marked reduction in temperature. It is thought that the mature eels return to the Sargasso sea to spawn using a route close to the western European coast line before being carried back to the Sargasso sea on a reciprocal current in the northern tropics. To date no workers have caught mature eels in the Atlantic Ocean on their migratory route to the Sargasso sea.
Migration of Glass Eels.
The glass eels are probably attracted to the estuaries of the major rivers by the presence of fresh water discharging into the sea. The fresh water can in some cases be detected in the surface water of the oceans hundreds of kilometres from the coastline. There is some evidence to suggest that there is a relationship between glass eel catches and river discharge volumes in the previous months.
Glass eels are very small and do not have sufficient biomass to undertake a successful active migration. The immigration of glass eels is under the control of the tidal movement of the water mass. The glass eels become active in the water column and are carried upstream on the rising tide. On the falling tide the glass eels become inactive and rest in the bottom of the river. This ratchet mechanism allows the glass eels to migrate inland on the tide, many kilometres a day. Low temperatures and high levels of freshwater (flood conditions) significantly reduce the efficiency of the immigration.
Active migration does not take place until temperatures are over 6°C. The periods of active migration are short and the distances travelled very small, it is also very rare during the day.
Species of Anguilla.
There are more than twenty different species of eels world wide.
The species for aquaculture are:-
A. japonica (one of the Asian species)
