A unifying hypothesis for the spawning migrations of temperate anguillid eels

Abstract

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.

Active swimming and transport by currents observed in Japanese eels (Anguilla japonica) acoustically tracked in the western North Pacific

“…Currently, it is not possible to verify whether the Sun culminating to north or south caused the meridional swimming tendencies of eels in this study. Potentially, these meridional swimming tendencies could be due to other orientation clues, such as the geomagnetic field, as discussed for temperate anguillid eels 17 , 45 . Nevertheless, in future studies, it would be worthwhile considering solar cues as a possible candidate factor in the orientation of eels, even when under faint underwater light conditions.…”

“…Several laboratory experiments of anguillid eels have shown that magnetic cues may be used by juvenile and adult eels to navigate during migration 41 – 44 , and this appears to be true for temperate anguillid eels that likely use a magnetic sense for their spawning migrations 45 . Whereas a recent comprehensive review indicated that no single cue or mechanism is used by animals to enable successful navigation over thousands of kilometers 46 .…”

Goldsinny wrasse (Ctenolabrus rupestris) have a sex‐dependent magnetic compass for maintaining site fidelity

“…Magnetic field-based migration behavior has been documented in many long-distance migrators such as salmon, eels, sharks, and turtles (Durif et al, 2021;Keller et al, 2021;Lohmann et al, 2007;Putman et al, 2014). In these species, the benefits associated with an ability to use a magnetic compass or map to cross hundreds to thousands of kilometers in pelagic water is clear.…”

Demographic history of two endangered Atlantic eel species, Anguilla anguilla and A. rostrata

“…Geomagnetism is considered highly important for navigation of eels during their larval spawning migrations (Durif et al 2021), and hence changes to the Earth's magnetic elds could have negative effects. The Brunhes-Matuyama magnetic reversal occurred ca.…”