Seasonal variations in time and space utilization by radio-tagged yellow eels Anguilla anguilla (L.) in a small stream

Seven yellow eels (572–643 mm, 318–592 g) Anguilla anguilla (L.) were tagged with surgically implanted radio transmitters (activity circuit, 1.6–1.7 g) and tracked in the Awirs stream, a small (width <5 m, depth from 0.1 to 1.2 m), densely populated (ca. 250 kg of eel ha−1) tributary of the Belgian River Meuse. The eels were positioned daily from late April to mid-August, and their diel activity was studied during twenty four 24-h cycles. During day-time, the eels were resting in rootwads or in crevices inside stone walls or in crevices in between rocks. They became more active in the late afternoon but generally did not leave their residence before sunset, except under overcast weather. Activity peaked during the first part of the night then progressively vanished, and always ended before sunrise. The area exploited during night-time never extended over more than 40 m2, except when the eel changed its residence. The intensity and timing of nocturnal activity and the extent of the daily activity area were dependent on water temperature (respectively P<0.0001, P<0.05 and P<0.0005), with eels showing little or no activity when the diurnal temperature did not exceed 13 _C. Eels showed higher agitation under full moon and maintained their activity later in the night (P