Theory 09/Marine Biology & Fisheries /Behavioural ecology/3 min
Migration Uses an Inherited Map
A juvenile fish that has never been there arrives at a specific river. Something is telling it where it is.
Long-distance marine migrants navigate with an inherited magnetic map rather than by following currents or learned routes, so displacement experiments should show corrective headings from the first migration, and population structure should follow magnetic rather than oceanographic features.
01The evidence
- An inherited magnetic map has been shown to guide ocean navigation in juvenile Pacific salmon, with fish responding to magnetic field parameters in a way that is consistent with a positional sense rather than with current-following.Current Biology
02The tension
Migration is explained with currents, learned routes and olfactory cues. Each works for part of the journey and none accounts for first-time navigation to a specific destination, which is why the map hypothesis has been resisted despite the behavioural evidence.
03The theory
The earth's magnetic field varies in intensity and inclination in a way that provides a two-dimensional coordinate system. An animal that can sense both can, in principle, determine its position without landmarks.
If the map is inherited, then a fish released in an unfamiliar place should take a heading that would bring it to its destination from there, not the heading it would have taken at home. That is a specific and unusual prediction, and it has been observed.
The prediction extends to population structure. If navigation is magnetic, then population boundaries should align with magnetic features rather than with current patterns, and artificial magnetic anomalies should distort migration.
The applied consequence is that magnetic noise from cables and installations could disrupt migration in ways that are currently unmeasured, and that stock boundaries derived from currents may be wrong.
04What this is not
Those models require experience or transport. This requires only a map, and predicts corrective headings on first migration and population boundaries that follow magnetism.
05Predictions
- P1Displaced juveniles should take corrective headings from unfamiliar locations.
- P2Stock boundaries should align with magnetic features better than with currents.
- P3Artificial magnetic anomalies should distort migration routes.
06What would refute it
Displaced animals navigating by currents or learning, with no map component, would leave the map hypothesis unsupported.
07What it would change
It would change stock assessment and add magnetic disturbance to environmental impact.
08Evidence base
- An inherited magnetic map guides ocean navigation in juvenile Pacific salmon — Current Biologyhttps://www.sciencedirect.com/science/article/pii/S0960982214000189