Plaice resting on the bottom of a flume respond to the current with a clearly defined pattern of behaviour. A simple hydrodynamic model which relates the slip-speed (Us) and lift-off speed (UL) to the physical forces acting on the fish has been verified experimentally using freshly killed fish and a rigid model. With an asymmetric semi-ellipsoid shape and a fineness ratio of 14 the plaice appears to be morphologically adapted to have minimum drag when heading into the current. As a result of this streamlined shape the lift force experienced by the fish is 10-20 times greater than the drag force, and the rheotactic behaviour therefore appears to be principally adapted to counteract hydrodynamic lift. Values of Us and UL, above which live fish must expend energy against the current, are exceeded for much of each tidal cycle in the southern North Sea. The model is of general application to benthic organisms in flowing water for Reynolds numbers between 104 and 106; outside these limits some modification of the constants is required.

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