THE DIRECT AND INDIRECT EFFECTS OF SUCTION DREDGING ON A RAZOR CLAM (ENSIS-ARCUATUS) POPULATION

Surveys were conducted in two shallow bays in the Orkney Islands, UK; Orphir Bay, an unexploited (control) site, and Bay of Ireland, a fishe d site, to investigate the effects of suction dredging on the resident razor clam, Ensis arcuatus, populations. A lower density and signific antly smaller mean length of razor clams were present at the dredged s ite compared with the control site. The age of individual razor clams was estimated using internal shell microgrowth patterns, visible in ac etate peels of polished and etched shell cross-sections. Ensis arcuatu s are relatively slow growing animals with the two study populations c haracterized by old individuals and an obvious lack of juveniles, indi cating populations with little resilience to disturbance. An analysis of the shell sections of razor clams from the Bay of Ireland revealed the presence of shell margin breaks, consisting of deep clefts in whic h sand grains were embedded in the shell matrix, whilst those from Orp hir Bay had fewer disturbances to shell growth. It is suggested the di sturbances to shell growth are the result of repeated suction dredging operations in the Bay of Ireland. In situ reburrowing experiments wer e conducted to determine the survival rate of E. arcuatus (<160 mm she ll length), returned to the sea after capture and to estimate the indi rect effect of dredging on the razor clam population. These individual s displayed a slow initiation of ''escape-digging'' which rendered the m vulnerable to attack from predatory crabs and fish, indicating that there is likely to be a low survival rate of any returned undersized c lams or ones that are disturbed and escape from the suction dredge. (C ) 1998 International Council for the Exploration of the Sea.