The ability to maintain position in a cut-rent (station-holding) was determined for the plaice Pleuronectes platessa on a simple form of habitat structure, substratum ripples. Twelve sinusoidal combinations of ripple wavelength and amplitude were tested, with wavelengths approximating 0.5, 1.2, and 5 times fish total length. Although ripple flow patterns varied, all provided regions of retarded flow in the ripple troughs. Therefore, ripples affected station-holding performance in the following three ways: (1) directly, by providing regions of retarded flow, thus creating flow refuges, (2) indirectly, by interfering with behaviors deferring the onset of active swimming, and (3) indirectly, by providing opportunities for new behaviors deferring swimming. The direct effects of retarded flow on performance versus indirect effects via swim-deferring behavior were evaluated. Flow retardation was measured as the velocity ratio (u(through)/u(free-stream)), where u(through) is the velocity measured 0.5 cm from the bottom of a trough and u(free-stream) is the velocity measured 10 cm above ripple crests. At a velocity ratio >0.4, direct flow-retardation effects were most important in determining observed performance. At a velocity ratio <0.4, indirect interference affected performance. Performance of plaice was maximized on ripples with wavelengths approximately twice their total body length.
