Estimating migration and mortality rates for European sea bass ( Dicentrarchus labrax ) using electronic tagging data

As animal movement and migration shape the internal dynamics and structure of populations, quantifying connectivity within populations is crucial for sustainable exploitation and conservation of wild species. New knowledge on individual movements and migrations of the European sea bass recently challenged the current view of the stock delineation. There was a need to extend these results from the individual to population scale by developing a spatially structured population dynamics model of tagged fish to estimate migration rates and mortality. A series of multistate spatial mark-recapture models, discrete in space and time and based on two alternative spatial structures have been developed and compared to analyse reconstructed individual trajectories of European sea bass tagged, from 2010 to 2022, with Data Storage Tags along the coast of France, Belgium, and the UK. The analysis of 328 tagged fish recoveries made from 2010 to 2022 allows the quantification, at population scale and in an integrated tagged-based model of both seasonal movement rates and fishing mortality rates for two of the current stock assessment and fishery regulatory areas: the northern stock area from the Irish Sea, Celtic Sea, English Channel, and southern North Sea (ICES divisions IVb–c and VIIa, d–h) and the Bay of Biscay (or southern) stock area (ICES divisions VIIIa–b). Significant movements are shown to occur between those areas, preferentially from the southern area to the northern area in relation with feeding in summer and in the other direction in relation with spawning in winter. Hence, in summer, an important fraction of the population breeding in the southern area (considered to be from the southern stock) mixes, in the northern area, with the population from the northern stock, which challenged the stock structure considered for the assessment and management of this species. Our results led to a revision of the European sea bass stock assessment carried out by the International Council for the Exploration of the Sea (ICES) to account for the mixing and movements between stocks.