Taxonomic Notes
The Mud Skate may be more closely related to members of the genus Rhinoraja and may need to be reassigned to this genus with further taxonomic resolution (Last et al. 2016).
Justification
The Mud Skate (Bathyraja taranetzi) is a small deepwater skate (to at least 77 cm total length) known from a widespread distribution in the North Pacific Ocean, from the Kuril Islands and Kamchatka, Russia through to the Sea of Okhotsk and the Bering Sea to the Aleutian Islands. It has been recorded on continental and insular shelves and slopes at depths of 17–1,063 m and is most common at 250–500 m. Across the North Pacific Ocean, skates are bycatch of industrial trawl, longline, and trap groundfish fisheries, and in some areas, skates comprise up to 10% of the total recorded biomass. The Mud Skate is a bycatch of fisheries targeting groundfish, including Pacific Cod (Gadus macrocephalus), Alaska Pollock (Gadus chalcogramma), and Greenland Halibut (Reinhardtius hippoglossoides). There is increasing international demand for skate products in countries including China, Japan, and Korea and this increased demand has increased skate retention and interest in developing skate targeted fisheries. There are catch limits set for skates (as a group) in Russian and United States waters. Areas of high skate egg density in the United States Bering Sea and Aleutian Islands are designated Habitats of Particular Concern which allows management intervention where fishing activity is deemed harmful to the habitat. This species has refuge in deep water outside of current fishing activities and catch and biomass trends from long-term demersal trawl surveys across its range indicate increasing (western Bering Sea) and stable trends (eastern Bering Sea and Aleutian Islands). The Mud Skate is therefore assessed as Least Concern.
Geographic Range Information
The Mud Skate is found in the North Pacific Ocean, from the Kuril Islands and Kamchatka, Russia through to the Sea of Okhotsk and the Bering Sea to the Aleutian Islands (Last et al. 2016, Love et al. 2021).
Population Information
The Mud Skate, along with the Okhotsk Skate (Bathyraja violacea) are the most abundant skates reported from the outer shelf and upper continental slope off the northern Kuril Islands and Southeastern Kamchatka (Savin et al. 2019). Across the North Pacific (western Bering Sea, Northwest Pacific Ocean), trends in catch-per-unit-effort (CPUE) have generally increased between 1977 and 2021 (Orlov and Volvenko 2022). Species-specific abundance data are available for the Aleutian Islands (AI), Eastern Bering Seas (EBS) slope, and EBS shelf over 22 years (2000–2022) based on demersal trawl surveys conducted by the U.S. National Marine Fisheries Service (Ormseth 2020). The Mud Skate is reported mostly from the EBS slope and AI. On the EBS slope, survey biomass estimates for this species have remained relatively stable between 576 and 978 tonnes (t) from 2002–2016. For the AI survey, biomass estimates were variable but overall stable from 1,165 to 2,970 t between 2000 and 2018, and on the EBS shelf survey, biomass estimates were low and variable without trend (55–880 t annually), from 2000–2019 and in some years, the species was not recorded (Ormseth 2020). Total modelled biomass estimates for the Mud Skate in the combined EBS shelf, EBS slope, and AI survey have been stable, varying from 2,122 and 2,706 t between 2002 and 2016 (Ormseth 2020).
The trend data from each source were analysed over three generation lengths using a Bayesian state-space framework (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 generation lengths (see the Supplementary Information). The trend analysis revealed an annual rate of increase of 0.15% although the past one generation length (11.6 years) had a slight annual decrease of 0.62%. This led to a median reduction of 27% over three generation lengths (35 years) and while Least Concern obtained the highest probability (44%), as it is less than 50%, the Near Threatened category was assigned as the most likely status. This probability is over a long period of three generations (35 years); it has incorporated the decline over the past one generation length of data and projected an estimated trend based on those reductions for a considerable period (35 years) beyond that of the time-series and thus indicates a trend of decline rather than stability. However, this long term slight decline is only for part of its range and in the western Bering Sea, CPUE has increased and the species also has significant refuge in deep water across its range outside of current fishing activities. Thus, it is estimated that across its entire range the population is stable.
Habitat and Ecology Information
The Mud Skate is demersal on rough substrates on continental and insular shelves and slopes at depths of 17–1,063 m and is most common at 250–500 m (Last et al. 2016, Love et al. 2021). It reaches a maximum size of 77 cm total length (TL) (Ebert 2005, Last et al. 2016). Size-at-maturity is smaller in the Kuril Islands (KI) than in the Eastern Bering Sea (EBS), with males mature at 50 cm TL and 58 cm TL, respectively and females mature at 59 cm TL and 61 cm TL, respectively (Ebert 2005, Ebert et al. 2009, Shelekhov et al. 2022). Reproduction is oviparous. Age-at-maturity and longevity are higher in the Kuril Islands (KI) than in the Eastern Bering Sea (EBS), with female age-at-maturity 12–13 years and 9.1 years, respectively and maximum age 19 years and 14 years respectively (Ebert et al. 2009, Shelekhov et al. 2022). Thus, in the KI and EBS, generation length is 15.8 and 11.6 years, respectively.
Threats Information
The Mud Skate is a bycatch of commercial trawl, longline, and trap fisheries targeting groundfish, including Pacific Cod (Gadus macrocephalus), Alaska Pollock (Gadus chalcogramma), and Greenland Halibut (Reinhardtius hippoglossoides). In the northern Kuril Islands and Southeastern Kamchatka, this species is concentrated on the edge of the continental slope at 200–300 m depth (Savin et al. 2019). It may be retained for the meat of the skate wings with retention rates of up to 30% in some fisheries. Post-release mortality is unknown but was 41% for deepwater Bathryaja species in southwest Atlantic trawl fisheries (Ellis et al. 2017).
In Russia, the fisheries usually operate to depths of 600–800 m (Ormseth and Matta 2011). There have been occasional experimental fisheries since 2016 that fish to 2,500 m depth (I.V. Volvenko unpub. data 2019) that may constitute a threat if expanded with the interest in developing targeted skate fisheries in the North Pacific. Skate fisheries in Russia are growing: the average annual skate catch reported to official statistics has grown nearly 3-fold, from 1,650 tonnes (t) in 2004–2008, 3,510 t in 2009–2013, and 4,860 t in 2014–2018; there is no species-specific skate catch reporting (Panchenko et al. 2020). In the eastern Bering Sea, skate egg cases, including the Mud Skate, are brought to the surface by commercial demersal trawl and longline fishing vessels targeting deepwater fish including Pacific Cod and Greenland Halibut (Stevenson et al. 2019). The Mud Skate is only commercially fished in the upper region of its bathymetric range and has significant refuge at depth from fisheries.
Use and Trade Information
In Russia, skates were previously discarded but more recently have been landed for export of wings to China, Japan, Korea, and the British Virgin Islands (Orlov and Volvenko 2022). International demand for skates by Korea is very high and has increased skate retention with Korea a major importer of skates from around the world (Hyun-Su et al. 2013). In the Bering Sea and Aleutian Islands fisheries, 27–51% (average 38%) of all skates were retained from 2011–2023, with larger individuals retained due to the higher market value, and while this may include the Mud Skate, there is limited species-specific information (Tribuzio et al. 2023).
Conservation Actions Information
There is a Total Allowable Catch (TAC) for skates in Russian and United States waters. In Russian waters, the TAC has changed little since the 1990s and in the 2020s is 11,2000–11,3000 t (Orlov and Volvenko 2022). In the United States Bering Sea and Aleutian Islands (BSAI) regions this species is managed as a skate complex. The BSAI TAC for the complex is based on the age structured model for Alaska Skate (B. parmifera) and the combined other skate survey biomass multiplied by an assumed natural mortality rate and has varied little from 2012–2023 and in 2023 was 27,441 t (Tribuzio et al. 2023). In 2015, six areas in the BSAI region of high skate egg density were designated Habitats of Particular Concern which allows management intervention where fishing activity is deemed harmful to the habitat. The 82 square nautical miles of habitat is split into different areas at the continental shelf/slope break, and aims to protect egg cases of various skate species. Targeted fishing for skates in the Gulf of Alaska has been prohibited since 2005 (Ormseth 2021). Further research is needed on population size and trends, and life history, and catch rates should be monitored.