Mþ and
M
fish were individually placed at different salinities (0, 10 or 30) for 28 days in small containers
equipped with shelters. Individual wet mass and pigmentary stage were determined on Day 1 and Day
28. In addition, dry mass and percent dry mass were measured on a sub-sample of 30 fish randomly
chosen on Day 1, and on all fish on Day 28. There was no difference in the loss of mass linked to their
adaptation to salinity between M and Mþ
fish, and so the results do not support the link between
migratory behaviour and salinity tolérance (or to a larger extent osmoregulation). However, M
fish lost
more wet mass than Mþ
fish irrespective of the salinity, maybe because of a higher standard metabolism,
which suggests that glass eels prone to loose more energy might choose to settle at sea or in estuary. In
addition, the highest percent dry mass after 28 days in seawater associated to the highest loss of mass in
28 days in seawater does not support a higher energy cost of osmoregulation at sea.