Silent uncrewed surface vehicles reveal the diurnal vertical distribution of lesser sandeel

Abstract Lesser sandeel (Ammodytes marinus) exhibits a peculiar diel vertical migration (DVM) during the feeding season, burying into the seabed at night and emerging during daytime to form schools that feed on zooplankton. Large schools may consist of a pelagic component searching for prey and a bottom component connected by collective bridge-like formations. However, the temporal variation in the schools’ vertical distribution is poorly understood. In this study, 38 and 200 kHz acoustic data recorded with Saildrones were used to examine the schooling dynamics during their main feeding season in May–June. A total of 1497 sandeel schools that were identified by linear discriminant analysis displayed two distinct vertical components throughout the season: one in the pelagic zone and one near the seabed. The pelagic component was distributed deepest at noon and had a similar pattern to zooplankton DVM, suggesting that sandeel followed the vertical distribution of their prey. Their diurnal ascension was greater in both distance and hours in May than June, suggesting a decline in feeding motivation towards the end of the feeding season. These findings were made possible with the long-term monitoring by silent Saildrones, which did not seem to affect the natural behaviour of sandeel schools.