“Moving back in” - Assessing habitat suitability and space use for re-introduced Thornback Ray in the Western Scheld and Belgian Exclusive Economic Zone

Abstract

The Thornback ray (Raja clavata) has virtually disappeared in the Dutch coastal areas since 1958. In an attempt to repopulate Raja clavata and other species in the Dutch coastal area the Dutch World Wildlife Fund (WNF), Blue Linked, Stichting de Noordzee, the Dutch Shark Society and the Dutch Anglers Association (Sportvisserij Nederland) set up a program named ‘Sharks and rays back in the North Sea’. As part of the program, thornback rays were artificially reared and released into the Eastern Scheldt and Western Scheldt areas. Some rays within this program got tagged and their location was monitored using an acoustic telemetric system. This paper uses data on 79 tagged rays that were released back into the Western Scheldt to model the spatial behaviour of these Raja clavata within the Western Scheldt and Belgian Exclusive Economic Zone. Of a total dataset of 79 juvenile rays, twenty were found to have sufficient data to represent natural resident behaviour inside of the study area through a weighted residency index (IWR). For these animals, the dynamic Brownian Bridge Movement Model (Kranstauber et al., 2012) was applied in order to estimate occurrence distributions. The outputs of the model were then used to compute .95 and .50 utilisation distributions (UD) showing spatial behaviour of the animals throughout the study period. The research concludes that overall, the Western Scheldt and Belgian Exclusive Economic Zone can be considered a suitable habitat for juvenile Raja clavata as the re-introduced rays showed residential behaviour. Especially the areas in the Western Scheldt between Vlissingen and Terneuzen and west of Terneuzen were intensively utilized by the animals, as well as the harbour of Zeebrugge at the Belgian coast. Male and female rays showed different utilisation distributions, with females showing more resting behaviour in the Western Scheldt and males showing more active behaviour moving out of the estuary. In winter the animals tended to stay within the estuary, as in summer they show more active movement behaviour towards the outer estuary and Belgian coastline. The study also highly recommends future research on the spatial relationship between Raja clavata presence and their five biggest predicting abiotic factors: water temperature, bathymetry, salinity, bottom shear stress and sediment type. Furthermore, the outcomes of this research can be used to design more detailed strategies in order to protect, maintain and potentially expand the population of Raja clavata in the Western Scheldt and surrounding areas.