Hydro-ecological relations in the Delta Waters

As birds are mainly top predators, they accumulate quite a lot of toxic
substances. The dramatic effects of chlorinated hydrocarbons,
insecticides and chlorinated biphenyls on many piscivores in general
and especially on the Sandwich Terns in the sixties, decimating their
pop~lations are well known (Koeman and Van Eenderen, 19721. The effects
of accwwlation of pollutants ere, hmever, not always so obvious,
neverthelens they wy severely affect populations. Pew data on
conoentrations of pollutanrs in birds in the Delta area are avaFLahlc.
However, experiments with Tufted Ducks clearly deman$trafed that when
fed With Zebra Mussels from the Raringvliet their reproductive oatput
was v eq much depressed IMarquenie, et al. 1986; Seholten and Poekema,
1988). Clearly pollution is a major factor affecting birds in the Delta
area and, as its effects are vey diverse and by now not very well
onderstood, much mere attention should he paid to this problem in the
Eutnre.
Babitat loss is ~f course a very impartant factor influencing birds.
The effects are obvious. In the remaining areas bird densities
increase, causing a higher predation pressure and hence a smaller food
supply. This tpgether with an inereased level of social iateractions
between individuals can reduce quite aubsfantiallp the food intake
lead* td death. detailed revietv af the effecrs of habitat loss bn
wader populatione is given by Goss-Custacd, 1985. It shauld be nated
that hahicat lams is not only cased by physical removal of an anea but
also by deterioration of the food supplp e.g. due to changing
environmental condition#. In the Westerschelde the increased dredging
activities cause the intertidal flats to become more sandy shee in the
larger channels higher current speeds occor, influencing the
sedimentation patterns. The benthic fauna and the numbers of waders are
much smaller on send- than on mudflats. This is especially in the
Wesrerschelde a severe threat.