Movements of yellow- and silver-phase European eels (Anguilla anguillaL.) tracked in the western North Sea

Abstract

Eighteen female European eels (4 yellow- and 14 silver-phase) captured in fresh waters and estuaries and fitted with 300 kHz transponding acoustic tags were tracked by sector-scanning sonar in the western North Sea for times up to 58 h and distances up to 136 km. All eels but one were released on the bottom after 1-22 h adaptation in a cage on the sea bed. Tidal currents were recorded continuously at two depths and two locations in the vicinity of tracking of 15 eels, and movements of eels through the water could be calculated by vector subtraction. The four yellow eels generally moved shoreward, while silver eels exhibited shoreward, offshoreward, or along-tide movements. Two silver eels used selective tidal stream transport to move northward, as evidenced by the close match between over-ground distances and calculated tidal excursions on transporting tides, and by the higher positions in the water column on transporting tides. When in midwater, most eels exhibited periods of well oriented directional swimming, showing only modest variation in the directional component of calculated 15-min vectors of swimming through the water. At other times, directions of 15-min vectors were widely scattered, and the eels drifted with the tide. One of the eels using selective tidal stream transport swam in an apparently unoriented manner on four successive transporting tides, but another added well oriented swimming to transport by the tide. Eels in midwater swam at modest mean speeds of 35-58 cm s −1 (0.45-0.75 L s −1 ). Eels were quite mobile vertically both on time scales of hours and of minutes. One clearly showed diel vertical migration, while two others showed the reverse pattern. Eels using selective tidal stream transport timed their vertical movements only imprecisely to times of tidal stream reversal. All 18 eels made contact with the sea bottom (ignoring the release at the bottom) and came near the sea surface during the tracks. Some eels spent most of the time near the surface with frequent descents. Only a few spent much time on or near the bottom, and they made frequent ascents. Some remained mostly in midwater but with ascents and descents from there. Holding the eels in the laboratory and aboard ship prior to tracking makes interpretation of observed movements relative to natural behaviour difficult. However, oriented swimming in midwater is consistent with previous reports in the literature of homing of displaced eels. The use of selective tidal stream transport has not been observed in other studies of adults of the European eel but has been reported in studies of estuarine movements of American eels (Anguilla rostrata).