LONG-TERM EFFECTS OF MECHANICAL HARVESTING OF LUGWORMS ARENICOLA-MARINA ON THE ZOOBENTHIC COMMUNITY OF A TIDAL FLAT IN THE WADDEN SEA

More than half of the annual catch of about 30 million lugworms Arenic ola marina from the Dutch Wadden Sea originates from digging machines which make 40-cm deep gullies in a few restricted tidal-flat areas (Te xel, Balgzand) in the westernmost part of the Wadden Sea. Four success ive years (1978-1982) of frequent disturbance by a lugworm dredge of o ne of the 15 sampling stations involved in a long-term study of the dy namics of the macrozoobenthos on Balgzand allowed a study of long-term effects of mechanical lugworm digging. Within an area of about 1 km(2 ), a near-doubling of the annual lugworm mortality rate resulted in a gradual and substantial decline of the local lugworm stock from more t han twice the overall Balgzand mean at the start of the 4-year digging period to a value close to this mean at the end of the period (when t he dredge moved to a richer area). Simultaneously, total zoobenthic bi omass declined even more by the almost complete extinction of the popu lation of large gaper clams Mya arenaria that initially comprised half of the total biomass. Of the other, mostly short-lived, species only Heteromastus filiformis showed a clear reduction during the dredging p eriod. Recovery of the biomass of the benthos took several years, part icularly by the slow re-establishment of a Mya population with a norma l size and age structure.