Updated: Aug 3
You might be surprised to learn that there are sharks in British waters.. But it is true! In fact, the UK boasts as many as 40 different species, depending on the time of year. But even more startling still is the recent discovery that there are now sharks in the River Thames. Yes, you read that right - there are indeed sharks in the Thames! Conservation efforts and improving water quality has allowed no less than three different species of sharks to return to the Thames, which they once called home. This is incredible news, as it suggests there has been some ecosystem recovery in the river and may mean that the UK can contribute to the conservation of some seriously threatened species of sharks!
The Thames was Once Thought to be Biologically Dead
Bisecting England from its source not far from the Welsh border in the west, The Thames stretches almost 350km, passing through Oxford, Reading and London, before emptying into the North Sea at the east coast (ZSL, 2021).
Thanks to centuries of development and urbanisation, the Thames has been subjected to significant pollution. In the late 1800s, the waters became a significant public health risk, as slaughterhouse runoff and sewage flowed so freely into the river (ZSL, 2021).
In 1957 the tidal region of the Thames was declared 'biologically dead'. Flanked by salt marshes and tidal flats, where the water depth fluctuates with the influx and efflux of tidal salt water, this area was once rich with birds and aquatic life. But the water quality had become so degraded that the area was thought to be 'nearly devoid of life' (ZSL, 2021).
Three Different Species of Sharks Live in the Thames
However, in recent decades, the ecosystems in the River Thames have made a remarkable recovery! Thanks so improvement in sewage treatment and regular surveying of water quality, today the Thames as actually thought to be one of the cleanest rivers of any urban area in the world! (ZSL, 2021).
The tidal region now boasts more extensive salt marsh habitats, with rich biodiversity - home to seals and many different wading birds. In the water, you can now find seahorses, and 115 different species of fish, including eels and three different species of sharks! (ZSL, 2021).
"Today, the river is a thriving ecosystem home to myriad wildlife as diverse as London itself"
- The Zoological Society London, 2021
Surveys have shown that the tidal Thames is now home to spiny dogfish (Squalus acanthias), starry smoothhounds (Mustelus asterias) and tope sharks (Galeorhinus galeus) (ZSL, 2021).
These are temperate water species that generally favour shallow, coastal habitats. All three are relatively small sharks; the smoothhound reaching around 1.4 metres in total length (TL), the dogfish reaching around 1.5 m TL and tope shark up to 2 m TL. None of these species is a threat to human beings; eating mostly small fish, shellfish and invertebrates (Compagno, 1984).
Conditions in the Thames are Improving!
So what has changed in the Thames to allow the sharks to come back? Surveys of the habitat and water quality have shown that dissolved oxygen and nutrient levels are improving in the tidal Thames (ZSL, 2021).
When sewage is released into waterways or fertilisers run off from neighbouring farmland, the added nutrients cause a shift in the aquatic ecosystem. Phospohorus and nitrates stimulate algae and other plant life to grow at a higher rate; drawing oxygen out of the water, and depleting dissolved oxygen levels to the point that aquatic life, like fish, cannot survive in the area. This is known as "eutrophication".
Thanks to improvements in waste management (especially in London), sewage is no longer expelled directly into the Thames, and eutrophication has been dramatically reduced. However, conditions in the Thames are still not perfect. Despite advances, sewage effluent is still contributing to nitrate levels in the Thames - which will continue to cause eutrophication and may threaten aquatic life (ZSL, 2021).
Climate Change Might be the Next Big Threat to the Thames Ecosystem
There are also serious concerns that impending climate change might once again upset the delicate balance in the Thames.
Whilst the water levels naturally fluctuate with the tides in the Thames estuary, on average, water levels are rising by around 4.26mm per year in some of the tidal areas thanks to anthropogenic climate change (ZSL, 2021).
This may not seem like much, but even a small increase could flood wetlands and render these critical habitats useless to the species that rely on them. Whatsmore, we cannot predict how extensive this habitat loss might be because it is difficult to calculate how far the water will continue to rise in the future. This is not only a threat to plants and animals living around the Thames, but also to the city of London itself (ZSL, 2021).
Increasing water temperatures as a result of climate change are also a serious concern. Water temperatures are rising by around 0.2 ⁰C each year in the Thames. Once again, this might not seem like a lot, but many aquatic species are very sensitive to temperature and can experience thermal stress if waters become to cold or too hot. This can affect how some animals grow and breed, and can even render the habitat completely inhospitable to some species (Knip et al, 2010; Schlaff et al, 2014; ZSL, 2021).
Environmental temperatures have been shown to be one of the most important factors determining the distribution of sharks throughout their natural range. If water temperatures continue to increase, it is very possible that the area will become intolerable and we will once again lose the sharks which have only just returned to the River Thames (Knip et al, 2010; Schlaff et al, 2014; ZSL, 2021).
The Thames is Now Home to Endangered Sharks
But enough of all the doom and gloom! We finally have some good news regarding our environment, and I for one think that should be celebrated! It is wonderful that we have sharks back in the Thames and the species that have chosen to return make these findings especially exciting... Because the sharks that have now started to use the Thames again are declining in the wild and at risk of extinction.
The spiny dogfish is listed as Vulnerable, after precipitous declines have left their populations severely fragmented. Dogfish have been extensively overfished in the past, for their meat and fins. In fact, if you have enjoyed a fish and chip dinner in the UK, there is a real chance that you have eaten spiny dogfish, as they are often inaccurately named when landed in fisheries (Hobbs et al, 2019).
Tope sharks have also suffered disastrous declines. Tope were historically fished for their liver oils, which are rich in vitamin A. More recently they have been targeted for their fins, which are valuable for shark fin soup. Now listed as Critically Endangered by the IUCN, it is thought that tope populations have declined by at least 70% worldwide. They are some of the most threatened sharks in the world!
Whilst it is, of course, vital that we manage fisheries, increasing the available habitat is also a critical facet to the conservation of endangered sharks. The regeneration of the Thames means that more habitat is now available to tope and spiny dogfish... Hopefully, the Thames can now be a stronghold for these threatened species, and can play some part in supporting them to replenish their struggling populations.
If you fish in the Thames and catch one of these sharks, please ensure you release them safely as soon as possible. If you would like to contribute to science and help us to keep monitoring the sharks of the Thames, check any sharks you catch for a yellow tag on their fin. You can record the details of the tag and contribute what you have found through the Zoological Society website!
Compagno LJV (1984). Sharks of the world. An annotated and illustrated catalogue of shark species known to date. FAO species catalogue, 125:4. Access online.
Hobbs CAD, Potts BWA, Walsh MB, Usher J & Griffiths AM (2019). Using DNA barcoding to investigate patterns of species utilisation in UK shark products reveals threatened species on sale. Scientific Reports, 9:1028. Access online.
Knip DM, Heupel MR & Simpfendorfer CA (2010). Sharks in nearshore environments: models, importance, and consequences. Marine Ecology Press Series, 402:1-11. Access online.
Schlaff AM, Heupel MR & Simpfendorfer CA (2014). Influence of environmental factors on shark and ray movement, behaviour and habitat use: a review. Reviews in Fish Biology and Fisheries, 24, 1089–1103. Access online.
ZSL (Zoological Society London) (2021). The State of the Thames 2021 Report: Environmental trends of the Tidal Thames. Access online.