Distribution and abundance of skates (Bathyraja spp.) on the Kerguelen Plateau through the lens of the toothfish fisheries

Three species of skate, Bathyraja eatonii, B. irrasa and B. murrayi, are commonly taken as incidental by-catch in Patagonian toothfish (Dissostichus eleginoides) longline and trawl fisheries, and the mackerelicefish (Champsocephalus gunnari) trawl fishery on the Kerguelen Plateau (KP) in the southern IndianOcean. Data from fishery observations for 1997–2014 shows that the three skates were widely distributedacross the Kerguelen Plateau, showing different spatial distributions, linked mainly with depth. Off HeardIsland and McDonald Islands (HIMI), in the southern part of the KP, B. eatonii and B. irrasa were mostabundant to the north and northwest of Heard Island, out to the edge of the Australian Exclusive EconomicZone (EEZ), and were caught down to depths of 1790 m and 2059 m respectively. The smallest species, B.murrayi, occurred mainly in the shallower waters down to 550 m, and was most abundant to the northand northeast, close to Heard Island. Around Kerguelen Islands, in the northern part of the KP, skateswere most abundant between the 500 m and 1000 m contours circling and extending from the islands.Catch rates were modelled using zero-inflated GAMs and GLMs. The catch rates of skates from thetrawl fisheries in the Australian EEZ surrounding Heard Island and McDonald Islands have shown littleevidence of depletion on the main trawl fishing grounds, although there is evidence of a decrease inthe average total length of B. eatonii. The marine reserves and the conservation measures employed bythe Commission for the Conservation of Antarctic Marine Living Resources in the HIMI fisheries, appearto provide effective protection for the skates, at least in the shallower waters where the trawl fisheriesoperate. B. irrasa taken in the deeper waters where longline fishing occurs have shown a slight declinein catch rate over the years of the HIMI fishery. Although all skates are returned to the water from thisfishery, survival rates are unknown and careful monitoring should continue to assess the status of thesestocks. There appears to be little change in the abundance of the skate species at Kerguelen in the timeperiod.This study provides the first review of skate by-catch across both the HIMI and Kerguelen fisheries.Ongoing monitoring of species specific by-catch levels and further research to determine the importantlife history parameters of these species are required, particularly for B. irrasa which is taken in both trawland longline fisheries.