Male European eels are highly efficient long distance swimmers: Effects of endurance swimming on maturation

European eels (Anguilla anguilla) migrate ~ 6000 km towards their spawning area in the Sargasso Sea. Based on the recent discovery that males swim even more efficiently than females, it was predicted that males also would be able to swim ~ 6000 km within six months. Additionally, eels do not mature naturally in captivity due to strong neural inhibition. Earlier, it was hypothesized that swimming exercise is a natural trigger to induce sexual maturation and may even result in full maturation. In the present study two groups of farmed male silver eels were subjected to either endurance swimming or resting for up to 6 months. It was found that male eels were able to swim continuously for a total distance of 6670 km within 6 months. The body weight decrease in swimming and resting males after 6 months was similar (< 30 g) underlining the extreme low energy cost of swimming. In contrast to our expectation long-term swimming did not induce sexual maturation in farmed silver eels, suggesting that swimming alone is not sufficient as a trigger for sexual maturation. In conclusion, male eels are efficient long distance swimmers and likely able to cover the distance to the Sargasso Sea within the expected time span of 6 months.