Seahorses, the distinctive horse-headed small fish typically found in tropical seas and on coral reefs, are thought to be breeding in the River Thames in London, the Environment Agency announced yesterday.
A juvenile short-snouted seahorse, Hippocampus hippocampus, was recently found in the river at Greenwich and may be evidence of a breeding colony, the agency said.
Growing up to 15cm (six inches) long, the species is more commonly found in the waters of the Mediterranean and Canary Islands, although it is sometimes encountered around the British coasts, especially in the south and in the Thames Estuary.
The last sighting of a seahorse in the Thames was at Dagenham in 2008 – much further down river than the one caught at Greenwich, which is only five miles from Westminster.
"The seahorse we found was only five centimetres long, a juvenile, suggesting that they may be breeding nearby," said Emma Barton, Environment Agency Fisheries Officer.
"This is a really good sign that seahorse populations are not only increasing, but also spreading to locations where they haven't been seen before. We routinely survey the Thames at this time of year and this is a really exciting discovery.
"We hope that further improvements to water quality and habitat in the Thames will encourage more of these rare species to take up residence in the river."
The species adds to the growing numbers of wildlife returning to the Thames, which a survey in 1958 showed was "biologically dead", with no viable fish populations from Kew in the west to Greenwich in the East.
But London's river has undergone a dramatic transformation, beginning with the secondary treatment of sewage in 1964, and it now supports more than 125 fish species such as eels, pike, sea bass, flounder and roach.
Last year the Thames was awarded the International Theiss River prize which celebrates outstanding achievement in river management and restoration.
Two of the world's 35 seahorse species have been found in British waters, the other being the long- snouted seahorse, Hippocampus guttulatus.
Most of the remainder are found in tropical seas, and many of them are endangered, because they live in the most vulnerable of marine habitats, the coastal environment – coral reefs, estuaries, mangrove swamps and seagrass beds – which are all being hard hit by pollution and coastal development.
They are also threatened by over-fishing, being targeted for use in traditional Asian medicine, as live pets and for the souvenir trade. Every year an estimated 30 million seahorses are traded by between 70 and 80 countries.
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