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European eel migrate from freshwater or coastal habitats throughout Europe to their spawning grounds in the Sargasso Sea. However, their route (~ 6000 km) and orientation mechanisms are unknown. Several attempts have been made to prove the existence of magnetoreception in Anguilla sp., but none of these studies have demonstrated magnetic compass orientation in earth-strength magnetic field intensities. We tested eels in four altered magnetic field conditions where magnetic North was set at geographic North, South, East, or West. Eels oriented in a manner that was related to the tank in which they were housed before the test. At lower temperature (under 12°C), their orientation relative to magnetic North corresponded to the direction of their displacement from the holding tank. At higher temperatures (12-17°C), eels showed bimodal orientation along an axis perpendicular to the axis of their displacement. These temperature-related shifts in orientation may be linked to the changes in behavior that occur between the warm season (during which eels are foraging) and the colder fall and winter (during which eels undertake their migrations). These observations support the conclusion that 1. eels have a magnetic compass, and 2. they use this sense to orient in a direction that they have registered moments before they are displaced. The adaptive advantage of having a magnetic compass and learning the direction in which they have been displaced becomes clear when set in the context of the eel's seaward migration. For example, if their migration is halted or blocked, as it is the case when environmental conditions become unfavorable or when they encounter a barrier, eels would be able to resume their movements along their old bearing when conditions become favorable again or when they pass by the barrier.

Anguillid eels grow in freshwater but spawn in the open ocean. The cues that guide eels over long distances to the spawning area are unknown. The Earth's magnetic field can provide directional and positional information and is likely used by catadromous eels during their spawning migration; as magnetosensitivity and compass orientation have been reported in eels. To test whether this is theoretically possible, we compared the migratory routes of five species of temperate eels that undertake long migrations with the geomagnetic field of their distribution/spawning areas. We found that, regardless of the species and although routes are different between life stages, larvae of those species always drift along paths of increasing magnetic inclination and intensity, while adults follow reverse gradients. This is consistent with an imprinting/retracing hypothesis. We propose a general navigation mechanism based on larvae imprinting on a target magnetic intensity (or inclination) at the hatching area and on the intensity (or inclination) gradient during larval drift. Years later, adults retrace the magnetic route by following the gradient of decreasing total intensity (or inclination) values that occurs towards lower latitudes. As they reach the target value, adults switch to compass orientation to stay on the target isoline and reach the spawning area. The proposed mechanism fits for all temperate eels examined. Knowledge about navigational strategies of eels is important to evaluate the effectiveness of management strategies that involve stocking of juveniles displaced from one area to another to rebuild local populations. [ABSTRACT FROM AUTHOR]

This symposiumissue of the ICES Journal of Marine Science contains 16 contributions from the second International Eels Symposium held during the American Fisheries Society (AFS) Annual Meeting (August 2014, Québec City, Canada). This symposium followed the first International Eels Symposium held in 2003, which emphasized the international scope of the Anguilla conservation problem. This second symposium reviewed a decade of research on biology and life history, genetics and genomics, reproduction, oceanic biology, early life history, population dynamics, assessment and management, eel passage at hydroelectric facilities, stocking, and threats, focusing primarily on Atlantic anguillids. The symposium finished with a panel discussion that emphasized the need to develop inter-jurisdictional governance approaches for panmictic eels as a way to promote recovery of the various species. [ABSTRACT FROM AUTHOR]

At the onset of sexual maturation, European eels Anguilla anguilla exhibit high locomotor activity which may correspond to migratory restlessness. We measured activity of captive eels and determined whether it correlated with downstream runs of silver eels as well as changes in environmental factors. Groups of eels at different stages of the silvering process (yellow to silver stage) were tagged and placed in separate tanks supplied with either river or tap water. Activity was measured by means of a flat-board antenna placed vertically in the middle of the tank at the surface of the water. Wild migrating silver eels were caught in the nearby river. Activity of eels in the river water tanks increased 1 to 2 days before downstream migrating eels were caught in the trap, and concurrently with a rise in turbidity and a decrease in conductivity. Activity of eels in the tap water tank showed a different pattern, which did not correspond to downstream runs. A peak in activity corresponded to a drop in tap water pH. It is concluded that eels do show periods of high locomotor activity at the onset of migration and this could be used to predict downstream migration. Movements are triggered by changes in water composition (as opposed to changes in discharge, atmospheric pressure and lunar cycle) measured using turbidity and/or conductivity as proxies. If eels are able to detect such small changes in water conductivity (80 μS cm-1), they may use it to find their way to the estuary. [ABSTRACT FROM AUTHOR]