Long-term indirect effects of mechanical cockle-dredging on intertidal bivalve stocks in the Wadden Sea

1. There is world-wide concern about the effects of bottom-dredging on bent hic communities in soft sediments. In autumn 1988, almost a third of the 50 -km(2) intertidal system around the island of Griend in the western Dutch W adden Sea was suction-dredged for edible cockles Cerastoderma edule and thi s study assessed subsequent effects. An adjacent area not directly touched by this fishery and an area from which the mussel Mytilus edulis beds were removed, served as reference areas. 2. Sediment characteristics, together with the total stock size and settlem ent densities of Cerastoderma, Baltic tellin Macoma balthica and soft-shell ed clam Mya arenaria, were documented during 11 successive autumns before ( August-September 1988) and after (August-September 1989-98) the suction-dre dging event in fished and unfished areas. Four other areas in the Dutch Wad den Sea, where changes in densities of juvenile bivalves from 1992 to 1998 were measured, served as additional reference locations. 3. Between 1988 and 1994, median sediment grain size increased while silt w as lost from sediments near Griend that were dredged for cockles. The initi al sediment characteristics were re-attained by 1996. 4. After the removal of all Mytilus and most Cerastoderma, the abundance of Macoma declined for 8 years. From 1989 to 1998, stocks of Cerastoderma, Ma coma and Mytilus did not recover to the 1988 levels, with the loss of Ceras toderma and Macoma being most pronounced in the area dredged for cockles. D eclines of bivalve stocks were caused by particularly low rates of settleme nt in fished areas until 1996, i.e. 8 years after the dredging. 5. A comparison of settlement in the short (1992-94) and medium term (1996- 98) after cockle-dredging in several fished and unfished areas spread over the entire Dutch Wadden Sea, showed a significant negative effect of dredgi ng on subsequent settlement of Cerastoderma. Macoma also declined, but not significantly. 6. We conclude that suction-dredging of Cerastoderma had long-lasting negat ive effects on recruitment of bivalves, particularly the target species, in sandy parts of the Wadden Sea basin. Initially, sediment reworking by suct ion-dredging (especially during autumn storms) probably caused losses of fi ne silts. Negative feedback processes appeared to follow that prevented the accumulation of fine-grained sediments conducive to bivalve settlement.