Adapting the banks of rivers, canals and ditches may prove a good tool to combat the American crayfish, a trial in Leiden has shown.

By making the banks flatter, the crayfish find it more difficult to dig holes and are more exposed, making it easier for some fish and birds to pick them off. Two years into the trial, far fewer crayfish have been found on the banks of the test location, a ditch in Leiden.

The crayfish – Procambarus clarkii, or red swamp crayfish – are thought to have travelled from their native United States in the ballast tanks of large freighters and are well adapted to local waterways. They are particularly common in Utrecht, Noord-Holland and Zuid-Holland.

First spotted in Dutch waters in 1985 and largely without any natural enemies, crayfish numbers are now thought to be running into billions. Their presence is said to be a threat to water flora and fauna – and they also damage river banks.

“In places with steep banks, American crayfish proliferate without being accessible to other animals in the food chain,” Leiden city ecologist Wouter Moerland told local broadcaster Omroep West. “If we don’t act we’ll end up to 15 adult crayfish per square metre without counting the little ones,” he said.

Catching the crayfish is no longer an option, he said, but by making the banks flatter more space is given to increase the number of animals which eat the crayfish, such as pike and grebes,” he said. “Nature-friendly banks have been found to harbour six to seven times fewer crayfish than banks where nothing has been done.”

The Leiden trial is similar to other nature-based initiatives to manage crayfish numbers. In Utrecht, predatory fish like catfish are being pitted against the crayfish and while and water-clearing plants like bladderwort are used to encourage species to return to grazed bare waters.

Government inaction

However, according to regional water boards, junior nature management minister Jean Rummenie (BBB) is not doing enough to support these efforts and they accuse him of ignoring their attempts to discuss the matter. The junior minister’s “contours of a plan” lack concrete measures, the water boards told the NRC.

In a letter to Rummenie earlier this week, the Hoogheemraadschap Delft said the ministry should compensate them for “tens of millions of euros” worth of damage to dykes and support them financially for the cost involved in combating the pest.

Time is of the essence, the Hoogheemraadschap said, because in Delfland the holes in the dykes are creating water level problems for farmers and untold ecological damage.