ZSL studies Thames eel habitats in decline

The European eel (Anguilla anguilla) and European eel fisheries are in major decline, with glass eel recruitment to Europe believed to have declined up to 99 per cent towards the edge of its range. Reasons for the decline are likely to include overfishing, barriers to migration, habitat loss, pollution, infestation by the Japanese eel parasite (Anguillicola crassus, the swim bladder nematode) and changes to ocean currents as a result of global warming.

Decline in eel fisheries cause concern

The eel fishery (elvers) is the most valuable commercial inland fishery in England. It is also the largest inland fishery in Europe, providing income for 25,000 fishers. Concern that the current fishery is not sustainable was raised from the International Council for the Exploration of the Sea (ICES) to the European Commission. The Environment Agency (EA) acted on this concern by putting together a National Eel Management Strategy in 2001. The EA is currently evaluating the state-of-play and future direction of this strategy.
Conservation Biologist, Renata Kowalik, taking water quality data at one of the sample sites

There are many unknowns with regard to eel population figures, life strategies and eel migratory and spawning behaviours. A DEFRA-funded project is being carried out by King’s College London titled ‘Development of Biological Reference Points for the Management of the European Eel’ in an attempt to establish baseline data on population densities, growth rates, glass eel recruitment and silver eel output. (Only one detailed long-term European glass eel recruitment study has been conducted. This consists of data collected following a dam erected in the 1930s in Den Oever, Netherlands).

Restricted access to potential rearing areas is thought to be a significant factor in the eel decline, and there is no doubt that man-made obstructions to migration and dispersion are limiting eel stocks in many parts of the UK and throughout Europe. In its Management Plan, the Environment Agency seeks to encourage and fund the construction of eel passes to restore access to areas where eels have been restricted by artificial barriers.

The eel is an important species to maintain a balanced riverine ecology. Eels are important prey species for species listed in the UK Biodiversity Action Plan, notably otters and bittern. It is important, therefore, to ensure that eel populations are sufficient to sustain predation by these species.

What happens next?

ZSL will determine which tributaries off the Thames were historically important as eel habitat. Many of these areas are no longer home to the species, due to the number of barrages, tidal flaps, weirs and dams that obstruct the eels’ upstream journey. ZSL will map the population distribution in the tributaries by using eel traps. This will enable potential restrictions to eel movements to be identified. Recommendations will be made to install or improve existing eel passes to minimise obstruction, as such the project seeks to re-establish eels into these areas.

The data output from this study will set conservation targets to ensure the sustainable management of the eel stocks. The project will increase knowledge and improve awareness of the declining eel populations. By assessing the distribution of the elvers within the tidal Thames catchment and identifying restrictions to the movement of the elvers, improvements steps are being taken to ensure that the historic and infamous Thames eels will remain a viable fishery and part of the ecology of the River Thames.