RHEOTAXIS IN JUVENILE SOLE SOLEA-SOLEA (L) - INFLUENCE OF SALINITY AND LIGHT CONDITIONS

The effects of water currents, salinity and light regime on the orient ation and swimming activity were studied in juvenile wild and hatchery -reared sole of two size ranges (10-20 mm; 21-30 mm) in an oval flume with a straight working section and a thin bed of fine sand. Short-ter m experiments using increasing water velocities were carried out at lo w salinities (S=26, 18 and ii) similar to those found in estuarine are as where recently metamorphosed juveniles settle and develop. Long-ter m experiments (24 h) were carried out with constant water velocities ( 3 cm s(-1) and 5 cm s(-1) in juveniles of 10-20 mm and 21-30 mm, respe ctively) and 4 water salinities (S=35, 26, 18 and 11). The behaviour o f juvenile sole was dependent on current speed regardless of light con ditions. In the short-term experiments, the number of rheopositive fis h increased with increased current. At low salinities, regardless of t he origin and the size of the animals, the relationship between water velocity and rheopositive reactions was not significantly different un der different light conditions. In long-term experiments, most juvenil es remained oriented against the current during the nycthemeral cycle but the percentage of rheopositive-responding fish was higher during t he dark phases or in total darkness than during the illuminated phases or under constant illumination. The interrelations between current, s alinity and light are discussed and a hypothesis is proposed to explai n the migration of juvenile sole towards coastal areas and estuaries.