Risk Screening and Distribution of the Invasive Amphipod Dikerogammarus villosus (Sowinsky, 1894) in the River Adda (Northern Italy)

In 2016, Dikerogammarus villosus (Sowinsky, 1894) (Crustacea, Amphipoda) was recorded for the first time in the River Adda, one of the main tributaries of the major Italian river, river Po. Here we investigate its distribution, population density and size classes distribution in the main course of the river, in the territory of the South Adda Regional Park. Furthermore, we defined its level of invasiveness using two different risk screening methods, in order to obtain a more comprehensive evaluation of its potential impacts: AS-ISK (Aquatic Species Invasiveness Screening Kit) and Harmonia(+). Finally, we compared the resulting invasiveness level with the output of the GISS (Generic Impact Scoring System) method, also used by IUCN. Our data confirm that the invasive amphipod is well-established in the downstream part of the River Adda, and its distribution seems related to hydrodynamism. Interestingly, its level of invasiveness was evaluated as low by Harmonia(+) and high by AS-ISK: this discrepancy is due to different evaluations of environmental and socio-economic impacts of a non-native species carried out by the two methods. Finally, we propose some possible actions to reduce the spreading rate of this invasive amphipod in this area.