First evidence of European eels exiting the Mediterranean Sea during their spawning migration

Abstract

The migration route and the spawning site of the European eel Anguilla anguilla are still uncertain. It has been suggested that the Mediterranean eel stock does not contribute to spawning because there is no evidence of eels leaving the Mediterranean Sea. To test this hypothesis, we equipped eight female silver eels from the south of France with pop-up satellite tags during escapement from coastal waters. Once in deeper water, the eels quickly established diel vertical migration (DVM) between the upper and lower mesopelagic zone. Five tagged eels were taken by predators within the Mediterranean, but two eels reached the Atlantic Ocean after six months and at distances greater than 2000 km from release. These eels ceased their DVM while they negotiated the Gibraltar Strait, and remained in deep water until they reached the Atlantic Ocean, when they recommenced DVM. Our results are the first to show that eels from Mediterranean can cross the Strait of Gibraltar and continue their migration into the Atlantic Ocean. This finding suggests that Mediterranean countries, as for other EU states, have an important role to play in contributing to conservation efforts for the recovery of the European eel stock.

Figure 1. Map of western Mediterranean Sea and Eastern Atlantic Ocean, showing the approximate tracks of the individual tagged eels, based on a restricted number longitude and latitude estimates, see Methods for details.

Crosses mark the pop-up position of each tag. Black crosses denote the tags that surfaced at the programmed date, while red crosses indicate the pop-up positions of tags attached to eels taken by predators. The map was drawn in Esri ArcMap 10.1, using GEBCO bathymetry ( http://www.gebco.net/) and ESRI map ( http://www.esri.com/software/arcgis) data.

Figure 2. Sample plot of eels ID no. 133979 (left column) and 133986 (right column) showing a 4 day portion of vertical behaviour from the Mediterranean (upper panel) and the Atlantic Ocean (lower panel).

Black symbols show depths where temperature data were not available due to variability in transmission conditions of the Microwave Telemetry X-tag; transmitted depth values may have no associated temperature value and vice versa.

Figure 3. Recorded depth and temperature of eels while passing the Gibraltar Strait into the Atlantic Ocean (tags 133979 and 133986).

The grey area shows the period from when the eel enters the Strait, based on the cessation of DVM, to the recurrence of DVM when the eel leaves the shelf slope and enters into the Atlantic Ocean. The grey symbols correspond to depth values and the red to the temperatures.

Figure 4. Example of predation (tag 133984).

After predation (where temperature rises), the predator stays at depth for approximately 11 hours. Maximum recorded body temperature is 34.9 ^oC. The grey symbols correspond to depth values and the red to the temperatures.