The effect of thermal shock during diel vertical migration on the energy required for oceanic migration of the European silver eel

• Diel vertical migrations (DVMs) were highlighted in migrating European silver eels.
• We experimentally simulated the variations in water temperature that occurred during DVMs.
• Simulated ascending DVMs showed an increase in cost of transport in females (+ 16%).
• Simulated ascending DVMs showed an increase in cost of transport in males (+ 73.9%).
• Simulated descending DVMs showed an increase in cost of transport in females (+ 17.9%).

Recently, diel vertical migration (DVM) was observed in European silver eels (Anguilla anguilla) migrating towards the Sargasso Sea. However, because European silver eels do not feed during this migration, their energy consumption requires accurate optimization due to their limited fat reserves. In this study, changes in water temperature were experimentally induced to simulate those experienced by silver eels during DVMs to estimate their effect on oxygen consumption (MO2). Therefore, the oxygen consumption of eels at 8 °C (deep waters) and 14 °C (surface waters) was measured, and the effect of rapid thermal changes (shocks) from 8 °C to 14 °C, simulating ascent, and from 14 °C to 8 °C, simulating descent, both movements mimicking DVMs, were assessed.Firstly, a single thermal shock induced an increase in MO2 in both sexes at both temperatures.Secondly, the cumulative effect of these two factors (single temperature and thermal shock) was analysed to mimic natural DVMs, and showed that an increase in MO2 was linked to DVMs in females (+ 16% at ascent, and 17.9% at descent) and for males during DVM ascents (+ 73.9%). These data are used to discuss the effect of DVMs on the cost of transport in European eels during their breeding migration across the Atlantic Ocean.