Identifying spatiotemporal variations in Four-spot megrim (Lepidorhombus boscii) fishery nurseries to enhance management and stock assessment

Esther Abad ^1* Francisco Izquierdo ¹ Jorge Landa ² Francisco Velasco ² Manuel Hidalgo ³ Maria Grazia Pennino ⁴

• ¹ Oceanographic Center of Vigo, Spanish Institute of Oceanography (IEO), Vigo, Spain
• ² Oceanographic Center of Santander, Spanish Institute of Oceanography (IEO), Santander, Cantabria, Spain
• ³ Oceanographic Center of the Balearic Islands, Spanish Institute of Oceanography (IEO), Palma de Mallorca, Balearic Islands, Spain
• ⁴ Spanish Institute of Oceanography (IEO), Madrid, Madrid, Spain

The identification of recruitment areas and other essential fish habitats of exploited stocks is a fundamental requirement for the development of marine spatial planning and ecosystem conservation measures. Reducing recruitment mortality is particularly relevant in the northern continental shelf of the Iberian Peninsula and is one of the key requisites for the future sustainability of trawl fisheries. In this study, the distribution of nursery areas of four-spot megrim (Lepidorhombus boscii) was analyzed using time series of scientific bottom trawl survey data to determine whether the recruitment areas were persistent over time. Four environmental variables were considered as potential predictors of recruits' distribution which may influence the habitat selection of this species, sea bottom temperature, sea bottom salinity, bathymetry and type of sediment. Spatial analyses revealed a specific depth preference for four-spot megrim, with higher abundance in shallow waters with respect deeper ones, being the optimal range between 150 and 300 m. More importantly, our findings revealed significant spatial-temporal variation in four-spot megrim nurseries, suggesting that static spatial management approaches may become ineffective due to environmental variability. Based on the obtained findings, the abundance recruits index was divided into two spatial areas in the a4a stock assessment model currently used to provide scientific advice for this species. The results of the new assessment model showed differences in the biological reference points (BRPs) compared to the current ones, highlighting the importance of considering spatial structure in the assessment process to achieve a more accurate evaluation of stock status and enable more efficient and sustainable management, setting thus the basis for a potential implantation of a spatial stock assessment of the species.