Sharks and Skates of Scotland Report: Thornback skate (Raja clavata)

Population status

Scotland and Northeast Atlantic: Stable. The trend analysis of the North Atlantic modelled biomass for 1950–2021 (71 years) revealed annual rates of increase of 0.66%, with the highest probability of no major reductions in population over the past three generation lengths (33 years) (Finucci et al., 2024).

Global: Decreasing. Due to estimated and suspected declines in part of its range (parts of the Mediterranean Sea, Black Sea and Eastern Central Atlantic) but increasing trends in others, and high levels of exploitation, a population reduction of 20–29% over the past three generations (33 years)  has been estimated based on abundance data and actual levels of exploitation (Finucci et al., 2024). 

Conservation listings

Range and distribution 

Thornback skate occur in the Northeast Atlantic and northern part of the Eastern Central Atlantic, from Iceland and Norway to Northwest Africa, including the Azores, Mediterranean Sea, and Black Sea (Ellis et al., 2016; Last et al., 2016, Ebert and Dando 2021). In European waters two distinct populations of Thornback skate exist, one in the Northeast Atlantic and one in the Mediterranean. They are one of the more abundant elasmobranch species in the Northeast Atlantic. Thornback skate are widely distributed around the British Isles but are most abundant along the southern and western coasts of England and are comparatively less abundant in Scottish waters (Ellis et al., 2005). They were once found throughout much of the North Sea, but over the last century their range here has become restricted to the south-western area (principally the Greater Thames Estuary and the Wash). It is thought that this range constriction and decline in the Greater North Sea is due to unsustainable fishing mortality (OSPAR, 2010). 

Habitat 

Thornback skate are an inshore demersal species that are found in relatively shallow water over a range of substrates, including mud, sand, shingle, gravel and rocky areas. Around the British Isles, they have been found at depths of 7-192 m (Ellis et al., 2005), and in the Northeast Atlantic, in general, they are most abundant between depths of 10-60 m (Ellis et al., 2016). Evidence from tagging studies suggests that they are a relatively sedentary species and stay within 30-50 nautical miles of their residing area (Walker, 1997). However, adults have been found to migrate from offshore, relatively deep (30 m) water in winter months to shallower inshore waters (10 m) in summer months, possibly to mate (Walker, 1997). High abundances of juveniles have been recorded in Luce Bay and the Solway Firth on the Scotland-England border, as well as in the Thames Estuary, north-eastern English Channel, northern Bristol Channel and Cardigan Bay (Ellis et al., 2005). 

Biology and Ecology

Thornback skate can live for up to 12 years and grow to a maximum length of 110 – 130 cm TL (Ellis et al., 2016). In British waters, length at first maturity is estimated to be 74-78 cm TL for females and 47 cm TL for males (McCully et al., 2012), or approximately 5 years of age (Ellis et al., 2016), with fecundity estimated to be 100 – 150 eggs per year, although it may be as low as 48 (Ellis et al., 2005; Serra-Pereira et al., 2011). Eggs are typically laid between February and September, with a peak in May-June, eggs take 16-20.5 weeks to hatch, and newborns are 10-13 cm long (TL). Empty egg cases are commonly seen washed ashore and are known as “mermaids’ purses”. Thornback skate are usually sedentary during the day and feed at night on fauna such as crustaceans (swimming crabs and brown shrimp) sand-eels, herrings, sprats and small flatfish (Snowden, 2008).  

Human interactions