Impact of mechanical clam harvesting on benthic habitat: an evaluation by means of Sediment Profile Imaging

1. Manila clam (Tapes philippinarum) harvesting in the Venice Lagoon has increased considerably in the last decade, owing to recently developed collection methods. However, these techniques have negative effects on benthic communities and on the structural and functional characteristics of the sediments. 2. A field survey was carried out in 2000 in the central basin of the Venice Lagoon to evaluate the efficacy of sediment profile imaging (SPI) in investigating disturbances caused by fishing activities and to compare the modifications of bottom sediments induced by different fishing gear (the ‘rusca’, currently used by local fishermen, and a rotating drum). 3. An environmental index, the organism–sediment index, derived from SPI analysis was applied. The efficacy of the SPI camera method in evaluating the disturbance of soft bottoms caused by clam harvesting was confirmed, as was the high degree of disturbance of sediment and benthic communities by mechanical clam harvesting. 4. The experimental hauls strongly modified the sediment features by resuspending the top layer of sediment and bringing the deep anoxic layer near the bottom. These effects could have a severe impact on the biogeochemical cycles and on the possibility of recolonization by benthic organisms in the short term. However, there was less disturbance when the rotating drum fishing gear was used.