A floating cage system for rearing freshwater mussels Anodonta anatina and Unio mancus in Piedmont region (NW Italy)

Abstract

This research represents the first attempt in Italy to develop a cage system for adult freshwater mussel stocking, for extensive farming, that was realized from 2003 to 2005 in NW Italy. Two autochthonous species of Italian freshwater mussel were used: Anodonta anatina and Unio mancus. 5133 mussels were collected and successively stocked in floating cages in 5 different sites in a lake. This study was planned in order to determine the effect of main rearing factors for potential farming: stocking density and cage position in the water column (depth). For each species, the effects of three rearing densities and two water depths were tested. Mussel growth was measured on a monthly basis in each experimental unit throughout the study. The survival rate at the end of the experiment for both species considered was 98.4%. U. mancus resulted to be more resistant to rearing conditions and during the summer the highest mortality was registered for A. anatina. Summer mortality was between 4.8% and 20.5%. Highest mortality in U. mancus was recorded in September 2004 at 1.5 m (3.6±2.4%), while the highest mortality for A. anatina was recorded in September 2003 as 13.3%±3.8% at a depth of 1.5 m. This research showed that cage stocking is a suitable method and the optimal position of cages is at the lake thermocline. The proposed species proved to be easily utilizable for future farming or bioremediation projects, and can readily be reared up to 135 kg m-3.