The Undulate Skate (Raja undulata) is a medium-sized (to at least 114 cm total length, and possibly 120 cm total length) skate found in the Northeast and Eastern Central Atlantic Ocean, from the southern British Isles to Senegal, and the Mediterranean Sea. It occurs in coastal waters to depths from nearshore to 100 m, and sometimes to ~200 m. The Undulate Skate is taken in demersal trawl, set net (gillnet and trammel), and longline fisheries. Following an earlier ban (introduced in 2009) on landing this species from European Union (and United Kingdom) Atlantic waters, the populations in most areas increased. This ban has been phased out in some areas, thus allowing a resumption of commercial exploitation of Undulate Skate. The species is reported infrequently in the Mediterranean Sea, but it is unclear if this is due to low abundance or a lack of appropriate data collection in shallow, coastal habitats. There are few data available for the coasts of northwestern Africa (Eastern Central Atlantic), an area where sharks and rays have been exploited by semi-industrial fisheries since the 1950s. There is high distribution overlap with intensive fishing pressure, increasing trends in parts of its range and suspected declines in others, and a lack of species-specific management across much of its range. The Undulate Skate is inferred to have undergone a population reduction of 20–29% over the past three generations (33 years) based on abundance data and actual levels of exploitation, and it is assessed as Near Threatened (close to meeting Vulnerable A2bd).

The Undulate Skate is found in the Northeast and Eastern Central Atlantic Ocean, from the southern British Isles to Senegal, and parts of the Mediterranean Sea (Ellis et al. 2012, Last et al. 2016, Ebert and Dando 2021, Dia et al. 2023).

There are no data available on the absolute global population size of the Undulate Skate. The Undulate Skate has a fragmented, coastal distribution (ICES 2021). Whilst locally abundant in certain areas, there is no indication from available historical sources that this species was distributed more widely in the past (Ellis et al. 2012). Population trend data are available from two sources: (1) the English Channel (ICES unpub. data); and (2) Mediterranean Sea (MEDITS unpub. data). The trend data from each source were analysed over three generation lengths using a Bayesian state-space framework (a modification of Sherley et al. 2020, Winker et al. 2020). This analysis yields an annual rate of change, a median change over three generation lengths, and the probability of the most likely IUCN Red List category percent change over three generations (see the Supplementary Information).

Within the Northeast Atlantic, ICES provide advice for stocks of Undulate Skate in the following areas: (1) south-west Ireland, (2) English Channel (3) Bay of Biscay, (4) Cantabrian Sea, and (5) Portuguese waters. Catches (landings and discards) have increased in the northern and central Bay of Biscay steadily since 2009 suggesting an increasing stock biomass following the prohibition on landing Undulate Skate. Similarly, a short time-series in Portuguese waters has indicated that the bycatch allowance for this stock has resulted in much lower fishing effort and recent stock stability (2017–2021). Fishery-independent surveys in the English Channel (the largest stock) both suggest a rapid increase in stock biomass from the mid-2010s to 2022. The average biomass index of large (>50 cm TL) specimens from the last five years (2017–2021) of the survey is greater than ten times that seen at the start of the time-series (1990–1994) (ICES 2021). The trend analysis of the English Channel modelled biomass for 1990–2021 (31 years) revealed annual rates of increase of 15.71%, with the highest probability of no major reductions in population over the past three generation lengths (33 years).

In the Mediterranean Sea, data are collected during the MEDiterranean International Trawl Survey (MEDITS) and these data have generally been analysed and reported in a range of publications with differing spatial and temporal scales. The Undulate Skate is infrequently recorded across much of the region and it is unclear if this is due to low abundance or a lack of appropriate data collection in shallow coastal habitats. In the southern Adriatic Sea, the species is considered rare and is only found in deep waters (Soldo and Lipej 2022). The trend analysis of the Mediterranean Sea modelled biomass for 1994–2021 (27 years) revealed annual rates of decrease of -1.34%, with the highest probability of >80% reductions in population over the three generation lengths (33 years).

The population trend of Undulate Skate off northwestern Africa (northern part of the Eastern Central Atlantic) is unknown. It may be noted that both overall catch and effort have continued to rise in the major shark and ray fishing countries of this area, including Mauritania (Lamrabott et al. 2024, Leurs 2024). Across West Africa, total artisanal fishing effort increased 10-fold between 1950–2010, while the catch-per-unit-effort (CPUE) of artisanal and industrial vessels declined by 34% over this period (Belhabib et al. 2018). Many regional fisheries are now characterised by severe overexploitation and declines in abundance of marine resources (e.g., Gascuel et al. 2007). While these levels of declines are not species-specific, they are informative for understanding the broader levels of decline in sharks in the region. Thus, it is suspected that Undulate Skate has declined in this part of the species’ range.

Due to increasing trends in parts of its range and inferred declines in others, and high levels of exploitation, a global population reduction of 20–29% was inferred over three generation lengths (33 years) based on abundance data and actual levels of exploitation. Therefore, the Undulate Skate is assessed as Near Threatened (close to meeting Vulnerable A2bd).

The Undulate Skate is demersal in coastal waters to depths from nearshore to 100 m, and sometimes to ~200 m (Ebert and Dando 2021). It reaches a maximum size of 114 cm total length (TL), and possibly 120 cm TL. Male and female maturity occurs at 73.6–78.1 cm TL and 76.2–86.2 cm TL, respectively (McCully et al. 2012, Serra Pereira et al. 2015) and size-at-hatching occurs at ~14 cm TL (Last et al. 2016). Reproduction is oviparous. Female age-at-maturity is 8.84 years and maximum age is 13 years (Villagra et al. 2022), resulting in a generation length of 11 years.

The Undulate Skate is a locally important commercial species that is taken as targeted catch, or retained from bycatch in mixed demersal trawl and set net (gillnet and trammel) fisheries targeting flatfish and demersal teleosts, and is also taken in inshore, demersal longline fisheries (ICES 2021). In the Northeast Atlantic, skates have been landed since the 1800s, and became increasingly marketable by the late 1800s/early 1900s (Ellis et al. 2010). Fisheries were largely unregulated, and fishing quotas for skates in the North Sea were only introduced in 1999, and further applied to all vessels operating in European Union Atlantic waters in 2009 (EC 1999, 2009). This ban has been phased out in some areas, thus allowing a resumption of commercial exploitation of Undulate Skate. Earlier management efforts treated skates as one skate complex, and species-specific measures were only first introduced in 2007 (Ellis et al. 2010). Long-term extensive and intensive fishing pressure has led to documented population reductions, localized extinctions, and widespread changes to the shark and ray community structure across the range of the Undulate Skate in the Northeast Atlantic (Dulvy and Reynolds 2002, Marandel et al. 2019). The effects of fishing have been greatest on large-bodied species (Dulvy et al. 2000, Ellis et al. 2010).

In the Mediterranean Sea, the Undulate Skate has been subject to fishing pressure across much of the species' known depth and spatial distribution. Skates are caught with demersal trawl, trammel net, and longline gear (e.g., Carbonara et al. 2020, Ferragut-Perello et al. 2023). The Undulate Skate is reported infrequently in the Mediterranean Sea (Geraci et al. 2021). Skates (Raja spp.) are the most fished elasmobranchs in parts of the Mediterranean Sea and neighbouring areas. For example, in the Gulf of Cádiz, skates represented up to 50% of regional landings (Biton-Porsmoguer and Lloret 2020).

There is little information available for the Undulate Skate across the remainder of its Atlantic range, where it is reported to occur off Morocco and as far south as Mauritania (Dia et al. 2023). Sharks and rays have been exploited by semi-industrial fisheries in the Eastern Central Atlantic since the 1950s (Walker et al. 2005). Over the years, this has expanded into targeted shark and ray fisheries across many countries and is likely increasing fishing pressure on coastal shark and ray populations (Walker et al. 2005, Diop and Dossa 2011).

In general, fishing effort and the number of fishers has intensified in recent decades across the range of this species. Reports indicate that the diversity and average body size of many important commercial coastal, demersal, and pelagic fishery species have markedly declined with many stocks now considered to be overexploited (CCLME 2016, Polidoro et al. 2017). The direct cause of decline for many of these stocks has been attributed to overcapacity within both the industrial and artisanal fisheries and destructive fishing practices (GCLME 2006, CCLME 2016).

Climate change may influence the distributions of various European fish populations, including skates, in a manner that may influence the assessment of population trends and the potential viability of subpopulations (Simpson et al. 2011, Sguotti et al. 2016).

The Undulate Skate is utilized for its meat and is commercially valued for its wings. Products of the Undulate Skate may be marketed under a generic "ray" or "skate" label or mislabelled as a different species (Griffiths et al. 2013, Giagkazoglou et al. 2022). Unmarketable parts of the ray may be used to supply bait for pot fisheries (Bullimore et al. 2001).

The species is likely to be used across the remainder of its range where retention is allowed or unregulated. Skate meat is consumed across West Africa, which may also be dried, or dried and smoked and exported across West Africa to supply countries such as Guinea, Ghana, Nigeria, Mali, and Burkina Faso (e.g., Jabado et al. 2021).

In the European Union and United Kingdom waters of the ICES area, there are Total Allowable Catches for Undulate Skate stocks in the Bay of Biscay, Iberian waters and English Channel, and it remains either a prohibited species, or a species that cannot be retained, for some other specified areas (ICES 2021). There are some local and national management measures in place across the species range. For example, there is a minimum conservation reference size (or minimum landing size) of either 40 cm or 45 cm (disc width) for skates and rays caught in the inshore waters of parts of the coasts of England and Wales. In Portugal, there is a limit on landing skates (Rajidae) in the nominal May-June spawning period, whereby skates and rays should not be landed (except where skate and ray landings are less than 5% of the landings). Portugal also has a minimum landing size (52 cm total length) for all Raja spp.

Further information on its ecology and life history of Undulate Skate is required, as well as information to better understand its distribution and interactions with fisheries in both the Mediterranean Sea and off northwest Africa.