WET Hogsmill

Some of the UK’s favourite and most iconic river wildlife species have been in severe decline for decades. A project launched in September 2023 on the Hogsmill River in south west London, named the WET Hogsmill project, will see the reintroduction of water voles on the river, while creating new habitats for both European eel and native (brown/sea) trout.

Run by the South East Rivers Trust, this is one of 63 nationwide funded by £14.5 million in Natural England Species Recovery Grants.

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Bringing back the water vole

The Hogsmill river is the first tributary of the non-tidal Thames and is a rare chalk stream, one of just 220 in the world.

Chalk streams are listed as a UK Biodiversity Action Plan Priority habitat. Despite its rarity and importance, the Hogsmill has suffered from a wide range of pressures, leading to decline and loss of priority habitats and species over the last century and beyond.

Water vole numbers have declined sharply since the end of the 20th Century; they are the UK’s fastest declining mammal having suffered a 97% decrease in populations.

Once ubiquitous and found in their millions, they are now considered to be on the brink of extinction, owing to loss of habitat and the introduction of invasive American mink. Water voles were once prolific on the Hogsmill but became locally extinct in 2017.

The South East Rivers Trust (SERT) will be working with Citizen Zoo, a conservation charity, to release 200 water voles across two sites. This will supplement 101 water voles previously released by Citizen Zoo in 2022, helping to increase the genetic diversity of the population on the Hogsmill.

Water voles have been in huge decline nationwide in the past century

Reviving the European eel and trout

European eel are also critically endangered with levels declining by 90-98% from historic figures. Eel migrate up rivers during their life span and recent surveys on the upper Hogsmill recorded just one eel in 2016 and three in 2022.

Creation of suitable habitats is a proven technique to support species recovery. The WET Hogsmill project will support the creation of a large wetland and backwater while also creating a more complex instream habitat which is favoured by eel.

Trout have been extinct on the Hogsmill since the 1900s as they have been unable to recolonise it owing to 19 predominantly obsolete weirs barring their passage.

These weirs have proven to be significant barriers to fish passage, preventing them from reaching critical spawning grounds in the river’s headwaters.

Over the past 11 years, SERT has made 18 of these weirs passable for fish and other species by either removing them completely or installing technical fish passes or easements. In 2024 the final remaining weir will be made passable for multiple fish species including trout and eel.

By spring 2025 eel and native trout will once again be able to access and migrate throughout the whole river for the first time in more than 200 years.

A silver eel, part of the life cycle of the European eel

Habitat restoration and education

The WET Hogsmill project will involve a range of river habitat restoration measures to support the sustainability of these fish populations, including introducing river gravel to the headwaters which is essential for successful trout spawning. There will be a variety of local volunteering opportunities associated with the river habitat works.

SERT will also provide a wide range of complementary community education and engagement activities which will be undertaken with local school children, members of the public and local businesses. These include installation of an interactive nature trail, classroom and outdoor education sessions, and lunch and learn sessions with local businesses to encourage both individual and corporate social responsibility.

The Natural England Species Recovery Grant has awarded £393,000 to SERT for this work, starting in August 2023 and ending in March 2025.

Pupils charting wildlife during an education session © South East Rivers Trust