Dipturus batis (Linnaeus 1758), Common skate (Rajidae)

Author: Ronald Fricke, Germany

1. Description of the habitat/autecology of the species

The common skate Dipturus batis (synonym: Raja batis) is a demersally living skate species living on soft bottoms below 10 m depth, adults usually below 30 m depth. They are slow-growing and long-lived and do not become sexually mature until after 10 years age. Mating occurs in spring, and the egg-capsules are laid during summer; the species is oviparous and deposits its egg-case in algae and seagrass beds. About 40 eggs per individual laid annually. The young may tend to follow large objects, such as their mother. The species feeds on moderately sized benthic organisms including crustaceans, bony fish and polychaete worms (Fricke, 1987: 50; Froese & Pauly, 2005). Maximum total length 285 cm, maximum total weight 97.1 kg.

2. Distribution (past and present)

Distributed in the western HELCOM area; mainly in Kattegat but occasionally reported in the southern Baltic Sea.

3. Importance (sub-regional, Baltic-wide, global)

This species is neither of local nor global importance in HELCOM area, although historically it was of local importance.

4. Status of threat/decline

This species is threatened and its populations have been significantly declining throughout its distribution range in HELCOM area and in other European waters (Anonymous, 2004b; ICES, 2006). In the HELCOM area, this species is classified as critically endangered (CR) according to IUCN criteria, and as a HELCOM high priority species (HELCOM, 2007). The species is listed as critically endangered in Sweden and by George (2003; personal communication 2004), and as threatened migrant (TM) in Germany. The species is included as a prioritary species on the OSPAR list (Anonymus, 2004a). Worldwide classification as endangered (EN) by McEachran & Dunn (1998), but considered as critically endangered (CR) by IUCN, who stress that this species should become a priority species in conservation plans (Anonymous, 2004b). It is forbidden to fish for and land common skate in Swedish waters.

5. Threat/decline factors

Due to its slow reproduction rate and large size the common skate is sensitive to fishing pressure already at a young age and it is threatened because of by-catch in demersal fisheries. It may also be threatened by habitat loss due to effects of sand and gravel extraction and trawling and eutrophication (the species needs clean oxygenated sand bottoms where it can bury and breathe) (Anonymous, 2004b). It is rare, and highly sensitive to human activities, but not considered as a keystone species.

6. Options for improvement

Dipturus batis would benefit from restrictions to benthic fisheries (trawling etc.), and a restrictive fisheries management. Sand and gravel extraction should be restricted, too. Marine protected areas without fisheries pressure and sand/gravel extraction would serve for the recovery of the populations. As major threats for the species occur outside the HELCOM area in the neighbouring OSPAR area, OSPAR could be requested to consider providing additional protection for this species.

7. References

Anonymous 2004a. 2004 Initial OSPAR List of Threatened and/or Declining Species and Habitats  (References number 2004-06). OSPAR Convention on the Protection of the Marine Environment of the North-east Atlantic, Meeting of the OSPAR Commission (OSPAR), Reykjavik, 28 June-1 July 2004.

Anonymous 2004b.  Case study 3, the common skate, Dipturus batis.  IUCN Internet Publication. Available in:  http://www.iucn.org/places/medoffice/medredlist/pdf/dipturus_batis.pdf

Fricke R. 1987. Deutsche Meeresfische. Bestimmungsbuch.  Hamburg (DJN), 219 pp.

Froese R. & Pauly D. (eds) 2005. FishBase. Availble in: www.fishbase.org, version (11/2005).

George M.R. 2003.  Die Ost- und Nordsee als Lebensraum für Haie, Rochen und Chimären. Meer und Museum, 17: 15-24. 

HELCOM 2007. HELCOM Red list of threatened and declining species of lampreys and fish of the Baltic Sea. Baltic Sea Environmental Proceedings, No. 109, 40 pp.  Available in: http://www.helcom.fi/stc/files/Publications/Proceedings/bsep109.pdf

ICES. 2006. Report of the Working Group on Elasmobranch Fishes (WGEF), 14–21 June

2006, ICES Headquarters. ICES CM 2006/ACFM:31. 291 pp.

McEachran J.D. and K.A. Dunn 1998. Phylogenetic analysis of skates, a morphologically conservative clade of elasmobranchs (Chondrichthyes: Rajidae). Copeia (2):271-290.