Semi-diurnal rhythmic activity in deep-sea benthic fishes in the Bay of Biscay

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Abstract
A large baited trap, supporting an Edgerton flash camera system was used several times in the Bay of Biscay between 2000 and 4700 m. Colour photographic sequences (one shot every 3 minutes for about 20 hours) reveal that the presence of fishes around the trap is almost always periodic. This behaviour is synchronous with periodic changes in the direction of weak currents, as indicated by yarns hanging in the trap. Fishes appear outside the trap shortly before low tide on the shore of the Bay. One or two hours before high tide a period of about 6 hours begins with no fish around the trap. They come back a little before the next low tide. This is consistent with various recent records of semi-diurnal tidal currents in the depth of the Bay. It is the only recovered geophysical cycle in the deep-sea and it has clear consequences on the biology of fishes and other benthic animals.