Features Research
Simulated migration enhances sexual maturation of farmed European eels

Sexual maturation of European eels (Anguilla anguilla) occurs during and/or after the approximately 6,000km migration from their freshwater habitats in Europe across the Atlantic to spawning grounds in the Sargasso Sea.

At the Aquaculture Europe Conference held last fall in Rotterdam, the Netherlands, Arjan Palstra from the Institute for Marine Resources and Ecosystem Studies (IMARES) of Wageningen University described what is likely the first study to simulate a mixed-sex, group-wise freshwater migration under mimicked photothermal conditions. Palstrca and collegues Daan Mes and Ron Dirks used farmed silver eels to determine the effects of this simulated ‘migration’ on the progression of sexual maturation

Silver eels swam in a flume against a flow of 0.57 m/sec (optimal swimming speed for males) under an 8:16 hour light:dark regime in freshwater at 11.5°C, covering an estimated 689km in two weeks, then in complete darkness in seawater at daily fluctuating temperatures between 11.7°C and 10.1°C mimicking migrations in the Atlantic for nine weeks, covering an estimated 3,103km. The total ‘distance’ the eels are estimated to have swum in the flume was 3792km.

Results of the study indicated that the freshwater migration significantly increased plasma testosterone levels in both migrating males and females, but did not enhance sexual maturation further as no significant increases in gonad weight, gonado-somatic index (GSI) nor eye index (EI) were observed.

The subsequent seawater migration significantly increased gonad weight and GSI of the migrant males, and particularly of the females (1.40 ± 0.06 vs. 1.00 ± 0.10) vs. non-migrating controls, indicating an advancement of sexual maturation. The EI was significantly higher in migrant males (14.0 ± 0.6) as compared to their controls (12.3 ± 0.4). Plasma levels of the gonadotropins FSH and LH were not elevated in the simulated-migrant eels.

The results showed that simulating migration under conditions mimicking those likely experienced by migrating eels in nature had significant stimulating effects on early maturation. It brought farmed silver eels to a similar state of maturity as wild eels that are ready to embark on their oceanic spawning migration, suggesting that simulated migration may be used to condition farmed eels for use as broodstock for further hormonal stimulation with gonadotropins.

For more information contact Arjan Palstra by email at: arjan.palstra@wur.nl