A multidisciplinary study of the immediate effects of mechanical clam harvesting in the Venice Lagoon

In the Venice Lagoon, clam (Tapes philippinarum) harvesting is carried out mainly by means of a gear locally called “rusca”, developed by local fishermen. The rusca consists of an iron cage, an outboard engine propeller, which produces a water flow directed onto the bottom suspending sediments and fauna, and a net bag where the clams are collected. The aim of this study was to provide an analysis of the immediate effects of rusca fishing on a wide spectrum of ecosystem compartments: sediment biogeochemistry, sediment resuspension, and macro- and meiofauna community. Rusca fishing produced a V-shaped furrow (about 60 cm wide and 7 cm deep) and a plume of resuspended sediment with a significant increase (up to two orders of magnitude greater than undisturbed areas) of suspended particulate matter (SPM) and increased Ctot, Corg, Ntot, and sulphide concentrations in the water column. Experimental rusca hauls significantly reduced macrofauna density, while no significant effect on meiofauna was detected. Results are also discussed in terms of basin-scale impact, attempting to compare natural and anthropogenic disturbance.