Brown shrimp Crangon crangon was sampled during several surveys in the Westerschelde estuary. Diurnal samples were taken in the brackish subtidal, seasonal samples in the marine and brackish subtidal, and monthly samples in the brackish intertidal part. The diet of C. crangon was studied through stomach content analyses, which were sometimes complicated due to the presence of sand grains and unidentifiable remains. On average 36% of the stomachs examined were empty, 33 % had few {highly digested) remains (mainly from crustaceans), and 31 % contained many (partly digested) prey items. Still, clear differences in density and feeding of C. crangon were recorded at several scales. Density, feeding intensity and prey diversity showed mixed tidal- diurnal patterns, with a dominance of the tidal pattern, and more feeding and a higher trophic diversity at high water. On a seasonal scale, density and prey diversity were highest during summer, both sub- and intertidally. Stomach fullness was higher in (early) spring, although this was part I y influenced by the moment of sampling compared to the tidal phase. C. crangon had a higher prey diversity in the intertidal (21 out of 32 prey categories), but with an overall dominance of small crustaceans, i.e. mysids, amphipods and copepods. During the 24-h surveys, mysids (mainly Neomysis integer and Mesopodopsis slabberi) were the most important prey items, day and night (>60 % in percentage occurrence (%F), abundance ro/0N) and weight ro/0G)). Seasonally, mysids were important prey items in spring and summer, both in the marine (>55 %F-N-G, mainly Gastrosaccus spinifer) and brackish (>40 %F-N-G, N. integer and M. slabberi) subtidal. In the latter, mysids were replaced by amphipods (20 %F-G, 60 % N. Corophium and Bathyporeia) in autumn and winter, which paralleled the seasonal pat- terns in prey abundance and prey availability. Feeding behaviour was dependent of habitat type, rather than body size. Larger shrimps did not necessarily feed on larger mysids or amphipods. In the brackish intertidal, mysids were infrequently preyed upon. In this subhabitat, shrimps were on average 10 mm smaller than in the subtidal, and in terms of abundance, copepods (30 %F-N) were gradually replaced in the diet by amphipods (>25 %F-N-G) with increasing size of C. crangon. Other less important prey items for the different surveys were ostracods, cyprid larvae, shrimps, bivalves (mainly Mytilus spat), polychaetes (mainly Nereidae), gastropods (mainly Littorina), and fish. Daily consumption amounted to 0.01-0.12 g ashfree dryweight (ADW) per day. It is suggested that C. crangon mainly foraged in the hyperbenthal waterlayer (mostIy during night), supplemented with some food from the epibenthal layer during the day. The presence of many mysid statoliths in the stomachs, additionally proved the importance of mysids in the shrimp diet, but possibly led to overestimated consumption values. Minimum and maximum values (either based on measured prey weights in the stomachs or on recalculated prey weights prior to ingestion) were on average 1 to 5 % bodyweight per day in the subtidal, and 4 to 14 % BW d-1 intertidally. Yearly consumption was 1 to 8 g ADW m-2yr-l, both sub- and intertidally. Shrimp predation could have a large impact on the mysid population, as on average 10 to 35 % of the mysid standing stock was consumed per day. |
