How to protect fish in the Tidal Thames

Joanna Barker

Following ZSL’s two-year Smelt Conservation Project, Joanna Barker describes what we have found out about the cucumber-smelling European smelt (Osmerus eperlanus) and how we are using this information to better conserve fish in the Tidal Thames.

I will always remember 2 April 2015 – this is the first time that we caught smelt fry (juvenile European smelt) in the Tidal Thames as part of the Smelt Conservation Project funded by the Heritage Lottery Fund.

Measuring at just 4mm in length, these tiny fish had just hatched from their eggs and started the long journey to adulthood. Following this discovery, we continued to survey for smelt over the next two years; battling hailstorms, cold March mornings and the navigation challenges of working on the Tideway.

Thames fish
Completing ichthyoplankton surveys close to Wandsworth Bridge to find smelt fry and eggs

Why study smelt?

The European smelt is a small predatory fish that inhabits cold-water estuaries. Once common in the UK, it has suffered significant declines since the early 19th century due to water pollution, over exploitation and destructive river engineering.

Improvements to water quality in the latter half of the 20th century have allowed smelt to return to 36 water courses in England including the Tidal Thames. It is protected under a variety of regulation and can act as an indicator for good water quality due to its sensitivity to pollution.

Smelt egg
Smelt egg in “pre hatching” stage sampled at Wandsworth Bridge

What did we find out?

During the two-year project we caught a total of 455 smelt fry and 28 smelt eggs, using two survey methods: ichthyoplankton netting (towing a micromesh plankton net just below the water surface to catch very small smelt fry) and seine netting (encircling an area of water by the foreshore with a sheet of net to catch all the fish in the enclosed area).

These data have been used to narrow down the most likely smelt spawning location to a 600m stretch of the Thames by Wandsworth Bridge. We have also shown that smelt are likely to spawn over an elongated period of 5 weeks from the beginning of March and provided information on smelt growth rates in the region.

Thames fish
Juvenile smelt caught in Chiswick during a seine netting survey

How are we using this information to conserve fish?

Over the course of the project, we realised that fish were often under-represented in local strategy documents and in some Biodiversity Action Plans. One reason for this could be due to the complexity of estuarine fish ecology, which is difficult to simplify for the non-technical user.

To fill this gap, we produced a Guidance Document  for Developers, Planners, Biodiversity of Environmental Officers in Local Government and Ecological Consultants to provide a single point of reference for information relating to fish conservation in the Tidal Thames.

This document was developed through feedback from multidisciplinary stakeholders and signposts the reader to relevant documents that provide more detail. We plan for it to be used to throughout the planning process to better conserve fish in the region. Download a copy here.

Find out more about our Thames fish conservation work


Select a blog

Careers at ZSL

Our people are our greatest asset and we realise our vision for a world where wildlife thrives through their ideas, skills and passion. An inspired, informed and empowered community of people work, study and volunteer together at ZSL.

Nature at the heart of global decision making

At ZSL, a key area of our work is the employment of Nature-based Solutions – an approach which both adapt to and mitigates the impacts of climate change. These Solutions, which include habitat protection and restoration, are low-cost yet high-impact, and provide multiple benefits to people and wildlife. We ensure that biodiversity recovery is at the heart of nature-based solutions. 

ZSL London Zoo

A blog for lovers of ZSL London Zoo, bringing you extraordinary animal facts and exclusive access to the world's oldest scientific zoo.

ZSL Whipsnade Zoo

Do you love wildlife? Discover more about our amazing animals at the UK's biggest zoo!


We're working around the world to conserve animals and their habitats, find out more about our latest achievements.


From the field to the lab, catch up with the scientists on the cutting edge of conservation biology at ZSL’s Institute of Zoology.


A day in Discovery and Learning at ZSL is never dull! The team tell us all about the exciting sessions for school children, as well as work further afield.

Artefact of the month

Every month, one of the pieces held in ZSL’s Library and at ZSL Whipsnade Zoo will feature here as Artefact of the Month.

Wild About

Read testimonials from our Members and extracts from ZSL's award winning members' magazine, Wild About.

Asia Conservation Programme

ZSL works across Asia, from the famous national parks of Nepal to marine protected areas in the Philippines. Read the latest updates on our conservation.

Remote Sensing in Ecology and Conservation

An Open Access journal for research at the interface of remote sensing, ecology and conservation.