Behavioural variability in the vertical and horizontal oceanic migrations of silver American eels

The oceanic spawning migration of American eels Anguilla rostrata has remained a complete mystery until the first direct observations were provided by recent tracking experiments. Here we increased the number of observations to better document the variability of migration patterns. Trajectories of 17 eels equipped with satellite tags and tracked for 14 to 58 d over an approximate distance of 630 to 2750 km were reconstructed. Thirteen eels were tracked beyond the continental shelf, including 5 eels into the Sargasso Sea. Similar trajectories were found among the tagged eels: they swam against currents towards the eastern part of the spawning area. Estimated net migration speeds in the open ocean ranged from 34.8 to 54.2 km d−1. In coastal waters, 2 vertical behaviours were observed: (1) repeated up and down movements without a circadian pattern within the first 50 m of the water column and (2) diel vertical migration, with eels swimming to greater depths during daytime (down to 974 m). Eels tracked in the Sargasso Sea all exhibited diel vertical migrations. Eels exhibited significantly greater swimming activity at night, the function of which remains unknown. The daytime and nighttime migration depths varied significantly among individuals and could not be explained by any of the physical factors examined. Some eels appeared to track particular isotherms. Eels tracked in the Sargasso Sea in 2014 ascended to depths shallower than 100 m at night, while eels tracked in 2015 were deeper.