Mucking in for the East End Eels

Next week I’ll be beginning a 3 month stint as a voluntary eel surveyor for the Canal & River Trust in London. For many people the idea of pulling on a pair of waders and splashing about in a river probably seems an odd way to spend two evenings a week. Indeed the majority of people I’ve spoken to about this latest conservation endeavour of mine have met my enthusiasm with a confused (perhaps even concerned . . .) expression. In the paragraphs below I attempt to give some sort of reasoning as to why I think this is an interesting and worthwhile thing to do.


The European eel (Anguilla anguilla) is a smooth, slippery fish that closely resembles a snake. It is a species to which the adjectives ‘creepy’ and ‘slimy’ are commonly appended, and I’ve no trouble conceding that they are not the best looking of creatures. Their life cycle, however, is undeniably fascinating. Of all European freshwater fish, the life history of the European eel is perhaps the most interesting. The European eel is what is known as a catadromous species. This refers to the fact that they are born in saltwater, they then migrate into freshwater as juveniles where they mature into adults before migrating back into the ocean to spawn.

European eel (Anguilla anguilla). Photo credit: GerardM

European eel (Anguilla anguilla). Photo credit: GerardM

Within this catadromous life cycle, the eel goes through a number of different life stages in which the morphology of the eel changes dramatically. Spawning takes place in the winter and early spring, and it is believed that the Sargasso Sea, just off the coast of North America, is the spawning ground for all European eels. After spawning, the eggs hatch and young, leaf-shaped larvae, called leptocephalus, emerge.  These larvae are essentially planktonic in that they rely on the Gulf Stream to transport them to Europe. This journey to Europe is some 3000 miles and can take as long as three years. It is only when the larvae reach the European continental shelf that the cylindrical shape typically associated with eels is revealed. The larvae undergo a metamorphosis in which they are transformed and become near transparent. They are now termed glass eels. The glass eels manoeuvre their way into European freshwater habitats and pigmentation is initiated such that they change in colour and become darker, these pigmented glass eels are now known as elvers.

Elvers migrate upstream and occupy a wide variety of freshwater and estuarine habitats, where they predate invertebrates and fish. Elvers essentially become freshwater residents, as they remain in these freshwater habitats for more than 20 years. During this extended freshwater adult phase the term yellow eels is applied. Remarkably, yellow eels are known to cross overland to access alternative waterbodies. On a damp night, yellow eels can survive for several hours out of water. Yellow eels reach sexual maturity towards the end of the 20 or so years spent in freshwater. Their colouration changes to a silvery colour and they begin the migration back towards the Sargasso Sea on dark; typically occurring on moonless, stormy nights. During this time they are known as silver eels. Upon returning to the sea, the common eel lives in mud, crevices, and under stones and spawning takes place once more.

Given the extensive migratory life of the eel, it is perhaps unsurprising that they are a  very long-lived species; the maximum life span is some 85 years.

Life cycle of the European eel. Credit: Salvor Gissurardottir

Life cycle of the European eel. Credit: Salvor Gissurardottir

The epic migration of the eel across the Atlantic and through the freshwater systems of Europe is interrupted only when they come to physical obstructions, such as dams, weirs and lock gates. Mitigation involves the creation of specially made passes that help the eels circumvent the barriers. This summer sees the opening of an eel pass in the River Lea in Hackney, London. This pass aims to allow the species to make their way around the weir (a barrier across a river that aims to alter the flow of the water) at this site so that they can set up a home in East London. The Canal & River Trust – a charity that looks after British waterways (and whom I volunteer with) – is working alongside the renowned conservation NGO the Zoological Society of London (ZSL) to monitor eel numbers at this new eel pass. Ensuring eel passes, such as the one in place in Hackney, are working is vital in enabling these fascinating creatures to complete their extraordinary and complex life cycle.

But there is more to the situation than an inherent ecological interest. The European eel is classified as Critically Endangered on the IUCN Red List of threatened species. Over the past 40 years or so there has been an apparent disappearance of older eels. There has also been a 90% decline in the recruitment of the glass eel stage over entire full geographic distribution. There is no indication of population recovery.  A full explanation for the recruitment and population decline is not fully understood. There are certain factors however that are known to instigate eel decline. These factors involve disruption to migration routes (e.g. dams and weirs, but also loss of freshwater habitats). A particularly influential factor is heavy fishing at all stages of the European eel life cycle. The popularity of the eel in Asian cuisine brings huge demand, and glass eels are even farmed. It is recommended by Seafood Watch, a widely known sustainable seafood advisor, that consumers should avoid eating European eels.

In my opinion the European eel is one of the more interesting creatures to visit the UK, and it is certainly one of the most poorly understood. The threats are recognised but the relative magnitude of each factor is unknown. Unfortunately, the diversity of threats confronting the eel mean that unless conservation action is taken it may be too late to ever properly to get to know this fascinating creature.

Slimy and gross? I prefer weird and wonderful.

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Billy Mills

Billy Mills

A postgraduate student at the University of Oxford studying M.Sc. Biodiversity, Conservation, & Management.
Billy Mills

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