European Eels in the River Brent

European eel in the River Brent

The European eel is classed as Critically Endangered, although many members of the public would be surprised to hear this. The number of young eels arriving from the sea has taken a dramatic plunge since the 1980s, with reports suggesting that this recruitment of juveniles has declined by 90-99%. This elusive fish species starts its life in the Sargasso Sea in the western Atlantic, before it migrates over 5,000 km to the freshwater of the European continent. Once they arrive in Europe, they ascend into estuaries and begin their upstream migration into rivers, where they can live for more than 20 years. There are many gaps in our current knowledge on this migration, as well as their life-history as a whole, with parts of the eels’ life still being a mystery to us.

Gabby Needler is studying for a Masters degree in Ecology, Evolution and Conservation at Imperial College London and she has chosen to complete her five month research project surveying European eel populations throughout the River Brent in west London under the supervision of ZSL and the Environment Agency. Read her blog below to find out what this research involves! 


European eels in the River Brent

There are a number of factors that are thought to be causing the decline in European eel populations, including water pollutants, climate change, overfishing and obstructions to migration pathways. There are EU regulations in place to help preserve the species, and the UK has been following an Eel Management Plan since 2010, which has shown partial success already.

For my Masters degree I am carrying out a research project with ZSL, looking at conservation priorities of the European eel in the river Brent. I am monitoring eel numbers across twelve sites in Hounslow and Ealing to analyse the effects of water temperature, flow and river obstacles on eel migration dynamics.

Previous monitoring carried out by ZSL have shown the tidal section of the Brent River, where it joins the Thames, has a much higher number of elvers migrating through it than in other tributaries. However, survey records from further up the Brent catchment have found low mean densities, throwing into question how far up the river the elvers are getting.

There are four major obstructions in this stretch of river, of the likes of sluices and weirs, that can cause issues to the eels’ migration, potentially reducing the habitat available to them. This can be combated by installing eel passes on the obstruction, giving the eels a hand in travelling past them and continuing their migration upstream. Three of these four obstacles currently have eel passes installed, but the fourth and final obstacle, a weir in Greenford, currently has no structure to aid the eels over the concrete structure.

My research has involved assessing the population levels throughout the 5km stretch of the river before it flows into the Thames. Using habitat collectors, a structure similar to a mop-head attached to a float and rope, and a team of volunteers, we are monitoring the eel populations and their movement up the river. The eels use the habitat collectors as refuge from daylight, meaning that when they are gently pulled from the water during the day, the eels present can be counted and measured, allowing for population density estimates to be calculated at each site.

So far we have found juvenile eels at the sites lower in the river, nearer the tidal section, and we are eagerly anticipating the surge in young migrating elvers over the coming weeks.

The main hope for the outcome of this project is to investigate the influence that these obstructions are having on eel migration dynamics, as well as give an insight into how well the already installed eel passes are working and if an eel pass is needed on the last obstruction in the section. This will hopefully allow for further conservation priorities to be identified within the Brent catchment for this critically endangered species.

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