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 ver y shallow waters. Food availability is assumed to be an important factor th at retains them in such areas. To study the effects of a biotic factor (foo d) and an abiotic factor (light) that strongly influence behavioural mechan isms, 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 kin ds of apparatus and methods were used to record variations in swimming acti vity: (1) a phototaxis device to study orientation reactions in horizontal tanks; (2) actographs with infrared photoelectric barriers fitted around ve rtical cylindrical tanks; and (3) video cameras and cylindrical tanks. Obse rvations were performed in total darkness and under dark-light regimes. Dif ferent types and quantities of food were provided to the fish. Larvae and j uveniles of turbot exhibited a positive phototaxis from 1 to 1000 mu W cm(- 2). At intensities lower than or equal to 0.1 mu W cm(-2), they did not exh ibit clear reactions toward or away from the light. Turbot larvae and juven iles kept in total darkness did not show a clear rhythm of activity. Under natural illumination as well as in artificial LD conditions of similar peri odicity, larvae swam by day and night. Live food (Artemia nauplii or juveni le 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 wit h a nocturnal maximum. Food given at night did not induce swimming changes as long as food density remained low. At higher prey concentrations, increa sed activity during feeding was followed by reduced activity for more than 24 hours. A similar response pattern was noted when active food was given i n large quantities during the day: juveniles displayed an immediate increas e in activity, which subsequently decreased. Regular food supply during the day induced an activity rhythm with a diurnal maximum. Despite slight diff erences, 6-7 cm juveniles behaved in a similar way. Our results clearly ind icate the importance of trophic conditions on the swimming activity rhythms of larval and juvenile turbots and the lesser role of light conditions. Th ey are compared with results in the literature describing the behaviour of the same species, and with juvenile sole, whose behaviour appears to be dif ferent. (C) 1998 Elsevier Science B.V. All rights reserved.