It started with an unwanted pet. There's now thousands invading a valley

Thousands of invasive crayfish have been trapped at Chadwick Lakes as authorities step up efforts to control a species that is damaging the valley’s fragile ecosystem.

Crayfish at Wied il-Qlejgħa, as it is locally known, have been “extremely problematic” since they were first identified almost 10 years ago, according to the Energy and Water Agency that manages the site.

Nature Trust together with the Environment and Resources Authority and the Energy and Water Agency are trying to catch as many crayfish as they can and control population numbers.

Nature Trust, which does the brunt of the work of catching the fish, uses minnow traps, a type of cage that allows the crayfish to get in but not out.

Thousands have been caught since the initiative began but the authorities are uncertain how many are left.

Crayfish, which are indigenous to southern America and northern Mexico, are considered an invasive alien species in the pristine Rabat valley. The red swamp crayfish (Procambarus clarkii) was first identified in Malta in 2017 by members of the University of Malta’s Conservation Biology Research Group.

Unlike some alien species that do not badly impact the environment they settle in, the crayfish is an invasive species that has a negative effect on species that are indigenous to the area and can disrupt entire native ecosystems.

ERA officer Jurgen Gatt told Times of Malta that crayfish are negatively impacting the population of indigenous species as they eat frog tadpoles, dragonfly eggs and even vegetation.

“Because crayfish eat frog tadpoles, the population of frogs – an indigenous and protected species in Malta – is declining,” Gatt said.

Dragonflies and beetles, as well as their larvae, also fall prey to the crayfish. Their burrowing behaviour can also severely damage watercourse banks.

A fast-growing species, crayfish can lay between 100 and 500 eggs, and most of them hatch. 

And with no predators to naturally control their numbers, crayfish are free to reproduce, unless the authorities step in.

“In habitats the crayfish is native to, there are predators that control its numbers, but here there are none, so the population explodes if it is not controlled by us,” the ERA officer said.

Thousands caught

With thousands already caught, Gatt said authorities do not know how many crayfish remain in the area, but there are encouraging indications that the numbers are getting smaller, as well as their size.

“We are seeing the average size of each crayfish getting smaller,” Gatt said.

In general, crayfish grow up to 12 centimetres, with juveniles being around five centimetres. The fact that the crayfish are smaller means they are getting caught at a younger age, he explained.

“We might not be able to get rid of the species completely, but we are doing our best to control it.”

The invasive species are usually found in water or muddy areas, and in Malta’s dry summers, crayfish dig holes along the banks for shelter and stay there.

Gatt believes the crayfish at Chadwick Lakes were originally unwanted pets.

“Most likely, these animals were brought in as pets. When they became unwanted, they were let go here, thinking it is something good.”