As with all fresh water species of eel the life cycle is dependant on a warm onshore tropical current. Where no such current exist, such as the West coast of South America, there are no eels.
It is generally accepted that the European eel spawns in the Sargasso sea and the leptocephali are carried across the Atlantic in the North Atlantic currents to Europe. The actual mechanism of the drift and the time span involved is a matter of considerable debate. The leptocephali metamorphose into glass eels on the continental shelf before migrating to the coastal areas. The Anguilla anguilla can be found in the most Northern extremes of Europe. It has been identified in the Canary Islands so the presumption is that the glass eels are swept southwards in the colder Canaries current but seem to be carried away from the African continental shelf. Our own investigation would indicate that the eel is not recognised between Agadir and Daker on the West coast of Africa.
It is likely that the eels will take some years to reach maturity in temperate waters. There are records indicating that some large mature females are more than 20 years old. ICES has inferred that the recovery plan for eels is based on three generations over 60 years. This may be the case in temperate climates but could be much shorter in eutrophic and/or warmer waters. It is generally accepted that the period in fresh water could be in the region of five to six years and perhaps shorter in a Mediterranean climate. (There is plenty of evidence from aquaculture that part of the glass eel population can reach weights of 400 gms in 8 months).The mature eels typically start their migration seaward in the autumn. Atypical weather patterns, low water flows etc have been reported to alter migration periods. The migration is possibly stimulated by increased water flows, reduction in temperature and dark nights. Disrupted 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. For the UK 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.
The identification of the anguillid species is difficult and morphological identification is open to misinterpretation. This will make the regulation of exports and imports by CITES quite difficult to mange. Genetic finger printing will open up a new opportunity to identify species. There are reports of non native species of glass eels being imported from America, Indonesia, Mozambique. There continues to be reports of Imports of native species from non approved approved countries such as North Africa. The imports represent a health risk to our resident wild and farmed population.
Glass eel counts. Anguilla anguilla – The count can be as low as 1800 pieces and as high as 4500 pieces per kilo. Generally the glass eels are larger at the start of the season say 2300 pieces and as the metamorphosis takes place they become smaller. A count of say 3300 pieces at the end of the season. The average count would be 3000 pcs per kilo. However there can be significant differences in the number of pieces per kilo between year classes.
Anguilla japonica and Anguilla rostrata – The number of pieces per kilo is considerably higher than Anguilla anguilla. The count being between 4500 and 6500 pcs per kilo.