Swimming stimulates oocyte development in European eel

In this study, we subjected eels from Lake Balaton (Hungary) to a swimming period of I week and 2 or 6 weeks. Most eels were silver and were 13-21 years old. Time dependent changes in morphometrical parameters and developmental characteristics of the oocytes were determined. Already after I week of swimming, the gonadal mass increased and oocytes became larger, filled with large numbers of lipid droplets. After 2 and 6 weeks of swimming we found in addition a significant enlargement of the eyes, which is a sign of sexual maturation. In contrast to the resting eels, that had oocytes in the primary growth phase (stage 1-2); the swimming eels had oocytes in stage 3; the cortical alveolus or lipid droplet stage. The results indicate that lipid mobilisation induced by swimming is a requirement for the natural incorporation of lipid droplets in the oocytes, a crucial step in oocyte maturation. As the Balaton eels responded stronger to swimming than young farmed eels, it is suggested that older eels are more sensitive for maturation triggers. (c) 2007 Elsevier B.V. All rights reserved.