Alien crayfish species in Latvian inland waters

Survey of data on Latvian freshwater crayfish species distribution and occurrence in different waterbodies (404 lakes, 368 rivers and 47 reservoirs, nine quarry reservoirs) across the territory in the last decades from 1988 to 2018 were performed with the aim to compare field data with previous research, inquiries and grey data. The most frequently occurring species was native noble crayfish Astacus astacus and the introduced narrow-clawed crayfish Astacus leptodactylis, with incidence of 26 and 5% from surveyed waterbodies, respectively. The invasive alien crayfish species Orconectes limosus and Pacifastacus leniusculus occured in 23 (2.8%) and 10 (1.2%) of waterbodies, respectively. The history of invasion of alien crayfish species was analysed. Survey results demonstrated that in the last decades noble crayfish occurrence decreased, or their presence was not detected due to low population density. The occurrence of other species, especially invasive spiny-cheek crayfish, was increasing.