An aggressive species of crayfish which has been invading England's waterways is being tracked with radio transmitters in a bid to better understand them.
The Environment Agency said virile crayfish, which are non-native, prey on native wildlife and spread crayfish plague, a disease deadly to native white clawed crayfish.
The North American predators have recently been spotted in waterways in east London after first being found on the River Lee near Enfield in 2004.
They have since colonised more than 17km (10.6m) of the river and connected waterways, spreading into Hertfordshire.
The Environment Agency has fitted small radio transmitters on the backs of the unwelcome guests, with preliminary results showing that virile crayfish are moving upstream at a rate of 500 metres (1,640ft) per month.
This is substantially faster than their cousin, the signal crayfish, which is also non-native.
The UK's only native crayfish, the white clawed crayfish, was wiped out along the River Lee following the invasion of the signal crayfish in the 1980s and the associated spread of crayfish plague.
Adam Ellis, environmental monitoring officer at the Environment Agency, said: "Whilst rivers in England and Wales are cleaner than they have been for decades, there is still a lot to be done in order to return them to full health. This includes the control of invasive species like virile crayfish.
"By tracking the colonisation of the River Lee by virile crayfish, we will better understand how this species impacts the environment and our native wildlife.
"However, one of the most important ways to protect our wildlife is to stop the spread of non-native invasive species. We're appealing to the public not to release unwanted pets into the wild."
Anyone who catches a crayfish must follow strict guidelines on how to handle them in order to prevent their spread and the transmission of crayfish plague.
It is believed that virile crayfish arrived in the UK after an aquarium owner released them into an east London pond.
The rise of invasive species is a major challenge in meeting tough new EU targets on the ecology of rivers and lakes. They cost the UK economy an estimated £1.7 billion every year, according to the Environment Agency.