Behaviour of European eel <i>Anguilla anguilla</i> transitioning from the yellow resident to the silver migratory stage in marine waters

Current knowledge on the continental shelf and transatlantic spawning migration of the European eel (Anguilla anguilla) is based on silver eels outmigrating from fresh water. However, an unknown proportion of eels grow in marine coastal waters during the continental part of their lifecycle and therefore start their spawning migration while they are already in marine waters. To study the onset of the European eel's spawning migration in marine waters, we tagged yellow eels in Sandnesfjord, Norway. One hundred yellow and pre-migratory eels (stage FII or FIII) were tagged with acoustic transmitters in August, September or October in 2018-2019 and their subsequent movements were recorded within a hydrophone array until May 2023. Twenty-two eels left the fjord in a period spanning from early October to early December, which partly coincides with the usual downstream migratory period among European eels. These eels were never detected again and had thus presumably transitioned to the silver eel stage. This transition occurred from a few months to up to 3 years after tagging. In all silver eels, an increase in vertical and horizontal movements within the fjord preceded their exit from the fjord. This sudden increase in activity occurred only for a few days before they left the fjord in most individuals, but this period was as long as 1 month in other individuals. Five of the silver eels undertook migrations up to several kilometres to a tidal stream before leaving the fjord. Eel migration was largely nocturnal, independent of moon or tidal phase. We conclude that the relatively short migration season and individual migration duration within the fjord, combined with the absence of significant environmental, physiological or anthropogenic stressors, may contribute to higher survival rates during the continental phase of the spawning migration compared to eels that start their spawning migration from fresh water, but this needs to be investigated in future studies.