An Experimental Test of the Impact of the Thermic Shock Encountred during the Diel Vertical Migrations of Silver Eels Anguilla Anguilla during Oceanic Spawning Migration

European eels (Anguilla anguilla) undertake a ~5000-km spawning migration from Europe to the Sargasso Sea (Tesch 1977). As details of this marine migration remain unknown, the EELIAD project was built to access the oceanic biology, ecology and movements of eels. This project showed that the eels travelled up to 50 km per day and exhibited a consistent diel vertical migration (DVM), ascending to warmer shallower waters at dusk and descending into deep, colder water at dawn, from ~200–900m and 14–8°C every day (Aarestrup 2009). To date, the causes of these DVMs remain unknown. Aarestrup et al. (2009) hypothesized that the observed DVMs reflects thermoregulation: “The daily ascent into shallower warm water may serve to maintain sufficiently high metabolism and swimming activity, whereas descent to deeper waters may permit the eels to keep their average temperature below 11°C, delaying gonadal development until reaching the Sargasso Sea”. To corroborate this assumption, we need to know the impact of the abrupt changes in temperature linked to these DVMs on the eel respiratory metabolism. A strong effect of the DVMs on this metabolism would lead to conclude at a strong hidden gain for the eels.