Rheotaxis in larvae and juvenile sole ( Solea solea L.): Influence of light conditions and sediment

Abstract

An oval channel with an experimental section containing a sediment bed of thin sand was used to examine the effects of current on the orientation and the patterns of behaviour of larvae and juvenile sole. Behaviour of wild and laboratory-reared fish was compared. Experiments were carried out at constant temperature (12 °C) and salinity (35%.) under different light conditions and water velocities. Compared with the random distribution observed in still water, most larvae and juvenile headed into the current from low water velocities; the number of rheopositive animals increased with increasing speeds from a threshold depending on their age and whether they were in the water column or on the bottom. Larvae and small juveniles began to respond at currents of about 1 cm·s −1; larvae swimming in the water column began to be swept back at currents above 4 cm·s −1, although most remained heading into the current. Near the bottom, most juveniles oriented upstream from a speed of ~ 1 cm·s −1. Often, they swam off and just over the bottom and some buried. Juvenile sole rheotaxis persisted in the dark. Observations, confirmed by statistical analysis, showed that light conditions modify the relation between rheopositive responses and current speed. The percentage of rheopositive responses was higher in dim light or total darkness than under bright light. The implications of these observations for fish behaviour in the field are considered. The experiments indicate that the effects of current, combined with those of light and endogenous rhythmicity may control larvae and juvenile sole migrations from spawning areas to coastal areas. Nevertheless, swimming activity and orientation with respect to currents differ between day and night and are probably related to vertical, and mainly nocturnal, migrations.