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

Eighteen female European eels (4 yellow- and 14 silver-phase) captured in f resh waters and estuaries and fitted with 300 kHz transponding acoustic tag s were tracked by sector-scanning sonar in the western North Sea for times up to 58 h and distances up to 136 lan. All eels but one were released on t he 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 ca lculated by vector subtraction. The four yellow eels generally moved shorew ard, while silver eels exhibited shoreward, offshoreward, or along-tide mov ements. Two silver eels used selective tidal stream transport to move north ward, as evidenced by the close, match between over-ground distances and ca lculated tidal excursions on transporting tides, and by the higher position s in the water column on transporting tides. When in midwater, most eels ex hibited periods of well oriented directional swimming, showing only modest variation in the directional component of calculated 15-min vectors of swim ming through the water. At other times, directions of 15-min vectors were w idely 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 swimmi ng 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 m igration, 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 botto m (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 frequen t 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 de scents from, there. Holding the eels in the laboratory and aboard ship prio r to tracking makes interpretation of observed movements relative to natura l behaviour difficult: However, oriented swimming in midwater is consistent with previous reports in the literature of homing of displaced eels. The u se of selective tidal stream transport has not been observed in other studi es of adults of the European eel but has been reported in studies of estuar ine movements of American eels (Anguilla rostrata). (C) 1999 International Council for the Exploration of the Sea.