Disentangling estuarine conditions that shape the temporal dynamics of anadromous European (Alosa spp.) spawning migration across environmentally contrasted estuaries

Anadromous fish travel through estuaries when migrating between their marine growth habitats and their freshwater spawning grounds. In these dynamic transitional environments, strong currents and extreme dissolved oxygen and turbidity levels can result in stressful conditions for fish, impeding longitudinal connectivity. This study examined how estuarine hydro-environmental variables influence the temporal dynamics of spawning migration in threatened European shads ( Alosa alosa and Alosa fallax , undistinguished) across five environmentally contrasted estuaries along the French Atlantic coast. In the first part, focusing on the Loire estuary, catch per unit effort (CPUE) from commercial fisheries were consistent with individual tracking data from a 2011 telemetry study, revealing more intense migration activity during neap tides than during spring tides. Consistently, statistical models (Boosted Regression Trees) built over a longer period (2011-2018) showed higher CPUE under conditions of low turbidity, elevated oxygen saturation, and moderate ebb current velocity. In the second part, the temporal patterns of shad migration and hydro-environmental conditions were compared across estuaries. A fortnightly periodicity (14 to 15 days) was detected in the Loire and Gironde estuaries, in line with the neap-spring tidal cycle. This periodicity was not observed in the Seine, Vilaine and Adour estuaries, where oxygen and turbidity levels were less extreme during spring tides. These findings support the hypothesis that the estuarine turbidity maximum zone can act as a partial barrier to migration during spring tides, thereby structuring the long-term migration dynamics in macrotidal turbid estuaries. • ETM act as a partial barrier during European shads spawning migration • Extreme oxygen and turbidity conditions occur during spring tides • Migration follows a fortnightly cycle (14-15 days) in turbid estuaries