Locomotory activity of the black goby,Gobius nigerL. (Pisces, Gobiidae), under artificial light conditions, including a false dawn and dusk

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Abstract
Under controlled conditions in the laboratory the activity of the black goby was measured by means of IR light-source/photoreceptors over a 60 day period. The fishes were mainly active at light intensities of 65 and 160 lux, but the activity was not completely suppressed during the dark period (0 lux). At light intensities lower than 0.5 lux the activity was high, probably as a result of a prolonged twilight period, because activity was high both at dawn and dusk. When the light was switched on suddenly an activity peak occurred whereas no such peak was recorded following the light-off stimulus. Under conditions where cover was available the gobies showed reduced activity. Under twilight conditions no distinct activity peak was recorded, but instead a burst of activity occurred followed by a gradual increase in activity. It was difficult to draw any parallel between the laboratory results and the observations from the natural habitat.