Long-Term Patterns in the Wader Usage of an Intertidal Flat in the Oosterschelde (Sw Netherlands) and the Impact of the Closure of an Adjacent Estuary

(1) The wader populations of the Roggenplaat, a 16.5 km2intertidal flat in the Oosterschelde estuary (Dutch Delta area), were monitored by a standardized monthly census of their high-water roosts, to evaluate the effect of the loss of 55 km2of feeding grounds in the adjacent Grevelingen estuary, because of its enclosure in May 1971. (2) A comparison of annual maximum numbers and bird-day totals between 1964-71 and 1971-79 revealed abrupt increases of the oystercatcher (Haematopus ostralegus L.) and the bar-tailed godwit (Limosa lapponica L.), which can unequivocally be attributed to the closure of the Grevelingen. (3) The 'carrying-capacity' of the Roggenplaat (and presumably also of the entire Oosterschelde) was apparently not fully exploited until 1971. (4) Shifts in the average seasonal abundance indicated that upper limits have been reached in oystercatcher and some other species since 1971. (5) This may result in reduced future population numbers, since in 1987 35% of the intertidal areas in the Oosterschelde were lost because of the completion of huge hydraulic engineering projects.