Depth selection behaviour during activity cycles of juvenile plaice on a simulated beach slope

Juvenile plaice Pleuronectes platessa on a sand slope in a laboratory tank showed depth-selection behaviour consistent with offshore migration by day and onshore by night, as seen in natural conditions. Plaice stayed deep (0.5 m) and avoided the shallowest water (< 10 cm depth) during light periods, but ventured up the slope and into the shallows in darkness. The freshly caught fish showed circatidal cycles of activity, but showed no change in depth selection between periods of high (time of expected high water) and low activity (expected low water). This suggests that changing direction of movements in tidal migration is controlled by responses to changing environmental conditions. Further analysis showed movements of fish up the slope to be slower, covering less distance and with shorter pauses in between than moves down the slope. Frequency, distance travelled and speed of moves up and down the slope did not change with tidal or light-dark cycles, and so such modulation could be ruled out as a mechanism for migration.