At the onset of sexual maturation, European eels Anguilla anguilla exhibit high locomotor activity which may correspond to migratory restlessness. We measured
activity of captive eels and determined whether it correlated with
downstream runs of silver eels as well as changes in environmental factors.
Groups of eels at different stages of the silvering process (yellow to
silver stage) were tagged and placed in separate tanks supplied with either
river or tap water. Activity was measured by means of a flat-board antenna
placed vertically in the middle of the tank at the surface of the water.
Wild migrating silver eels were caught in the nearby river. Activity of eels
in the river water tanks increased 1 to 2 days before downstream migrating
eels were caught in the trap, and concurrently with a rise in turbidity and
a decrease in conductivity. Activity of eels in the tap water tank showed a
different pattern, which did not correspond to downstream runs. A peak in
activity corresponded to a drop in tap water pH. It is concluded that eels
do show periods of high locomotor activity at the onset of migration and
this could be used to predict downstream migration. Movements are triggered
by changes in water composition (as opposed to changes in discharge,
atmospheric pressure and lunar cycle) measured using turbidity and/or
conductivity as proxies. If eels are able to detect such small changes in
water conductivity (80 µS cm−1), they may use it to find their way
to the estuary.