Jonathan Baxter on his job monitoring the fish populations of the River Thames

There are few sights as beautiful and serene as the River Thames snaking its way through Oxfordshire’s sumptuous countryside. But the tranquil scenes above the water’s surface are in stark contrast to the hustle and bustle below, as the river’s teeming fish population goes about its business.

The freshwater Thames is home to a hugely diverse community of fish species – from minnows growing to just 3cm in length, to huge pike and carp weighing 30lb or more.

How do I know this? Partly because I’m a keen angler, but mainly because as an environment monitoring officer for the Environment Agency I’m part of the team that carries out annual surveys of the river’s piscine inhabitants.

The surveys are done in batches and are carried out by the team based at our office in Crowmarsh Gifford. Recently we covered the stretch between Iffley Lock near the Rose Hill estate, in Oxford – where the river even now retains its Roman name of ‘Isis’ – to Benson Lock near Wallingford.

Iffley Lock and its downstream neighbour Sandford, were both originally built in the 1600s. Sandford also has the distinction of being the deepest lock on the non-tidal Thames, and as we found out when we had to drain the lock to refurbish it a few years ago, is inhabited by its own community of fish who have everything they need to live a life of contentment deep in the lock’s depths.

Near the next lock at Abingdon, is what is thought to be the oldest surviving lock chamber in Europe. It sits on the Swift Ditch just upstream of Abingdon, and was built around 1624.

These really are sumptuous settings for our surveys, and to be able to work in such surroundings is a privilege indeed, although the fish are so plentiful we have to work pretty much flat out to record them all. To give you an indication, at Abingdon alone we assessed no fewer than 800 fish.

The data we gather about these fish populations – the number found of each species, their size and age – gives us a really valuable insight into the overall ‘health’ of the river.

We use the data to assess how it’s affected by factors like changes in the chemical composition of the water, weather events like drought, floods, hot spells and cold snaps, and the range of habitat – or lack of it – in the river itself and along its margins.

Having carried out these surveys for more than 20 years, we’ve developed an extremely useful bank of knowledge to protect and improve water quality and all the plant and animal species that rely on it to. Much of this work, including the surveys themselves, is funded by the income from rod licences.

Its current state of wellbeing is a direct result of a strategic, catchment-based approach, considerable investment and more than a decade of hard work carried out through a massive programme of enhanced engagement and partnership. Consequently the overall picture for our rivers is the best its been for 20 years.

We use what we call a ‘boom boat’ to carry out our surveys. It’s not the most aesthetically pleasing piece of kit to be honest – something of a Heath Robinson – but it does the job very well.

It gets its name from the two arms or booms which extend from the front of it. Through these, we are able to pass an electric current into the water. This attracts and momentarily stuns fish in the area, enabling us to net them nice and easily from punts attached to side of the boat.

If you’re wondering by the way, the Thames is doing pretty well on the whole, and supports an abundance of bleak, dace, chub, bream, tench, pike, perch, barbel, gudgeon and carp.