From stable food and currency in the Middle Ages to our fastest declining freshwater fish
Eels have got to be one of the most fascinating and enigmatic creatures in our rivers. Their mysterious migrations, complex life stages and close links with both rivers and the open ocean make them a vital, but vulnerable, part of our freshwater and coastal ecosystems.
The European Eel has declined by around 90% since the 1980s and is one of only a few UK species considered to be ‘critically endangered’. It is disappearing faster from our rivers than any other fish despite being considered one of the toughest and most resilient of fish species.
But what do we actually know about eels - apart from the fact that they can spend decades in our rivers and waterways before they migrate back to the Sargasso Sea, where they once hatched and where they finally go to spawn and die?
Eels in history and culture
Eels were once so abundant in Europe, especially in the UK, that they were often used as a form of rent or currency in the Middle Ages. Records from monasteries and manorial rolls show tenants paying their dues in “sticks” of eels (a bundle of 25 fish). In fact, the Domesday Book (1086) contains references to eel rents being paid to landowners. Eels were even used as gifts to monarchs. As late as 2015, the city of Gloucester revived an old tradition and presented the Queen with 1,500 eels as a ceremonial rent payment dating back centuries.
Eels were also a staple food, particularly during Lent and other fast days, since they were considered “fish” and acceptable to eat when meat was forbidden. The River Thames was once full of eels, and from the 18th century onwards, “jellied eels” became a famous working-class dish in East London which is still available today in traditional pie and mash shops.
Because of their strange migrations and the fact that no one saw them breed, eels inspired myths for centuries. Often associated with transformation and magic, eels were believed to possess healing powers and were sometimes used in potions. Eels ability to survive out of water for a surprising amount of time and move between water and land symbolises adaptability and transformation. Even Aristotle believed they arose spontaneously from mud!
Eel life cycle
For a long time the lives of eels were covered in a veil of mystery. In many ways their life cycle is similar to that of migrating salmon except they reverse the process, living most of their life in freshwater but migrate to the ocean to breed. In the wild eels generally live for around 15-20 years, but some might live up to 80 years. The oldest recorded wild eel was 85 years old!
Adult eels never return from breeding in the ocean, this is a trip they make only once in the lifetime, dying at sea with only their offspring making the return journey back to freshwater rivers. Young eels start as tiny, transparent larvae which drift across the Atlantic towards Europe. As larvae approach coasts they metamorphose into glass eels, the juvenile stage that enters estuaries and rivers. These juveniles move upstream and grow into yellow eels, which can spend many years feeding in rivers, lakes and wetlands. As mature adults they undergo physiological changes, turn into silver eels and migrate back to the Sargasso Sea to spawn.
It is only in recent years details of this migration are becoming clearer. Eels are mainly nocturnal and secretive, feeding on a wide range of prey and using complex river-riverine corridors and bank-side structure to move and hide during the day. Their long freshwater residency and their oceanic breeding make them particularly sensitive to pressure on both freshwater and marine environments.
Current population status
The European Eel is in a critical state. With population estimates having declined by around 90% since 1980s and its one of only a few UK species considered to be ‘critically endangered’ on the IUCN Red List. Eels are disappearing faster from our rivers than any other fish despite being considered one of the toughest and most resilient of fish species. Long-term monitoring shows drastic declines both in recruitment (the numbers of young glass eels reaching European waters) and in many adult populations, and recent monitoring continue to show low larval and juvenile numbers, which make recovering the population uncertain.
There are many interacting threats to eel populations which reduce spawning escapement, cause poorer ocean survival, blocked river access and local mortality:
- Barriers and infrastructure - Dams, weirs, hydroelectric turbines and pumping stations block upstream migration and kill or injure migrants on downstream escapement. These impacts can dramatically reduce the number of eels that reach feeding grounds or sea to spawn.
- Overfishing and illegal trade - Glass eel and elver fisheries, legal and illegal, put pressure on already-low recruitment. According to a UK government source elvers (young eels) can fetch up to £4,000 per kilo on the black market. Fishing for eels is now largely illegal and yet suitcases full of live eels frequently leave the country for food trade in Asia, sometimes intercepted by customs and border control.
- Habitat loss and degradation - Drainage, loss of wetlands, and degraded waterways reduce feeding and shelter opportunities for juvenile and adult eels. Water pollution is a significant problem for eels due to their long life making them vulnerable to build-up of chemicals in their bodies, and eels have been found to contain heavy metals and other toxins.
- Diseases and parasites - The introduced swim bladder parasite Angullicola crassus and other pathogens damage fish health. They are likely to be reducing migration success and survival by hampering their ability to swim and reach its spawning grounds.
- Climate and ocean change – Changes in ocean currents, temperature and productivity may reduce larval survival and alter migration routes – factors that are still poorly understood but worrying given the declines observed.
Eel conservation work
The Game & Wildlife Conservation Trust’s (GWCT) fisheries department have been part of a project to monitor eels on the River Frome in Dorset for the past two years. In collaboration with Bournemouth University, the Environment Agency and the Centre for Environment, Fisheries and Aquaculture Science (Cefas) they have trapped and tagged with PIT or acoustic tags both juvenile eels living in the river and mature silver eels moving down river to start their migration back to their spawning ground.
Trapping has taken place in spring and autumn under strict welfare and licensing conditions. Tagged eels can be tracked using 35 acoustic receivers along the River Frome and in Poole Harbour, with additional receivers further out in the Channel. These devices will help researchers gather information on migration routes, habitat use, movement patterns and survival rates of tagged fish in different environments.
Eel research is a new focus for GWCT, which has monitored salmon on the Frome for 50 years. Despite their resilience and cultural significance, eels remain poorly understood and face severe declines. Long-term it is hoped that the data collected will solve some of the ‘mystery’ of the eel life cycle and support conservation of this endangered fish.
This work will also help the GWCT and partners to mediate between fishers, land managers, communities and policy makers to understand best practice and design locally appropriate and effective conservation measures. Recovering eel populations is not only about one species: eels are mobile ecosystem engineers and an important food web link between sea and rivers. Their decline signals problems across freshwater and marine systems that also effect other wildlife and the services rivers provide to people. Coordinated science, pragmatic on-the-ground actions and international cooperation are essential if we are to give eels a realistic chance of recovery.