Status Assessment 2021 - Common skate
| Assessment of Status | Distribution | Population size | Demographics, e.g. productivity | Previous OSPAR status assessment | Status | ||
|---|---|---|---|---|---|---|---|
Region | I | ? | ? | low | ● | Poor | |
| II | ←→2 | ←→2 | low | ● | Poor | ←→1 | |
| III | ←→2 | ←→2 | low | ● | ? | ||
| IV | ? | ? | low | ● | ? | ||
| V | ? | ? | low | ● | ? | ||
| Assessment of key pressures | Excessive mortality | Habitat damage | Prey availability | Threat or impact | |
|---|---|---|---|---|---|
Region | I | ? | ? | ? | ? |
| II | ←→2 | ? | ? | ↓2 | |
| III | ←→2 | ? | ? | ↓2 | |
| IV | ←→2 | ? | ? | ? | |
| V | ? | ? | ? | ? | |
Explanation to table:
Distribution, Population size, Condition
Trends in status (since the assessment in the background document)
↓ decreasing trend or deterioration of the criterion assessed
↑ increasing trend or improvement in the criterion assessed
←→ no change observed in the criterion assessed
? trend unknown in the criterion assessed
Previous status assessment: If in QSR 2010 then enter regions where species occurs ( ○) and has been recognised by OSPAR to be threatened and/or declining (● ). If a more recent status assessment is available, then enter ‘poor’/’good’
Status*(overall assessment)
red – poor
green – good
Blue cells –? status unknown, insufficient information available,
NA - Not Applicable
*applied to assessments of status of the feature or of a criterion, as defined by the assessment values used in the QSR 2023 or by expert judgement.
Key pressures
↓ key pressures and human activities reducing
↑ key pressures and human activities increasing
←→ no change in key pressures and human activities
? Change in pressure and human activities uncertain
Threat or impact (overall assessment)
red – significant threat or impact;
green –no evidence of a significant threat or impact
Blue cells – ? insufficient information available
NA – not applicable
1 – direct data driven
2 – indirect data driven
3 – third party assessment, close-geographic match
4 – third party assessment, partial-geographic match
5 – expert judgement
Confidence
Medium
Background Information
Common skate was nominated for inclusion on the OSPAR List in 2001, and was agreed due to the species’ rarity, sensitivity and decline. The previous assessment in 2010 noted that ‘common skate’ was a complex of two species (OSPAR 2010). The current, accepted taxonomic names are common blue skate Dipturus batis and flapper skate Dipturus intermedius. Earlier data ascribed to D. batis refers to the species-complex.
The larger-bodied D. intermedius may be the more vulnerable to overfishing and incidental bycatch. Four species (D. batis, D. intermedius, Norwegian skate D. nidarosiensis and longnosed skate D. oxyrinchus) may be misidentified, affecting the accuracy of survey and landings data. All but the latter species are on the list of prohibited species over large parts of their distribution in EU waters.
The status of this species is based on ICES advice to fisheries managers for fishing opportunities. This advice has been integrated into an assessment for conservation purposes.
Geographical Range and Distribution
The distributions and stock boundaries of both species are uncertain. However, the species complex occurs in all OSPAR Regions. Dipturus intermedius occurs in the northwestern North Sea, north and western Scotland (where it is the dominant species in coastal areas) and southwards to the Celtic Sea. Dipturus batis is locally common in the Celtic Sea, ranging northwards to the Rockall Bank and to Iceland. The main Celtic Sea distribution is expanding eastwards through the Channel to the southern North Sea and southwards to the northern Bay of Biscay. Tagging studies indicate limited dispersal from tagging sites, with most individuals remaining in the region where they were tagged. The southern geographical limits for both species are somewhat uncertain. The species is only caught occasionally in the Bay of Biscay and might not occur to any degree in Division 9.a. (ICES, 2020a)
Population/Abundance
The most recent ICES advice for North Sea follows earlier advice since 2008 that the species complex remains depleted (ICES 2019).
ICES provided its advice for the first time for the Celtic Seas eco-region in 2008, stating that the species complex was depleted (ICES 2008). No information has become available to ICES to suggest that the species complex has recovered. ICES now classifies its status in that area as unknown (ICES 2019), and no longer provides advice relating to fishing opportunities.
ICES is unable to provide advice on these species in the Biscay/Iberia region, due to a lack of data (ICES 2020b).
Condition
For common blue skate: The length at 50% maturity is estimated to be 115.0 and 122.9 cm in total length for male and females, respectively (Iglésias et al. 2010). The age at 50% maturity is estimated as 11 years (Iglésias et al. 2010). The maximum length and maximum weight (eviscerated) observed by Iglésias et al. (2010) were 143.2 cm and 15.2 kg. The species is known to forms discrete nursery grounds (Varian et al. 2011).
For flapper skate: The length at 50% maturity is estimated to be 185.5 and 197.5 cm in total length for male and females, respectively. The age at 50% maturity is estimated as 19-20 years (Iglésias et al. 2010). The maximum length and maximum weight (eviscerated) observed by Iglésias et al. (2010) were 228.8 cm and 78 kg. Electronic tagging studies have revealed that this species shows pronounced site fidelity to highly localised areas (Wearmouth and Sims 2009).
Figure 1: Geographic distribution map of common skate complex species. The distributions of the two species in the southern parts of the area shown, including the Mediterranean, are uncertain. Source: https://www.iucnredlist.org/species/39397/10198950#geographic-range
Threats and Impacts
Fishing pressure is considered to be the most important threat to populations of both species of skate. It has been prohibited to land both species from EU waters since 2009, which should reduce the mortality rates. Both species are bycaught in bottom trawl and set net fisheries however and discard survival, though likely to occur, has not been quantified.
ICES noted an increase in reported landings of long-nosed skate since the prohibition on landing “common skate-complex”, which may reflect some misreporting. The impacts of other fisheries (e.g. deep-water and recreational fisheries) have not been evaluated.
Common skate complex species predate on a wide variety of demersal fish and crustaceans, suggesting prey availability may not be limiting.
Measures that address key pressures from human activities or conserve the species/habitat
EU fishing regulations have listed Dipturus batis and D. intermedius as prohibited species in EU waters since 2009, which should reduce fishing mortality. Both species should be promptly released unharmed by fishers, and they cannot be landed. Regulation (EU) 2015/812 (EU 2015) requires that all discards of common skate in EU waters are recorded by commercial fishers. Catch rates of species in the complex have increased in scientific trawl surveys since the prohibition (ICES 2020a), suggesting that the measure has benefited the populations. The Loch Sunart to the Sound of Jura Marine Conservation Order (UK Legislation, 2016) lists “common skate” as the designation feature of this MPA, which should reduce fishing mortality and maintain habitat in an important area for the species.
Conclusion (including management considerations)
What was once regarded as a single species (common skate Dipturus batis) over much of the 20th century has been shown to be a complex of two species (Iglésias et al., 2010) which are now termed the common blue skate Dipturus batis and flapper skate D. intermedius (Last et al. 2016). Earlier data ascribed to D. batis refers to the species complex. The larger-bodied D. intermedius may be the more vulnerable to overfishing.
The common skate complex is considered to be ‘Critically Endangered’ globally by the IUCN, with both species considered to be Critically Endangered in European waters. While there have been positive signs in the stocks of both species in parts of OSPAR Regions II and III, in terms of increasing catch rates as an indicative measure of occurrence or relative abundance, both species are still infrequently recorded or absent from some former parts of their geographic range. Both species of the common skate complex are on the prohibited species list according to EU fishing regulations.
Given the revised taxonomy, it is recommended that both species be considered separately and, if accepted, listed separately by OSPAR. While there have been a number of scientific studies on these species since the OSPAR listing, especially in Region III, further studies on stock delineation, habitat use, and discarding could inform future management options.
Knowledge Gaps
Information on both species’ essential habitats for reproduction, nursery grounds, and feeding is needed to assess options for potential spatial management measures. Such work is required for both inshore habitats (e.g. sea lochs) and shelf seas. The quantities of discards and associated discard survival need to be quantified for relevant fisheries and métiers to determine the efficacy of the prohibited listing.
In addition, the southern limits (Region IV), and the bathymetric and geographical ranges of the two species (and other Dipturus) in offshore waters (Region V) needs to be better understood and documented. The status of other Dipturus species in the OSPAR Area could also usefully be evaluated.
Method used
The assessment is based on ICES examinations of distributions and catch trends, peer reviewed literature, and expert opinion.
EU. 2015. Regulation (EU) 2015/812 of the European Parliament and of the Council of 20 May 2015 amending Council Regulations (EC) No 850/98, (EC) No 2187/2005, (EC) No 1967/2006, (EC) No 1098/2007, (EC) No 254/2002, (EC) No 2347/2002 and (EC) No 1224/2009, and Regulations (EU) No 1379/2013 and (EU) No 1380/2013 of the European Parliament and of the Council, as regards the landing obligation, and repealing Council Regulation (EC) No 1434/98. Official Journal of the European Union, L 133: 1–20. http://data.europa.eu/eli/reg/2015/812/oj.
ICES. 2008. Demersal elasmobranchs in the Celtic Seas (ICES Areas VI, VIIa c, e k). ICES Advice 2008, Book 5, Section 5.4.39.
ICES. 2019. Common skate complex (Blue skate (Dipturus batis) and flapper skate (Dipturus intermedius) in Subarea 4 and Division 3.a (North Sea, Skagerrak and Kattegat). In Report of the ICES Advisory Committee, 2019. ICES Advice 2019, rjb.27.3a4, https://doi.org/10.17895/ices.advice.4835
ICES. 2020a. Working Group on Elasmobranch Fishes (WGEF). ICES Scientific Reports. 2:77. 789 pp. http://doi.org/10.17895/ices.pub.7470
ICES. 2020b. Common skate complex (Blue skate [Dipturus batis] and flapper skate [Dipturus intermedius]) in Subarea 8 and Division 9.a (Bay of Biscay and Atlantic Iberian waters). In Report of the ICES Advisory Committee, 2020. ICES Advice 2020, rjb.27.89a.
Iglésias, S. P., Toulhoat, L., and Sellos, D. Y. 2010. Taxonomic confusion and market mislabelling of threatened skates: important consequences for their conservation status. Aquatic Conservation: Marine and Freshwater Ecosystems, 20: 319–333.
Last, P. R., White, W. T., de Carvalho, M. R., Séret, B., Stehmann, M. F. W., and Naylor, G. J. P. 2016. Rays of the world. CSIRO Publishing & Cornell University Press, Comstock Publishing Associates, vii + 790 pp.
OSPAR. 2010. Background Document for Common Skate Dipturus batis. OSPAR Commission 2010 19 pp.
Varian, S., Dunagan, K., and Moloney, A. 2011. Heritage research project to investigate critical habitats for threatened species of shark and skate. Irish Heritage Council, Kilkenny. Report R00261. 25 pp.
Wearmouth, V.J., and Sims, D.W. 2009. Movement and behaviour patterns of the critically endangered common skate Dipturus batis revealed by electronic tagging. Journal of Experimental Marine Biology and Ecology 380: 77–87.
Sheet reference:
BDC2021/Common skate complex