Simulated migration under mimicked photothermal conditions enhances sexual maturation of farmed European eel (Anguilla anguilla)

Sexual maturation from puberty to adulthood in European eels (Anguilla anguilla) occurs during and/or after the ~ 6000 km reproductive migration from their freshwater habitats to the spawning grounds in the Sargasso sea. This is the first study to simulate an anorexic, mixed-sex, group-wise freshwater migration (2 weeks; 689 km) and subsequent seawater migration (9 weeks; 3,103 km) under mimicked photothermal conditions, using farmed silver eels. Silver eels swam under an 8 hour light:16 hour dark regime in freshwater at 11.5 °C, and subsequently in complete darkness in seawater at daily fluctuating temperatures between 11.7 °C and 10.1 °C mimicking the vertical migrations in the Atlantic Ocean. The aim of these two consecutive experiments was to determine the effects of a simulated reproductive migration on the progression of sexual maturation. 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, gonadosomatic 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. control groups, suggesting advancement of maturation. Also 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 remained near the detection limits of the assays and levels were not elevated in migrating eels. These results show that simulation of migration under mimicked photothermal conditions has significant stimulating effects on early maturation which are presumably under steroid control. This brings farmed silver eels to a similar state of maturity as their wild conspecifics that are ready to embark on their oceanic migration. Simulated migration may therefore be used to condition farmed eels for the use as broodstock eels for further hormonal stimulation with gonadotropins in maturation protocols.