Adult plaice (Pleuronectes platessa L.) in the southern North Sea use selective tidal stream transport to migrate between their summer feeding grounds and winter spawning grounds. The fish come up into midwater when the tidal stream is flowing in one direction, but remain on the bottom when it is flowing in the opposite direction. This behaviour can theoretically save energy, which can be used for growth or reproduction. Alternatively, it may provide a transport system by means of which fish that have no ability to navigate over long distances can be conveyed reliably between their feeding grounds and spawning grounds. We have attempted to identify whether plaice do use selective tidal stream transport in an energetically effective way by calculating the metabolic cost of transport for 12 individual plaice tracked in the open sea. We have deduced the net movement of the fish through the water from their geographical positions at the beginning and end of each of 34 extended midwater excursions and the predicted speed and direction of the tidal stream for the same period. Our analysis shows that plaice in midwater swim at a moderate speed (mean 0.6 ±0.05 L s-1; maximum 1.4 L s-1) through the water, and on average travel further over the ground than is to be expected by passive transport. The metabolic cost of transport is about 50 mg Oz kg-1 km-1, which is low compared with the costs of migration in other species, and is about 20% less than the cost of continuous swimming at the most effective speed (1.3 L s-1). Although most fish head downtide when in midwater, the orientation is not very precise: It is, however, sufficient to achieve 88% of the maximum energy saving that could be made with precise downtide orientation. Although our results do not allow us to conclude that plaice use selective tidal stream transport solely to save energy, an energetic analysis indicates that large plaice cannot save energy by using selective tidal stream transport in sea areas with slower tidal stream speeds (mean speed<0.2 m s-1). They should therefore not exhibit this behaviour in such areas if the mechanism evolved as a means of conserving energy. © 1990, Marine Biological Association of the United Kingdom. All rights reserved.
