CHANGES IN THE WATERBIRD POPULATIONS OF THE OOSTERSCHELDE (SW NETHERLANDS) AS A RESULT OF LARGE-SCALE COASTAL ENGINEERING WORKS

Between 1982 and 1987, the construction of a storm-surge barrier and t wo secondary dams in the eastern and northern parts of the Oosterschel de/Krammer-Volkerak area resulted in the loss of 33% of the 170 kM2 of intertidal area in the estuary. Consequences for non-breeding waterbi rds were evaluated on the basis of monthly high-tide counts during fiv e seasons before and three seasons after the construction period. In t he entire Oosterschelde/Krammer-Volkerak area, numbers of wintering wa ders decreased but those of ducks increased. Peak numbers and total nu mber of bird-days changed little, but the seasonal pattern shifted fro m a midwinter maximum to a peak in autumn. In the Oosterschelde (exclu ding the Krammer-Volkerak), where 17% of the tidal flats disappeared, species feeding mainly on open water remained stable or increased. Spe cies dependent on intertidal areas for foraging (mainly waders and dab bling ducks) generally decreased. Total density of intertidal foragers decreased slightly. In most intertidal species, the Oosterschelde win tering population showed a stronger decrease, or smaller increase, tha n was shown during the same period by numbers in Britain and Ireland w hich were taken as an index of the total W-European winter populations . Changes varied considerably between species, and were correlated wit h their distribution within the estuary. Species concentrated in the e astern sector, where most habitat loss occurred, declined more than sp ecies with a more westerly distribution. Results indicate that interti dal foragers forced to move from the enclosed parts of the estuary wer e not generally able to settle into the remaining intertidal areas. Bo th dispersal to adjacent areas (mainly by dabbling ducks) and mortalit y during severe winter weather (in some wader species) may have contri buted to the declines. Populations of intertidal foragers apparently w ere (and consequently still are) close to carrying capacity, and furth er changes in capacity, as foreseen from geomorphological changes stil l under way in the estuary, are likely to be reflected in bird populat ions. Numbers of waders moulting in the Oosterschelde in late summer d eclined strongly compared to numbers in other seasons. Increased distu rbance due to recreational activities may have played a role during th is time of the year. than that of the bird densities it is likely that at some times food is not limiting, at other times it is. On the othe r hand, consumption is very low in the subtidal compartment and specie s feeding here could potentially increase substantially in numbers in the Oosterschelde.