Seas (1972) (Capape 1976). However, a time series of comparative trawl surveys running from 1957 to 1995 in the Gulf of Lions, in the Eastern basin of the Mediterranean failed to catch any white skate specimens. This consisted of eight separate surveys, conducted by four survey vessels. A total of 1,359 tows were carried out on shelf and slope areas extending from the coast to 800 m in depth (which includes the known depth range of this species) (Aldebert 1997).
In the eastern Mediterranean, it was captured in the Adriatic (1963) and the limit of its eastern distribution is Greece (Capape 1976). This species was regularly captured by fisheries in Tunisian waters and was common in all Tunisian coastal waters but captures were more frequent in the north part of the Tunisian coast (Capape 1976). This species is now rare along the Tunisian coast (Bradai 2000).
In the Adriatic Sea, comparative trawl surveys indicate that Rostroraja alba was present in 4% of hauls of the "Hvar" 1948 survey (based on 138 valid hauls in depths of up to 400 m). Since then, two time series of trawl surveys have occurred in the Adriatic Sea. The Italian national trawl survey National Group for Demersal Resource Evaluation (GRUND) began in 1982, and the MEDITS survey began in 1985, both surveys are still ongoing. Rostroraja alba was captured sporadically and infrequently in both of these surveys, typically in <2.6% of hauls in any year. Of the GRUND surveys, it was captured in approximately 1.7% of hauls in the central, northern and southern Adriatic Sea (Marano et al. 2003).
In addition to the Adriatic Sea, the MEDITS survey actually operates on the north Mediterranean coast almost continuously from western Morocco and Spain in the west Mediterranean to the Aegean Sea in the eastern Mediterranean (Baino et al. 2001). Six trawl surveys are carried out each year in the coastal areas of four arbitrary geographically defined areas: Western (Morocco, Spain, France), Western Central (Tyrrhenian, Corsican, Sardinia and Sicily coasts), Eastern Central (Adriatic, Ionian and Albanian coasts) and the Eastern (Aegean Sea). A total of 6,336 tows were been performed between 1994 and 1999 in depths ranging from 10 to 800 m. This species was not recorded in the central and northern MEDITS surveys, but was captured in the southern MEDITS survey in 1998 and 1999, in 1.4% of the hauls. However, uncertainty exists as to the validity of the identification of this species. It is suspected that earlier surveys may have confused this species with Leucoraja circularis and or Dipturus oxyrhinchus (F. Tinti and N. Ungaro pers. comm. 2003). The white skate was recorded in only nine hauls in the western central area (Baino et al. 2001).
From other regions within the Mediterranean, only one specimen was caught with a bottom trawl net by a commercial vessel in Livorno (the Ligurian Sea) in 2003 (Serena et al. 2003). Also in the Northwestern Ionian sea, only a single individual of this species was reportedly caught in 1997 (Sion et al. 2003).
From the data presented it can be inferred that this species is now very rare and also the MEDITS survey suggests a substantial reduction in geographic range. In fact, this species was only recorded from the western area of this survey (Morocco, Spain, France). Evidence from the Gulf of Lions survey suggests its presence along French coast may be in doubt. As described above, Aldebert (1997) reported that the white skate was historically absent from the Gulf of Lions, however the MEDITS survey data casts doubt over the occurrence of this species across the length of the French coast. The current distribution of occurrence for this species represents a small fraction of its former range. From the former frequency of capture of this species in the western Mediterranean it can be inferred that this species has undergone a substantial but currently unquantifiable decline in abundance and extent.
|Current Population Trend:||Decreasing|
|Habitat and Ecology:||This is a benthic species of sandy and detrital bottoms from coastal waters to the upper slope region between about 40 to 400 m and exceptionally down to 500 m (Capape 1976, Stehmann and Burkel 1984, Serena 2005). Du Buit 1974 reports this species to be more prevalent in rocky habitats. Little is known of this species' life history and ecology. |
Estimates of size at maturity are 130 cm (males) and 120 cm (females) (Capape 1976). Maximum recorded size is 200 cm, even if is common between 60 and 150 cm of total length (Bauchot 1987). Gestation period is 15 months and females produce between 55 to 156 ova per year, egg cases are 160 to 200 cm in length and 130 to 150 cm in width (Stehmann and Burkel 1984, Serena 2005). Age at maturity, longevity, size at birth, average reproductive age, reproductive periodicity, annual rate of population growth and natural mortality are all unknown for this skate.
The white skate feeds on all kinds of bottom dwelling animals (Stehmann and Burkel 1984), especially fish, crustaceans and cephalopods (Bauchot 1987).
Northeast Atlantic: Rajids are an important component of demersal fisheries in the Northeast Atlantic (Holden 1977) and R. alba would be landed and sold if caught. Targeted fisheries have existed in certain areas, though these fisheries ceased after localised depletion. It may still be taken as by-catch in demersal fisheries off the Iberian Peninsula. Anecdotal information suggests that this species, including localized populations, have declined severely. In 2002, the ICES Study Group on Elasmobranch Fishes was asked to comment on the status of white skate and "considered that there was a high probability of population decline, both in the Bay of Biscay and Iberian coast, and in the Celtic Seas. For example, there was a directed long-line fishery in Douarnanez (Brittany) in the 1960s that collapsed (white skate are no longer listed on French fishery statistics), and a similar decline is thought to have occurred in the Irish Sea" (ICES 2002). As discussed in the population section, historical data (early 1900s) from English surveys suggest that they occurred in UK waters, though no recent records exist (Rogers and Ellis 2000). Their status further south is unclear, though they may be landed around the Iberian Peninsula (ICES 2006), accounting for about 0.3% of skate and ray landings in Portugal. It is also noted that this species may be confused with shagreen ray Leucoraja fullonica and sandy ray L. circularis, and improved market sampling is required to fully ascertain the status of this species in fisheries. Dulvy et al. (2000) also report this species to have undergone a decline and even disappearance from other shelf areas, notably the Irish Sea.
R. alba is captured as bycatch to the multi-species trawl fisheries operating within its range. 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.
As discussed in the population section, the historical occurrence of the white skate within the Gulf of Lions is unclear, however the change in characteristics of the fisheries operating in this area can provide an insight into the change in Mediterranean fishing effort over time. 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), since then effort increased to a total nominal horsepower of 19,940 hp (between 1974 and 1987). Aldebert (1997) reported that half of the fishing effort within the Adriatic Sea to have been displaced to target small pelagic fish, however this 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 likely to be 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.
There is a minimum landing size of 40 cm for skates and rays caught in the inshore waters of various parts of England and Wales, as implemented by Sea Fishery Committee bylaws. Though there are no species-specific management measures for this species, there is a total allowable catch (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). Given the low abundance of this species, species-specific management measures may also be warranted.
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 species is listed in the Appendix 3 of the Bern convention (Convention on the Conservation of European Wildlife and Natural Habitats), Appendix 3 of this convention requires "regulation of species populations to keep them out of danger". This species is also listed by the Barcelona Convention for the Protection of the Mediterranean Sea, this convention concerns specially protected areas and biological diversity within the Mediterranean Sea and lists the white skate on the Annex III, whose exploitation is regulated.
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|Citation:||Dulvy, N.K., Pasolini, P., Notarbartolo di Sciara, G. Serena, F., Tinti, F., Ungaro, N., Mancusi, C. & Ellis, J.E. 2006. Rostroraja alba. The IUCN Red List of Threatened Species 2006: e.T61408A12473706.Downloaded on 22 October 2016.|