Tidally modulated burrow emergence rhythms of Nephrops norvegicus

The Norway lobster (Nephrops norvegicus) is one of the most important fishery resources in Europe and a key species for investigating biological rhythms in marine crustaceans, due its burrowing behaviour and wide depth distribution across continental shelves and slopes. Its availability to trawling is influenced by population-level burrow emergence events that vary in timing throughout the day. These emergence patters are known to depend on depth-related light intensity thresholds, but the effects of other environmental modulators remain less understood. Among them, the role of tidal cycles is poorly characterized, particularly in areas where strong hydrodynamic forcing may interact with day–night cues. Here, we assess the effects of tidal dynamics on Nephrops emergence in the Bay of Biscay (NE Atlantic, France), using a temporal analysis of population densities (i.e. by counting partially or fully emerged Nephrops) estimated from archived video-transects from 2016 to 2020 by Underwater Television surveys at depths between 28 and 133 m. Tidal data (i.e. sea level anomaly) were estimated to every transect, and 24-h variations in Nephrops density were analysed using Generalized Additive Models. Our results revealed a strong positive correlation between burrow emergence and the semidiurnal tidal cycle, with peaks typically aligned with flood currents and reduced densities with ebb phases, independently of the photoperiod. This finding contrasts with results from field studies in other regions using similar methodologies to analyse populations activity rhythms. It suggests that light intensity and tidal cycles influence burrow emergence patterns, with the strength of this modulation likely shaped by local habitat conditions. Behavioural responsiveness to tidal rhythms may therefore represent an adaptive trait in shelf-dwelling Nephrops populations inhabiting tidally dynamic environments.