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The UK is home to a quarter of the world's population, but numbers are declining massively, and global extinction is a frightening but real possibility.

Over the last 20-30 years, river pollution and habitat loss have taken their toll on our only native crayfish. The tributaries of the Usk and the Wye are real enclaves for these lobster-like creatures, which grow up to 10cm long.

However, even these remaining refuges are under threat now from an alien invader. American signal crayfish were brought over to UK in the 1970s to be farmed for the food industry. They escaped from captivity and spread prolifically in our rivers, streams and ponds, leaving death and destruction in their wake.

The American species virtually wiped out the native crayfish in the south and east of England, and in the last few years, there has been increasing concern about their potential impact in Wales.

The alien species is on the increase here, so CCW and the Wye and Usk Foundation have embarked on a trapping programme to try and stop it completely taking over our river systems. In 2007 they eliminated 30,000 signal crayfish from the Wye catchment, but only time will tell whether these aggressive invaders are reproducing as fast as they're being removed.