DNA based monitoring of sharks, skates and rays, and risk-based evaluation of bycatch in Danish fisheries

Sharks and rays are important components of marine biodiversity. This together with their life history with slow growth, late maturation and large asymptotic size makes them candidates for biodiversity indicators under descriptor 1 of the Marine Strategy Framework Directive. However, the assessment of their abundance and occurrence in the fishery is complicated by expected poor species identification. Further, landings of skates and rays (Rajidae) have historically often been reported as “wings” in commercial landings, rendering the catches by species unknown. For sharks, many species are easier to identify visually, but some, such as Mustelus asterias (starry smoothhound) and Mustelus mustelus (common smoothhound) can be difficult to distinguish from each other. These difficulties mean that the Danish monitoring undertaken prior to the previous MSFD assessments could not support the evaluation of the impacts of fishing on individual species.

This report aims to ensure that the species composition of sharks and rays in the Danish commercial fishery can be assessed going forward, and that Denmark can fulfil the obligations of the common fisheries policy and the marine strategy directive. This was approached through developing and testing DNA-based monitoring of by-catch of sharks and rays in the Danish commercial fishery and providing a knowledge base for risk-based management in relation to unintentional by-catch of sharks and rays. The project is divided into two components, mapping of the species composition of rays and selected sharks in the Danish fishery and an analysis of the catches of skates and rays in the Danish fishery. The mapping of species composition is confirmed by tissue samples from rays and selected shark species during observer trips on commercial fishing vessels and samples at landing sites. A risk-based analysis combines results from the genetic analysis with the observer and landing data to rank fisheries and gear in relation to the extent of unintentional by-catch.

Species identification performed by trained scientific staff (i.e., scientific surveys and observer trips) was more accurate than that performed by fishers. Misidentification rates were highest for the spotted ray species (Raja brachyura, Raja montagui and Leucoraja naevus), and thornback ray (Raja clavata). The unspecified categories of ray and skate wings consisted of a relatively large number of species and these categories should therefore be avoided. Misidentification levels were similar for samples collected in Sweden and Holland, regardless of the country of origin of the fishers. Hence, there is a general challenge of misidentification across European countries. Shark species caught as part of scientific surveys were in general identified correctly. However, all specimens originally identified as M. mustelus were genetically identified to be M. asterias. This finding is in line with earlier published research and supports that M. mustelus is a more southern species, while M. asterias is likely the only one of the two species occurring in Danish waters.

Danish landings of rays and skates have more than doubled over the last ten years. Historically, rays and skates were landed mainly as a mix of species (Raja spp), or as common skate (Dipturus batis), but since 2021 almost all landings have been recorded by species. The genetic samples confirmed the species identification of most landed species and their distribution. R. clavata was generally identified correctly, while it seems more difficult to distinguish R. brachyura from R. montagui. Rajella lintea (sailray) was the dominant species landed in recent years. The species first occurred in the landing statistics in 2018 even though a previous investigation showed considerable landings twenty years ago. A small amount of Dipturus oxyrinchus (longnose skate) has been recorded in the landings statistics since 2021. The genetic samples confirm the species ID of D. oxyrinchus even though some specimens were landed as R. lintea. Species ID of Rajidae from surveys and discard sampling was, in general, correct with a few misclassifications of R. brachyura and R. montagui. The quality of species recording from landings in Denmark has improved considerably since 2021, but misclassification still occurs. Ideally, all catches of rays and skates should be identified to species level, and landings of especially the protected species (complex) Dipturus batis/intermedia need to be confirmed by tissue samples. The development in species identification means that the relative importance of species cannot be evaluated historically, but in 2022, 60% of the landings were R. lintea, while R. clavata was the second most important species with 19% of landings.

The use of genetic identification meant that the wider distribution of the species in landings than from surveys could be used to indicate actual species distribution rather than be disregarded as expected misidentifications. This allowed documentation of the distribution of R. lintea and D. oxyrinchus, which were previously poorly known in the North Sea. It was also documented that protected species such as D. batis are at present rare but not completely absent from Danish catches. While the original intention of the study was to identify high abundance areas for the species and use these both to aid in species ID and in risk assessments, the distribution of the rarer species examined turned out to be much wider than indicated by scientific surveys in both sampled depths and at deeper waters. Though high abundance areas could not be identified, deeper waters generally had higher occurrences of the larger Dipturus and Rajella species. These deeper waters have in later years exhibited an increase in landing value, indicating that they may experience increased fishing pressure and hence increased risk in later years. Among the remaining species, all but Amblyraja radiata (starry ray) and Raja undulata (undulate ray) have increased in the past 20 years, presumably as a result of the general decline in demersal fishing since 2000. A. radiata is widespread in the central and northern North Sea and northern part of the inner Danish waters and it was not possible to identify areas with greater risk of bycatch for this species.

The analyses provide the basis for assessing the mortality as a result of accidental bycatch for sharks and rays per species for assessments from 2021 onwards under the Marine Strategy Framework Directive. The knowledge about the fisheries, gear type and areas with a significant risk of accidental bycatch can on a longer time scale potentially be used during the preparation of action programs under the auspices of Denmark's marine strategy to ensure a reduction in catch where necessary and the achievement of good environmental status.