Publication Abstract

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

J.D. McCleave and G.P. Arnold

Eighteen femaleEuropean eels (4 yellow- and 14 silver-phase) captured in fresh waters and estuaries andfitted with 300 kHz transponding acoustic tags were tracked by sector-scanning sonar inthe western North Sea for times up to 58 h and distances up to 136 km. All eels but onewere released on the bottom after 1-22 h adaptation in a cage on the sea bed. Tidalcurrents were recorded continuously at two depths and two locations in the vicinity oftracking of 15 eels, and movements of eels through the water could be calculated by vectorsubtraction. The four yellow eels generally moved shoreward, while silver eels exhibitedshoreward, offshoreward, or along-tide movements. Two silver eels used selective tidalstream transport to move northward, as evidenced by the close match between over-grounddistances and calculated tidal excursions on transporting tides, and by the higherpositions in the water column on transporting tides. When in midwater, most eels exhibitedperiods of well oriented directional swimming, showing only modest variation in thedirectional component of calculated 15-min vectors of swimming through the water. At othertimes, directions of 15-min vectors were widely scattered, and the eels drifted with thetide. One of the eels using selective tidal stream transport swam in an apparentlyunoriented manner on four successive transporting tides, but another added well orientedswimming to transport by the tide. Eels in midwater swam at modest mean speeds of 35-58 cms^-1 (0.45-0.75 L^-1). Eels were quite mobile vertically both on timescales of hours and of minutes. One clearly showed diel vertical migration, while twoothers showed the reverse pattern. Eels using selective tidal stream transport timed theirvertical movements only imprecisely to times of tidal stream reversal. All 18 eels madecontact with the sea bottom (ignoring the release at the bottom) and came near the seasurface during the tracks. Some eels spent most of the time near the surface with frequentdescents. Only a few spent much time on or near the bottom, and they made frequentascents. 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 interpretationof observed movements relative to natural behaviour difficult. However, oriented swimmingin midwater is consistent with previous reports in the literature of homing of displacedeels. The use of selective tidal stream transport has not been observed in other studiesof adults of the European eel but has been reported in studies of estuarine movements ofAmerican eels (Anguilla rostrata).

Reference:

J.D. McCleave and G.P. Arnold, 1999. Movements of yellow- and silver-phase eels (Anguillaanguilla (L.)) tracked in the western North Sea. ICES Journal of Marine Science, 56(4):510-536.