Survival probabilities of thornback and spotted rays discarded by beam trawl and flyshoot fisheries

Abstract

Elasmobranchs (i.e. sharks, skates and rays) play an important role as predators in marine ecosystems. In the last century the abundance of several skates and rays in the North Sea declined. Their specific life-history traits, i.e. being long-lived, showing slow growth, late sexual maturity and producing a small number of young per year, make them vulnerable to fishing, pollution and changes in essential habitats, especially spawning and nursery areas. Since 2010 a recovery in the abundances of some species is observed. Landings of skates and rays in European waters are generally managed by a group-TAC, meaning several species are managed under a single Total Allowable Catch (TAC). In the North Sea this group-TAC applies to thornback (Raja clavata), blonde (Raja brachyura), spotted (Raja montagui) and cuckoo ray (Leucoraja naevus). Landings of starry ray (Amblyraja radiata) and common skate (Dipturus sp.) are prohibited. In Dutch fisheries, skates and rays are mainly caught as by-catch in the mixed demersal fishery for flatfish, with landings dominated by thornback ray. For these species the rays below minimum landing size and catches that exceed the vessel’s weekly trip limits are discarded. Since 2019 this practise of discarding has been restricted for all quota regulated species, including rays, by the implementation of a landing obligation under the Common Fisheries Policy (European Union, 2013). The objective of the landing obligation is to create an incentive for fishers to avoid unwanted bycatches and thereby reduce discards rates. The European commission granted a temporary ‘high survival’ exemption for rays and indicated that for this exemption to be renewed or extended beyond 2023, knowledge gaps regarding discards survival probabilities of rays, as laid out in the “Roadmap skates and rays”, need to be filled. The current study therefore aims to contribute to this roadmap by filling main gaps in our knowledge on the survival probability of rays when discarded by the most important Dutch demersal fisheries. Beam trawl and flyshoot fisheries are quantitatively the main contributors to ray catches by Dutch demersal fisheries with thornback and spotted ray as main species. These two métiers and species were therefore selected for the discards survival experiments. We conducted five trips with a beam trawler during which 184 thornback and 140 spotted rays were sampled and four trips with a flyshooter during which 134 thornback and 28 spotted rays were sampled. Survival probabilities were measured by captive observation of these rays up to 25 days post-capture. Control rays were used to separate potential effects of the experimental procedures on mortality from fisheries-induced mortality. All fishery operations were conducted in the southern North Sea (ICES division 27.4.c) and in the English Channel (ICES division 27.7.d) according to the regular commercial practices of the fishing vessels. Trips were allocated to different quarters over the year to account for the effect of variable environmental and fishing conditions on discards survival. Operational and environmental conditions during sea trips were recorded. In this study we established the following discards survival probabilities (95% CI): • 45.5% (37.9-54.5) for spotted ray discarded by tickler chain beam trawl fisheries with 80 mm cod-end meshes; • 49.6% (42.9-57.4) for thornback ray discarded by tickler chain beam trawl fisheries with 80 mm cod-end meshes; • 77.6% (63.3-95.2) for spotted ray discarded by flyshoot fisheries with 80 mm cod-end meshes; • 81.0% (74.4-88.2) for thornback ray discarded by flyshoot fisheries with 80 mm cod-end meshes. The experimental procedures did not cause any mortality since all control rays survived the experiments. All mortality among sampled rays can thus be considered to be fisheries induced. Survival probability was higher for rays discarded by flyshoot fisheries compared to beam trawl fisheries. Survival probability of discarded rays decreased with increasing seawater temperature and some evidence for decreasing survival with increasing catch processing time was found. Species and body length did not affect survival probability. Survival probability was highest for rays that were landed on deck in good condition. To further increase survival probability it is recommended to keep catch processing time as short as possible and to focus on gear modifications that reduce stressors inflicted upon fish during the catch and hauling process to increase the proportion of rays that is landed on deck in good condition. In addition, we recommend to pursue reduction of fishing mortality among rays by reducing the amount of caught rays by improved selectivity of fishing gear and by actively avoiding fishing grounds where rays occur in high densities.