Outdoors: The Eel, once common species, is now formally classified as critically endangered

How eels reproduce has been the subject of great debate since the earliest of times. Aristotle thought eels generated spontaneously from mud or slime and Pliny the Elder suggested that eels rub their skin against rocks and the pieces that came off turned into young eels.

Isaak Walton in the classic work The Compleat Angler, published in 1653, postulated that eels were generated by "… the sun's heat or out of the putrefaction of the earth".

The reason for such confusion lay in the fact that nobody had ever discovered a mature eel laden with eggs or milt and it is only in the last 90 years or so that it has been deduced that eels in fact embark upon an epic 5,000km migration from our rivers to the Sargasso Sea off central America to spawn.

In one of the most incredible of biological feats, the tiny glass-like larvae then gradually drift back to Europe on the Gulf Stream before ascending our rivers as elvers.

But a new mystery engulfing the European eel is causing huge alarm among biologists - our eel population has crashed, perhaps by as much as 80 per cent, and scientists are not sure why. A once common species, it is now formally classified as critically endangered.

The issue is of major concern because eels play a fundamentally important role in our freshwater ecosystems, a vital source of food for otters, herons and other fish-eating water birds.

According to Professor Peter Maitland, one of Scotland's foremost experts on freshwater fish, the evidence in the UK is variable as to the extent of the decline.

In England the fall has been substantial, with old established eel fisheries on the Severn and elsewhere reporting catches only a fraction of those a couple of decades ago.

In Scotland the situation is less clear - partly because of a lack of information - and some rivers still appear to have reasonable stocks.

"There are several theories for the decline in stocks," says Prof Maitland.

"Climate change is suggested as one, but it is difficult to understand how this could be affecting what is actually a fairly hardy species unless it is in the marine environment where both adults migrating west to the Sargasso Sea and elvers coming back east have very long journeys.

"Overfishing may play a part, for the eel is a highly prized species and much sought after in parts of Europe and further afield. Some of the fisheries are intensive and catch a high proportion of the stock in individual water systems."

Eels are very tough creatures indeed and can even leave the water at times to make their way from one body of water to another, slithering across damp grass on dark moonless nights.

This is why eels can sometimes be found in landlocked ponds and ditches, where they survive in poorly oxygenated waters that other fish would find difficult to live in.Ironically, this robustness may too be playing a part in its downfall.

"Curiously, part of the reduction in eel numbers in the UK may actually be due to the reduction in organic pollution in most rivers in recent years," says Prof Maitland.

"Eels are more tolerant than most fish of low oxygen levels and were able to exploit the rich feeding conditions available in many semi-polluted lowland rivers with little competition or predation from other species. Cleaner rivers probably mean fewer eels."

So, what can be done? "The main crux of the problem is that we really don't understand the cause of such a widespread decline," states Prof Maitland.

"Data from monitoring is critical to any scheme of improvement. In some systems barriers to eel migration are a major impediment and these could be modified to aid passage or eels could be transported over them during the elver migration. Reduction of catches or closure of some fisheries would also help."

This article was first published in The Scotsman, 19 February, 2011