Mapping the invasive crayfish distribution across Malta

Abstract

The freshwater ecosystems on the Maltese archipelago are extremely vulnerable to alien species invasion due to an already limited freshwater supply and the threatened species that inhabit it. This study discusses the effects and provides an updated population estimation of two invasive crayfish species, P. clarkii and P. virginalis, that have been found on the archipelago since 2016. The primary goal of this research is to provide updated information on the distribution and impacts that invasive crayfish are having on the Maltese freshwater system. These species are significant threats to local species as they tend to alter landscapes, encroach on habitat territory, and prey upon small amphibians such as the Maltese painted frog (Discoglossus pictus) and on aquatic invertebrates, some of which are rare and threatened. The fieldwork involved in this study was conducted at five different locations across Malta and Gozo from February to April of 2025. The crayfish were observed through visual counts, evidence of molting, as well as observation of their effects on the surrounding area. These effects range from decreased biodiversity to lower water quality. Additional research was done through literature review, lab identification, as well as correspondence with a local site manager at Chadwick Lakes. This research found that established crayfish have a high density in the Western Chadwick Lakes area as well as at Ghadira Ta’ Sarraflu on Gozo. Current efforts to manage populations are insufficient and lack the funding and resources necessary to properly do so. This study calls for increased public awareness of invasive species located in the Maltese freshwater environment as well as better management strategies accompanied by proper funding.