Alien crayfish have invaded Malta’s valleys, study finds

Alien crayfish species released into the wild by “irresponsible individuals” have invaded several valleys across Malta and Gozo, a two-year study has found.

The study documented the occurrence of at least five crayfish species that are not native to Malta.

According to lead researcher Alan Deidun, a professor from the department of geosciences at the University of Malta, the scale of invasion “is cause for concern”.

The study was published in the Journal of Crustacean Biology and was conducted between 2016 and 2017.

Alien crayfish were found at Wied Qlejgħa, known as Chadwick Lakes

The crayfish species were documented at Fiddien, Wied Qlejgħa, Baħrija, Għajn Żejtuna and Wied l-Isperanza in Malta, and Wied il-Lunzjata in Gozo.

The invasive species was also recorded at a number of freshwater pools, ponds and reservoirs including Ta’ Sarraflu and Għajn il-Papri in Gozo and Pembroke respectively.

The five alien crayfish species are all popular within the pet industry or are reared commercially for human consumption.

Researchers documented the presence of the red swamp crayfish, the marmorkrebs, the Galician or narrow-clawed crayfish,the signal crayfish and the Australian redclaw.

A sixth alien species, the bamboo shrimp, was also documented in a reservoir in Pembroke.

Individuals of the common yabby were documented from the island of Sicily for the first time.

The red swamp crayfish, a native of North America, was also recorded in Maltese valleys and freshwater pools

Most of the exotic species are native to freshwater habitats in the US, with the exception of the Australian red claw and the yabby, native to Australia, and the bamboo shrimp, native to south-east Asia. 

“The scale of the invasion of Maltese freshwater ecosystems, already susceptible to a clutch of other anthropogenic impacts, including diversion of water for irrigation purposes and pollution with pesticides and fertilisers, is cause for concern, especially considering that some of the invaded valleys, notably the ones at Baħrija and at Xlendi (Wied il-Lunzjata) are amongst some of the last haunts for the endangered freshwater crab and the painted frog,” Deidun said.

He said the fact that the non-indigenous crayfish species were also recorded in man-made reservoirs and ponds is evidence that the species were intentionally released by irresponsible individuals.

“These people are unaware of the ecological disaster such releases could unleash. Some of the alien crayfish species, in fact, can withstand prolonged periods of draught, and exhibit prolific rates of reproduction, besides being opportunists when it comes to feeding,” Deidun said.

Marmorkrebs

He urged aquarists and people to desist from releasing such exotic species into the wild and encouraged those encountering them to report their occurrence to his research team on [email protected].

The Environment and Resources Authority (ERA) has been alerted to the findings of the study.

The invasive nature of some of the alien crayfish species documented in this study has prompted the researchers to list the red swamp crayfish, the marmorkrebs and the signal crayfish in the EU’s Invasive Alien Species (IAS) list slated for direct intervention.

Captained by Deidun, the study involved Maltese researchers Arnold Sciberras and Justin Formosa, Italian researchers Bruno Zava, Gianni Insacco and Maria Corsini-Foka and Keith Crandall, a professor from the George Washington University in the US.

The full text of the paper can be accessed here.