Influence of light and feeding conditions on swimming activity rhythms of larval and juvenile turbot: Scophthalmus maximus L.: An experimental study

Turbot larvae are transported towards coastal nursery areas and live in very shallow waters. Food availability is assumed to be an important factor that retains them in such areas. To study the effects of a biotic factor (food) and an abiotic factor (light) that strongly influence behavioural mechanisms, experiments were carried out on laboratory-reared animals: larvae (1 cm), post-larvae (1.2 to 2.5 cm) and early juveniles (6 to 7 cm). Three kinds of apparatus and methods were used to record variations in swimming activity: (1) a phototaxis device to study orientation reactions in horizontal tanks; (2) actographs with infrared photoelectric barriers fitted around vertical cylindrical tanks; and (3) video cameras and cylindrical tanks. Observations were performed in total darkness and under dark–light regimes. Different types and quantities of food were provided to the fish. Larvae and juveniles of turbot exhibited a positive phototaxis from 1 to 1000 μW cm−2. At intensities lower than or equal to 0.1 μW cm−2, they did not exhibit clear reactions toward or away from the light. Turbot larvae and juveniles kept in total darkness did not show a clear rhythm of activity. Under natural illumination as well as in artificial LD conditions of similar periodicity, larvae swam by day and night. Live food (Artemia nauplii or juvenile mysids) induced an immediate increase in activity or the maintenance of a high level of activity, which decreased over the following days. Recently metamorphosed turbot kept under LD conditions exhibited a clear rhythm with a nocturnal maximum. Food given at night did not induce swimming changes as long as food density remained low. At higher prey concentrations, increased activity during feeding was followed by reduced activity for more than 24 hours. A similar response pattern was noted when active food was given in large quantities during the day: juveniles displayed an immediate increase in activity, which subsequently decreased. Regular food supply during the day induced an activity rhythm with a diurnal maximum. Despite slight differences, 6–7 cm juveniles behaved in a similar way. Our results clearly indicate the importance of trophic conditions on the swimming activity rhythms of larval and juvenile turbots and the lesser role of light conditions. They are compared with results in the literature describing the behaviour of the same species, and with juvenile sole, whose behaviour appears to be different.