River ecology

Last updated: 26 November 2016

River ecology

The tidal River Thames is not as clean as you might think. Sewage from London's overloaded sewerage network is polluting the river, affecting the fish, invertebrates, birds and aquatic mammals that live in and around the Thames Tideway.

The tidal River Thames stretches 110km, between the upstream tidal limit at Teddington Lock in the west, to the open sea in the east.

Over the last 30 years, we have seen the dramatic clean-up of the River Thames, making it today an example of a recovering ecosystem which is of great ecological importance. The sewage from the combined sewer overflows (CSOs) is the last major source of pollution into the capital's river.

Tideway's job is to ensure that the excellent progress in cleaning up the river is not reversed.

The importance of the Thames' tidal passage

The Thames Tideway is a vital link and migration route for many aquatic species. Fish usually migrate because of diet or reproductive needs. It is vital that the ecological condition of the Tideway is maintained and improved to support these fish in their lifecycle. The tidal River Thames is a valuable nursery for many commercially fished species in the North Sea.

As many as 125 different species of fish have been documented in the River Thames.

Currently, around 10,000 tonnes of sewage related litter enters the tidal River Thames from CSOs every year. This will be reduced by 90% once the Tideway Tunnel is in operation in 2023.

Sewage contains ammonia, which is poisonous to fish, invertebrates and other aquatic life.

When sewage enters the river the bacteria which use the material as food take up the dissolved oxygen from the water. Fish need oxygen to survive.


Marine facts

Fish

The Thames Tideway is an estuary, meaning it contains water of varying salinity, allowing a variety of marine, estuarine and freshwater fish species to thrive. Salmon, sea trout, dace, smelt and eels migrate through and within the Tideway every year.

The Thames Tideway is important, because it can support a range of marine and fresh water fish through different stages of their life cycle, while estuaries are important for species that rely upon their sheltered waters for protection, making them ideal for spawning adults and as nursery areas for young fish.

Species such as barbel, bream, carp, roach and perch can largely be found in the upper and middle tidal reaches of the River Thames, while other species such as dab, tub gurnard and dover sole can be found downstream in the more saline sections of the estuary.

Many other fish species, such as bass, flounder, thick lipped mullet, sea trout and smelt also move up and down the Tideway to different habitats depending on their lifecycle stage. The short-snouted seahorse has also recently been discovered in the river.

The River Thames is a key nursery area for millions of bass and flounder, which are both very important commercial and recreational angling fish. There is also salmon, allis shad, twaite shad, river lamprey, sea lamprey, dover sole and smelt.

The River Thames as a commercial fishery

The Thames estuary is not just of ecological importance. It also supports a commercial fishery. Species, such as dover sole, thornback ray and bass, as well as shellfish, such as the edible cockle and whelk, are caught and sold.

There are also many well-known local London delicacies which are associated with the River Thames. In the 19th century Londoners dined on 'jellied eels' and 'whitebait suppers', and a thriving industry evolved with local pubs and eateries serving these dishes.

Birds

The Thames estuary is one of the most important sites for waterfowl in the UK, supporting over 155,000 wintering waders and wildfowl.

These populations include species such as avocet, ringed plover, gadwall and shoveler, and feed on invertebrates in the intertidal mudflats.

Other water birds that can be found along the Tideway, within London, include the cormorant, black-headed gull, and herring gull. The mute swan is a familiar sight on the river, as well as many non-native geese including Canada geese, Egyptian geese, and bar-headed geese, and also ducks such as the native mallard.

Invertebrates

The Thames Tideway is also home to many invertebrates, both within the river and on its banks. These include molluscs, such as the nationally rare depressed river mussel, which occurs close to the tidal limit at Teddington, as well as species that live near the water’s edge, such as the two-lipped door snail and the German hairy snail.

Crustaceans such as the common shrimp are also found in the Tideway. Worms, including the common ragworm and the tentacled lagoon worm can also be found. Exotic invaders, such as the Chinese Mitten crab which arrived in ships ballast water, have colonised the Tideway.

Aquatic mammals

Additionally, many aquatic mammals, such as the harbor porpoise, bottlenose dolphin, and various seal species, have all been seen within the Thames Tideway in recent years.

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