Hydro-ecological relations in the Delta Waters

The mein diet of piscivores in the salbe waters consists of Cobiid
fish (Pomatosehistus microps, P. minutums, P. loaanei). In the
Grevelingen Meer the diet of Great Crested Grebe consisted for more
then 60%: (by weight) of Gobiidae (Domrnbos, 19$4). Other important
species were Herring (C1,u~eq harengus) and Shrimps (Grangmn crangon) .
The Three-spinod Stifkleback (6asterosteus aculeatus) is very abundant
in the Veerse Meer (Waardenburg and Meiler, 19881, but it is not known
ta &€it esent they $re eaten by birds. As GmbFid fish fenus the
majority of the diet and more PT less comparable data are available,
the average winter density is even h Table 2 together with tha
densities of pibcivofes. Clearly the highest densitieg of bath fish sad
birds are found ib the Veerse Meer. In the Voordelta the nobiid
densities ate varg varia%lo and prehably transparency. wMth is very
low, end wave action prevent Birds from feeding here. In the tidal
areas and the saline lakes a clear relation bstwesn ptscivore densities
and Gobffd densities is found. In the froshwatar lake6 ratber high
densities of piscivores are found. Perch (Perca fluviatilis), Boaeh
(Rutilus ~tilus) and other Cyprinidae as well as Gasterosteidae ere
inportant prey species in these lakes. Howevr;~, no density eqtimations
are available.
The bibmass of macrozoobenthos end the dessities of waders and diving
ducks ere given in Table 3. Waders occur -My on intertidal areas
here they feed on *any dzffetent species @f izverteh~afes. Same waders
are very speciUred, for instance Cystercatchers which feed mditrly 6zl
Mussels and Cackles; others suCh as Dunlin feed on a verierp of
polychaete worms (Rerefs sp.; Scoloplos armfger =Cc.) and small
crustaceans (e.g. Corophiua sp.) whereas Bar-tailed Godwits feed on
both worms, crustaceans and molluscs. Sometimes the diet ohansea with
the seasons. In autumn the diet of Curlew oonaists mainly of the
Sharecrab wbereas in winter, when the crabs move to deeper water,
Curlews feed l~iiinly on large worms (Lugwonus and Nereis sp.). In the
tidal basins an obvious relation between the densities of waders and
the available biomass extsts. A very high bkmass and density is fomd
fn the Ooeteyschelde, much lower valnes in the Westerschelde and
Kramer-Volkerak. The densities of waders in the other basins are much
lmer and as they arzt i?ee-g these in a veriety of habitats, the diet