Update on the signal crayfish, pacifastacus leniusculus (Dana, 1852) range expansion in Croatia: A 10-year report

Abstract

The signal crayfish (Pacifastacus leniusculus) is considered to be the most successful crayfish invader in Europe. It is present across the continental part of Croatia, with first records from the Mura-Drava river basin in 2008 and from the karstic Korana River in 2011. In both rivers, P. leniusculus presents an imminent threat to indigenous crayfish species (ICS). The aim of the study was to explore the range expansion of P. leniusculus 10 years after its first record in Croatia. Based on the data collected via an extensive literature search and our own fieldwork performed in 2018, we here demonstrated that P. leniusculus is the most successful non-indigenous crayfish species (NICS) in Croatia, with the invasion range covering the largest extent of Croatia compared to other NICS. Its dispersal rates in the rivers of the continental part of Croatia have remained among the highest in Europe. In the Drava River, recorded dispersal rate (21.3 km/yr) corresponded to the predictions made by the earlier study that estimated P. leniusculus dispersal over 10 years. However, due to the low number of caught individuals along the 128 km of the Drava River course, dispersal rates as well as invasion pathways (natural vs. human-mediated) need to be further explored. The predicted encounter of P. leniusculus with the invasive Faxonius limosus was not observed. In the Korana River, we observed a continued expansion of P. leniusculus invasion range in both upstream and downstream direction. Dispersal rates were higher in the upstream direction, potentially due to differences in competitive pressure by the native crayfish, Pontastacus leptodactylus, which abundances were 6 times higher at the downstream invasion front. In both rivers, a displacement of the native P. leptodactylus has been observed. Even though P. leptodactylus has been considered to be the most robust ICS, that successfully expands its range in water bodies of Croatia and Europe, it is also the species the most adversely affected by the range expansion of the P. leniusculus in the continental part of Croatia. Thus, the status of P. leptodactylus in Croatia should be monitored and re-examined.