The impact of hydraulic blade dredging on a benthic megafaunal community in the Clyde Sea area, Scotland.

A study was made of the impacts on a benthic megafaunal community of a hydraulic blade dredge fishing for razor clams Ensis spp. within the Clyde Sea area. Damage caused to the target species and the discard collected by the dredge as well as the fauna dislodged by the dredge but left exposed at the surface of the sea bed was quantified. The dredge contents and the dislodged fauna were dominated by the burrowing heart urchin Echinocardium cordatum, approximately 60 – 70% of which survived the fishing process intact. The next most dominant species, the target razor clam species Ensis siliqua and E. arcuatus as well as the common otter shell Lutraria lutraria, did not survive the fishing process as well as E. cordatum, with between 20 – 100% of individuals suffering severe damage in any one dredge haul. Additional experiments were conducted to quantify the reburial capacity of dredged fauna that was returned to the sea bed as discard. Approximately 85% of razor clams retained the ability to rapidly rebury into both undredged and dredged sand, as did the majority of those heart urchins Echinocardium cordatum which did not suffer aerial exposure. Individual E. cordatum which were brought to surface in the dredge collecting cage were unable to successfully rebury within three hours of being returned to the sea bed. These data were combined to produce a model of the fate of the burrowing megafauna dredged and dislodged in order to collect 10 kg of marketable razor clams