An experimental field study on the migratory behaviours of glass eel (Anguilla anguilla) at the interface of fresh and salt waters, with implications to the management and improvement of glass eel migration

Glass eels are known to reach the shores of Holland by using ocean currents and selective tidal transport (DM ¿ Downstream Movements). Most of the fish ladders that are installed at the interface of salt and fresh waters, however, are designed for fish that are attracted by freshwater currents and that actively move upstream (Upstream Migratory (UM) behaviours). From this, we questioned the efficiency of these fish ladders for glass eel immigration. We tested our hypothesis of UM versus DM behaviours by comparing the catch of glass eel in two experimental fish ladders: a siphon and an eel trap, appealing to DM and UM behaviours respectively. These traps were installed at two sluice complexes in Holland and operated at night during the glass eel season of 2005. These experiments were supplemented by direct scuba observations. The results showed that most glass eels gained access to inland waters by DM instead of UM behaviours. At low tide, none of the glass eel were observed to reach fresh waters as the water velocities in the cracks of the sluice doors were too high for upstream migration. At high tide, many glass eels were observed to reach fresh waters by DM-behaviours and small cracks in the sluice doors. These results suggested that DM-behaviours are important to glass eel migration at the interface of fresh and salt waters. Access to inland waters may be improved by providing small openings in or around sluice doors that allow for DM behaviours at night