Spatio-temporal dynamics of Allis shad (Alosa alosa L.) spawning in response to environmental variability

The Allis shad (Alosa alosa L.) is a threatened anadromous fish species whose populations have dramatically declined in recent decades due to multiple anthropogenic pressures, including habitat fragmentation, water quality degradation and overexploitation. In this study, we aimed (1) to identify the environmental and spatio-temporal drivers of spawning probability and intensity, and (2) to estimate the number of spawners over time in two major French river catchments, the Garonne and the Loire. Using generalized additive models, we found that these models performed well in predicting spawning dynamics. Spawning probability and intensity were significantly influenced by environmental variables (temperature, discharge and precipitation) and spatio-temporal factors (sampling site, river catchment, year and Julian day). Specifically, moderate increases in temperature during the pre-spawning period and low to moderate discharge on the day before or the day of spawning were associated with higher spawning probabilities and greater spawning intensity. In contrast, a temperature decrease a few days before spawning, excessively low or high discharge on the day before or the day of spawning and excessive precipitation on the spawning day were associated with lower spawning probability and intensity. Additionally, trends in the estimated number of spawners at spawning sites between 2001 and 2022 revealed a significant severe decline in both river catchments, with an average population decrease of approximately 90% in the Garonne and 75% in the Loire. These findings highlight the critical situation and the urgent need for management measures to mitigate the ongoing decline of Allis shad populations.