European Eels Declining Because of Nematodes

Parasitic Disease Transferred from Pacific Cousin Destroying Fishery

Aug 27, 2008 John Blatchford

Eel life-cycle, fisheries and farming have been hit by a parasitic worm that was accidentally introduced.

The number of young eels returning to European estuaries has declined disastrously in recent years. This will have serious consequences for eel farms and fisheries.

Eel Life Cycle

The European Eel (Anguilla anguilla) spawns in the Sargasso Sea. When the eggs hatch the small larvae (called leptocephali) drift in the Gulf Stream towards Europe. This journey takes a number of years, and they arrive in estuaries where they change into ‘glass eels’ while they wait for the river water to warm up. They used to move up European rivers in vast numbers, crossing damp grass and getting into virtually every small pond and stream.
Safely ensconced in fresh water the eels mature over a number of years (usually ten or so) and can reach over a metre in length.
Fully mature adults return to the sea, and make their way back to the spawning grounds. This journey (over 4,000 miles) requires physiological changes (they are now called ‘silver eels’), and since they will not feed on the way must be completed using the reserves the adults laid down while they lived in fresh water.

Eel Fisheries

Eels were caught all over Europe as they returned in vast numbers to the estuaries. These small ‘glass eels’ or ‘elvers’ were a seasonal delicacy in many places (notably around the River Severn in England).
The rest of these small eels were either sent on to eel farms, where they were fattened up for human consumption, or released into European rivers for re-stocking purposes.
Since eels have never been bred successfully in captivity all eel farms depend on wild-caught stocks of young eels. Now that young eel numbers have dramatically reduced most of those caught go to eel farms and fewer are eaten directly or used to re-stock rivers.

Reasons for the Decline in Eel Numbers

A parasitic nematode worm (Anguillicola crassus) was accidentally introduced from Asia seems to be one of the main causes for the decline in European Eel numbers. The worm was originally a parasite of Japanese Eels (Anguilla japonica), but as with many movements of parasites to new hosts, its effects on the European Eel seem more serious. The worm attacks the swim bladder of the European Eel, and since this is essential for their return to the spawning ground it has dramatic effects.
The problem is that it takes over ten years for adult European Eels to grow up and return to the Sargasso Sea to spawn. This decline in young eels is extremely serious for the future of all eel farms and fisheries, and whatever the cause(s) urgent action is required.

(See also 'Thames Eel Fishery')

The copyright of the article European Eels Declining Because of Nematodes in Marine Biology & Oceanography is owned by John Blatchford. Permission to republish European Eels Declining Because of Nematodes in print or online must be granted by the author in writing.

Adult Eel