Corbicula fluminea invasion in Lake Maggiore (Italy): a three-year field monitoring

Corbicula fluminea (Müller, 1774) was recorded for the first time in Lake Maggiore in 2010. Recently, possible introduction in the Lake was suggested as coming from a population settled in River Ticino outlet. This study focused on spatial and temporal variations in abundance, biomass and population structure of C. fluminea in Lake Maggiore during the period 2010-2013. Clams were quantitatively sampled at four sites (Angera, Arona, Brebbia and Feriolo) located in the Southern part of the littoral area, characterized by diverse type of habitats and different degree of anthropogenic disturbance. Biomass was calculated based on dry weight of subsamples of clams collected at sampling sites. Age of individuals was estimated from shell length. Invasions dynamics was reconstructed from spatial differences in size/age distribution patterns and the contribution of the last recruited cohort to total population density. The results revealed two main reproductive periods in this environment - spring and autumn. We found also that contribution of young-of-the year individuals was highest in the northernmost site Feriolo. A remarkably higher population density at Angera and Brebbia (with peaks of 1652 ind.m-2 in April 2011 and 2800 ind.m-2 in March 2012 respectively), together with the contribution of up to three-years-old individuals could stand the so-called "log-phase" (spread of an established population) of the invasion process. On the opposite, the size distribution and density observed at Arona (maximum of 136 ind.m-2) in April 2013 suggested a decline of Corbicula. However, the northward spreading of the species was confirmed in summer 2011 when clams were found at a new location on Lombardy shore, thus giving the possibility to study "lag-phase" (initial dispersal and establishment of self-sustaining population) of C. fluminea invasion dynamics.