Coastal to Riverine Entry Timing During the Spawning Migration of the European Shads (<i>Alosa</i> spp.): Drivers and Phenological Trends for the French Atlantic Coast Populations

ABSTRACT During the spawning migration of the anadromous allis Alosa alosa and twaite Alosa fallax shads, timing of river entry is decisive to ensure that arrival in the spawning grounds matches with favourable conditions for reproductive success. Identifying the environmental cues that drive the timing of river entry is therefore crucial to understanding the implications of climate change for shad populations and to implementing management measures for these threatened species. In this study, data from fisheries and fish counting stations located in the estuaries or low reaches of 10 rivers were combined to investigate the effects of coastal, river conditions and abundance on the timing of migration. Phenological trends were quantified at five sites with more than 20 years' monitoring, and we analysed whether these trends aligned with the period when river temperatures were in the most favourable range for offspring survival. The results indicated that the temporality of spring warming in coastal habitats and photoperiod were key drivers influencing river entry timing. Their relative influence varied between models predicting migration initiation, median and end dates. Significant shifts toward earlier and longer migration periods were quantified. At the site with the longest monitoring time series, the shift in migration timing increased the time lag between early shad arrival and the period of most favourable breeding temperatures. Therefore, further studies should assess the repercussions of earlier spawning migration on the phenology and success of reproduction and juvenile stages.