In order to evaluate the role saltmarshes can play for estuarine fishes and macrocrustaceans, samples were taken in two major saltmarshes of the Westerschelde. Samples were collected in February, April, June and August in the saltmarsh of Waarde. In the saltmarsh of Saeftinghe samples were collected in April, June and August. A passive fishing method was used to sample the organisms migrating in and out of the creeks. The gear used was a simple fykenet that was operated from a bridge across the saltmarsh creek. When water started to enter the creek on the rising tide, the net was placed on the creek bottom, its mouth facing the current. Ropes and weights kept the net in place and the net was hauled every hour. After the moment of high water the net was placed at the other side of the bridge, now facing the ebb current. Measurements of the current and waterheight allowed quantification of the catches in relation to the volume of water which passed through the net. Temperature, salinity and oxygen content of the water were also measured. Samples collected were preserved in a formalin seawater solution. Fish to be used for stomach content analysis were anaesthetised before fixation. In the laboratory, all animals caught were identified to species level, counted and measured under the binocular microscope. The dry weight of the detritus present in all samples was measured and used as an environmental variable. Stomach contents of two important fish species were analyzed. All prey items were identified to species level, counted and measured. Fullness indices were calculated for all fish. Numerical and gravimetrical analyses were used to estimate the importance of the different prey in the diet of the fish. The average fullness index of the rising and falling tide samples were then tested for statistically significant differences. For every hour sample density and biomass were calculated for all species found. The densities and biomass for all rising tide samples and the total volume of water were used to estimate the densities and biomass present at high water. The same was done with the falling tide samples. The average of both estimates was used as the sample densities and biomass values for the specific month. Most species caught were found to be common to both saltmarshes. Common species caught included, Neomysis integer, Mesopodopsis slabberi, Crangon crangon, Palaemonetes varians, Corophium volutator, Gammarus salinus, Carcinus maenas, Pomatoschistus microps and Platichthys flesus postlarvae. Densities and biomass for most species caught varied seasonally. For most species densities and biomass were low in February , when temperatures were also low. The high densities and biomass in April (Waarde) and in June (Saeftinghe) coincided with recruitment periods, optimal temperatures and high food abundance in the saltmarsh. In August densities declined again for most species but for some species, with late recruitment, maximum values were found. Community studies were done using multivariate techniques in order to determine the spatial and temporal structure in the data. All the techniques used yielded similar results. Temporal factors were more prominent in structuring the communities and spatial factors were found to be negligible. A comparison of the denities found in the saltmarshes with those in the estuary showed that all juvenile stages caught were more abundant in the marsh habitat. This shows that saltmarshes are preferred by these early life stages of fish, shrimp and crabs. In this way the nursery function of the saltmarshes is confirmed. Most species entered the marsh during the first hour of the rising tide and left it during the last hours of the falling tide. This tidal migration pattern showed that most species were using the saltmarsh habitat as long as possible. In this way they can profit maximally from a favourable habitat. The quantitative analyses of the stomach contents could prove that only P. microps leaving the marsh in the sample of October had more food in the stomach than those entering. For P. microps caught in April and D. labrax there were no differences detected. The results of the numerical and gravimetrical analyses could show qualitative differences in the diet of fish entering and leaving the marshes. Fish leaving the marsh clearly had been preying upon organisms which are common in the saltmarshes. This gives us the evidence that saltmarshes act as feeding grounds for fish. |
