Eels will be anesthetized by immersion into a clove oil solution (0 2 mL / L, Sigma. Fish will be measured (cm,) weighed (g) and tagged with a passive integrated transponder PIT Tag (ID100B implantable transponder, diameter. 2 12 mm; length 11 5 mm, Trovan ®, Dorset Identification b. v., Netherlands, Annexe 3.