To reproduce, European eels must undergo a long migration without feeding. During this migration they have to cope with many environmental factor changes, one of them being hydrostatic pressure. We focus on the effects of hydrostatic pressure on swimming energetics: does the pressure exposure modify swimming efficiency? By using a specially designed Blazka type swimming tunnel able to work under pressure, we have measured oxygen consumption of migrating male silver eels at different swimming speeds (from 0.2 to 1.0 BL/s) first at atmospheric pressure then at 101 ATA hydrostatic pressure. The results show that pressure increases the energetic swimming efficiency by decreasing oxygen consumption for a given swimming speed. Such a pressure effect could represent a remarkable adaptation enabling eels to spare their energy stores and swim for a long time.