An invasion of alien crayfish in Malta's natural freshwater streams is taking over the last haunts of some of the island's beloved species, a new study has found.
Recently published in the Journal of Crustacean Biology, the study has revealed the extent of the invasion of freshwater valley systems in the Maltese Islands by a number of introduced crayfish species.
The study, conducted over 2016 and 2017, was led by marine biologist Alan Deidun from the University of Malta's Department of Geosciences.
Prof Deidun on Wednesday warned about the scale of the invasion of Maltese freshwater ecosystems is a "cause for concern".
Read: Invasive crayfish spreading like hell
The freshwater network, he said, were already susceptible to a clutch of other man-made impacts, including diversion of water for irrigation purposes and pollution through release of pesticides and fertilisers.
Some of the invaded valleys were among the last haunts for the endangered freshwater crab and painted frog - two indigenous species on their last legs.
The fact that the non-indigenous crayfish species were also recorded from man-made reservoirs and ponds was evidence, according to Prof Deidun, that the invaders were intentionally released by "irresponsible individuals, unaware of the ecological disaster such releases could unleash".
Some of the alien crayfish species can withstand prolonged periods of draught, and exhibit prolific rates of reproduction, besides being opportunists when it comes to feeding, he said.
Prof Deidun urged aquarists and the public in general to desist from releasing such exotic species into the wild and encouraged those encountering such species to report their occurrence to his research team on alan.deidun@um.edu.mt.
The Environment and Resources Authority has been alerted of the findings of the study so as to consider future management options.
Researchers painstakingly documented the occurrence of at least five non-native (alien) crayfish species within four valleys in Malta - Fiddien/Wied Qlejgha, Baħrija, Għajn Zejtuna and Wied l-Isperanza) and one valley in Gozo, Wied il-Lunzjata.
It also found crayfish within a number of freshwater pools, ponds and reservoirs including Ta’ Sarraflu and Għajn il-Papri in Gozo and Pembroke in Malta.
The five alien crayfish species are all popular within the pet industry and are reared commercially for human consumption.
The researchers said these include the red swamp crayfish (Procambarus clarkii), the marmorkrebs (Procambarus virginalis), the Galician or narrow-clawed crayfish (Pontastacus leptodactylus), the signal crayfish (Pacifastacus leniusculus) and the Australian redclaw (Cherax quadricarinatus).
A sixth alien species, this time the bamboo shrimp (Atyopsis moluccensis), was also documented within the study from a reservoir in Pembroke, whilst individuals of the common yabby (Cherax destructor) were documented from the island of Sicily for the first time.
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 researchers found that the invasive nature of some of these alien crayfish species saw them included, as of August 2017, in the Invasive Alien Species (IAS) list slated for direct intervention by the European Union.
The study also involved colleagues from Malta (Arnold Sciberras, Justin Formosa) and Sicily (Bruno Zava, Gianni Insacco and Maria Corsini-Foka) and also Prof. Keith Crandall from the George Washington University in the US.