Weather related variability. Consequences on the swimming activity of a marine fish

Seabass, Dicentrarchus labrax (L.), movements were investigated between June and November 1993 using acoustic telemetry under semi-natural conditions. In the thermal range tested (5-27 degrees C), two decreases in activity appeared at 12 and 22 degrees C and swimming activity increased at night for oxygen content greater than 10 mg L-1. Light levels also modified swimming activity at the extremes: low or high values both decreased swimming. Moreover, swimming activity was demonstrated to be sensitive to wind and showed an increase for wind speeds higher than 8 m s(-1). Under cyclonic weather conditions, fish displayed higher activity levels than in anti-cyclonic conditions. We also observed that an individual, studied under contrasted ambiant conditions, showed more fluctuating activity levels than those displayed by several individuals studied in similar weather conditions. This emphasizes the weight of naturally combined environmental factors on fish behaviour. Finally, the behavioural responses produced by meteorological factors sustain the hypothesis of fish anticipating their habitat changes. ((C) Academie des sciences/Elsevier, Paris.).