Orientation of plaice (Pleuronectes platessa) in the open sea: evidence for the use of external directional clues

We studied the orientation of plaice (Pleuronectes platessa L.) in the open sea (Southern Bight of the North Sea) during May and June of 1979 and 1980, using continuous, on-line data from 6 fish equipped with transponding acoustic compass tags. During the day, fish swimming in midwater did not maintain their heading. At night, the same fish maintained their heading in a way that was consistent with active orientation to some external directional clue, but inconsistent with the use of an inertial guidance system. We believe this to be the first analysis to demonstrate the use of external guidance clues by fish in the open sea. Because fish were able to maintain a consistent heading in midwater by night, in the apparent absence of both visual and tactile clues, the external reference is likely to be geophysical. Possible clues include the electrical field generated by the flow of sea water through the geomagnetic field and the earth's magnetic field itself.