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|Utilization of estuarine salt marshes by postlarval brown shrimp Crangon crangon|
Dankwa, H.R. (1993). Utilization of estuarine salt marshes by postlarval brown shrimp Crangon crangon. MSc Thesis. RUG: Gent. 49 pp.
The utilization of an estuarine brackish salt marsh by postlarval brown shrimp Crangon crangon was investigated in the Westerschelde estuary. Between March 1990 and October 1991 monthly samples were collected with a stow net in an intertidal creek in the marsh of Saeftinghe. Fishing lasted a complete tidal cycle and the net was hauled every hour providing in general, three flood samples and four ebb samples. Oxygen concentration, temperature and salinity were measured hourly while the water height and the current speed were recorded every fifteen minutes. The latter two variables were used in the estimation of densities and biomasses. Densities and biomasses were estimated for every hour sampled based on the number of animals and the volume of water that passed through the net within that particular period. From these, estimates for each sampling period were made for the whole creek considering the volume of water that passed through the creek over the same period of time. This was done for each flood and ebb sample. The average of these estimates was used for a particular sampling date. The community structure of the marsh was analysed using three independent multivariate statistical techniques. The biomass and density data were fourth-root transformed prior to analysis. Thirty eight species were used in the analysis after eliminating the species which frequency of occurrence was less than three and those which were not representative of the targeted community. The relationship between the species composition and the environmental variables was analysed with Canonical Correspondence Analysis (CCA). Classification of samples into clusters according to species composition was done using Two Way INdicator SPecies ANalysis (TWINSPAN). Cluster Analysis was used to verify the TWINSPAN results. Data from the hyperbenthic survey of the Westerschelde (Hamerlynck et.al, in press) allowed for comparison of the abundance and the seasonal variations in occurrence of postlarval and juvenile shrimp between the marsh and the subtidal parts of the estuary . In order to compare the predation pressure upon postlarval shrimp in both habitats stomach contents of fish caught in both areas were analysed. The fullness indices of postlarval shrimp entering and leaving the creek system were also compared. With these data the nursery function of the marsh in terms of harbouring higher densities of postlarvae and providing refuge and food, was evaluated. The tidal migration pattern of postlarval shrimp was assessed by correlating the numbers of shrimp caught with the current speed. Sixty eight species were caught during the study period but only thirty eight were found to occur regularly in the marsh creek. Neomysis integer was the dominating species during most months both in terms of biomass and density. Mesopodopsis slabberi was very important in the fall period of 1990. In spring, postlarval Crangon crangon and postlarval Pleuronectes flesus were numerous but did not contribute much to the total biomass. Juvenile Pomatoschistus microps was abundant in early summer through fall while juveniles of Dicentrarchus labrax and Carcinus maenas were present in very low numbers in the same period but were important in terms of biomass. The marsh community was found to be fairly stable. However, any variation occurring in the community could be attributed to the effects of temperature and/or salinity. Biological factors such as spawning behaviour and predation might also play a role in structuring the community. Only two clearly defined seasonal communities were identified. The spring season was characterized by the presence of postlarvae or juveniles of several species eg. C. crangon, P. flesus and N. integer. The other community occurred from early summer through late fall and was characterized by juveniles of fish and crustacean species eg. D. labrax, P. microps and C. ma
- Hyperbenthos community in the salt marsh of Saeftinghe in 1990 and 1991