Guest post: Zoological Society of London

If you take anything from this blog, hopefully it will be that the Thames is no longer a dead and dirty river, but instead a living estuary that is teeming with hundreds, if not thousands of species supported by a thriving ecosystem.

Anna Cucknell is Marine & Freshwater Conservation Project Manager at the Zoological Society of London

The brown murk of London’s iconic river, caused by the high tidal flow scouring the muddy river bed, hides over a hundred species of fish that use the estuary to grow, spawn and migrate. Many of these species you would recognise from your dinner plate – tiny seabass, flounder and grey mullet are spawned or come into the river to grow in the Thames’ protected and food-rich environment, whilst the Critically Endangered European eel migrates up the estuary when tiny to grow in the upper Thames tributaries. Other species, such as the small, predatory smelt, which incidentally also smells distinctly of cucumber, may not be as common to most Londoners. This species is especially sensitive to pollution and is therefore monitored as an excellent ecological indicator of the health of the Thames – a ‘canary in the coalmine’ for water quality, if you like. Having left the Thames during the 1950s/60s, when the estuary was declared biologically dead due to its low oxygen content and pollution, of the 36 estuaries where smelt are now found around the coast of England, incredibly smelt are most abundant in the Thames.

Young Seabass on the Thames

A Smelt from the Thames

This array of fish species are supported by a plethora of small invertebrates like prawns and insect larvae, and in turn support many top predator populations including two species of seal, the harbour and the grey; Europe’s smallest whale, the harbour porpoise; as well as shark species such as smooth-hound and tope. The estuary is also an important habitat and overwintering ground to many species of bird, including the glorious kingfisher, the striking avocet and the noisy oystercatcher.

With all of this wildlife living in the Thames, it is understandable that when the concept of the Illuminated River project came to our attention here at ZSL, initially we had some concerns. Why? Although most people probably know us best as the organisation that oversees ZSL London Zoo, we’re also an international conservation charity that has been working to support wildlife in the Thames and far beyond for over a decade. And as part of this work, we’re all too aware of the damaging effects that light pollution can have on natural habitats.

Put simply, light levels dictate and affect many aspects of fish biology, from the secretion of several hormones that regulate growth and day-night-rhythms, to influencing reproduction. For example, it is well known that European eels – a Thames-dwelling species that is Critically Endangered according to the IUCN Red List, can be irritated by light pollution during their spawning migrations, whilst the perch’s circadian rhythms are affected by light pollution.

Given these potential impacts of light pollution on our fragile estuarine wildlife, ZSL was very pleased to learn that although Illuminated River would focus on the concept of creative lighting, actually it would have the overall effect of reducing current levels of light pollution infiltrating the Thames from London’s many bridges. Through a combination of modern technology, more focused lighting and planning around the wildlife’s biological rhythms, the happy irony is that the Illuminated River project should ultimately deliver a ‘brighter’ future of less light pollution impacting the Thames and its wildlife.

So when this exciting project is eventually launched, lighting up some of the city’s most beautiful and iconic bridges, don’t just use it as an excuse to admire the architecture. Let it also remind you of the huge variety of wildlife that has recolonised our once-dead waterway. And when the lights are dimmed on a bridge at certain times of year, remember that it’s just the artwork highlighting the spawning, shoaling or migrations of wildlife living invisibly in the murky waters below.