Unveiling European hake spawning patterns from fishery data

Abstract Understanding the spatial distribution of commercial fish species is critical for estimating the stock size and its temporal dynamics, and essential for establishing sustainable catch limits and develop adequate management strategies. Spatial data can also reveal suitable habitats and preferred spawning grounds and nursery areas. This study aimed to characterize the spatio-temporal variability of the spawning grounds of the European hake (Merluccius merluccius) in north Atlantic Spanish waters by combining different sources of fishery-dependent data. Hake holds a significant commercial importance in the northwest of Spain, where the species is targeted by both large- and small-scale fisheries of which two fleets using longlines and gillnets mainly catch large female individuals. By means of linking Vessel Monitoring System data, which provide accurate geographical position of the vessels, with first-sale notes and electronic logbooks, which enable the estimation of commercialized hake roe independently of the rest of the body, we studied the spatio-temporal dynamics of spawner females (2009–2022) to infer potential spawning grounds and analyse the dynamics and drivers of the reproductive activity using generalized additive mixed models. Results showed a clear reproductive pattern, characterized by a higher spawning activity in the first part of the year with a geographical expansion from northwest to northern waters. Spawning areas were persistent in the northwest coast of the Iberian Peninsula, while secondary spots were further detected to the west and eastern most extremes. Finally, a maximum of reproductive activity was obtained between 10°C and 12.5°C of sea bottom temperature, and it increased non-linearly with chlorophyll-a concentration. Overall, this study proposes a new indirect approach to identify spawning areas at large spatio-temporal scales, improves our understanding of hake spawning allowing for a better interpretation of the fluctuations of the species’ productivity, and provides the scientific basis for the implementation of new technical management measures.