Evaluation of shell damage to the clam Chamelea gallina eaptured by hydraulic dredging in the Northern Adriatic Sea

摘要

The impact of experimental hydraulic dredging was assessed on Chamelea gallina populations in two sites along the north-western Adriatic coast (Lido and Jesolo) by detecting and quantifying shell damage caused by fishing operations on both captured and discarded clams. Various levels of stress were applied, the highest being that used by commercial fishing vessels, which employ high water pressure and mechanised sorting and the lowest manual sampling of clams by scuba divers. Water pressure and sorting significantly increased shell damage, the highest levels always being observed in commercially dredged clams. At Lido, damage was mostly due to the action of the mechanised sorter; at Jesolo, the effect of high water pressure was more clearcut. Moreover, clams collected at Jesolo had both higher mean damage level and higher numbers of damaged individuals compared to the Lido samples. These differences seem to be mostly related to differing bottom features in the two sites. A positive relationship was observed between damage level and clam size: small-sized samples (length <17mm) were less damaged than medium-sized ones (25mm < length < 17mm) and commercial size clams (<25mm) showed the highest damage level. The severe and harmful physical impact of hydraulic dredging was apparent in captured and then discarded animals, a small fraction of which appears able to recover, as shown by the presence of clams with repaired shells.