Spatial distribution, migration, and population structure of North Sea rays

Samenvatting

Little is known about the migratory behaviour of sharks and rays in the Greater North Sea ecoregion. While several ray stocks were widespread in the early 20th century, the population declines resulted in a reduced distribution area of most rays, with the east coast of the UK and the Eastern English Channel being the hotspots for abundance for most larger species. The recovery in abundance also results in a larger distribution area, including parts of the Dutch EEZ. The question is how these different areas are connected in terms of migration and population genetics. To study migration of rays, we tagged 224 specimens of three ray species; spotted ray (Raja montagui), blonde ray (Raja brachyura), and thornback ray (Raja clavata). This tagging was done using CEFAS G5 Data Storage Tags (DSTs). Each tag was attached to an individual ray at the basis of the tail. To ensure minimum effect of the tag on the ray, a protocol was developed to attach the tag to a ray, with the goal of having a procedure for attachment that was as short as possible, and with minimum effect on the ray. DSTs measure and store temperature and depth of the individual that they are attached to, The G5 DSTs used in this study can be programmed to detach from the animal (‘pop-off’) after a set time. Because the DSTs are slightly buoyant, they will float to the surface and be taken to land by currents and winds. When the tags were found by fishers on live rays, or by beach combers, the temperature and depth records can be downloaded from the tag and analyzed. These temperature and depth profiles can subsequently be used to reconstruct the migration pattern of the individual rays. Because the pop-off time for the deployed tags has not been reached, we now have a limited set of returned tags: 30 tags were returned, with the majority being found on beaches. Most returned tags were retuned within 6 months after release. Of the 30 tags which were returned, 7 were reported by fishers and therefore both capture and recapture locations are known, shown in figure 4. These ‘traditional’ mark and recaptures give preliminary insights into skate movements in the North Sea and English Channel: While some tags were recaptured close to the release positions, others were found far from the release positions. One tag was released north of the Dutch Wadden Sea and was recaptured off the English coast. In some cases, the tags were recaptured in different ICES divisions from their release position. Mapping spatial distributions of ray populations using Integrated Nested Laplace Approximation (INLA) on research vessel survey data showed the changes in spatial distributions of Raja clavata and R. brachyura. Data was used from 1988, when the survey coverage of the Eastern English Channel was complete, to 2022. Data from three different surveys were used, estimating potential differences in catchability between the surveys in the model. Three size classes were distinguished in the model, with boundaries between classes being based on the length at which rays become vulnerable to the fishery, and length at first maturity. Both species increased in overall population abundance, with higher abundances in the eastern part of the southern North Sea. The range of spatial correlation in abundances was slightly smaller for R. brachyura compared to R. clavata. A large number of rays from different locations in the North Sea was genotyped, and combined with earlier genotypes from the species. Genotyping was done by means of “Genotyping by Sequencing” and by means of an Illumina SNP chip. No clear population genetic structure was found within the North Sea for any of the species, while structure did exist outside of the North Sea for R. clavata. To conclude, there are clear increases observed in the abundances of the demersal elasmobranch stocks in the Southern part of the Greater North Sea ecoregion. As a result, more rays are observed in the Dutch part of the North Sea. Meanwhile, no clear population genetic structure was observed for the stocks. This could be explained by the migration that is observed from tagging data, even with the limited set of tags that has yet been returned. The tags that have been returned show clear diel vertical migration patterns in all species. The number of returned tags is expected to increase substantially over the next two years, as more pop-off dates are reached. Such a full set of tag data can be used to reconstruct annual migration patterns, as was demonstrated by the tags analysed in this study.