Samenvatting

Salt marshes are known to be an important habitat type for numerous species of fish. As fish stocks are pressured around the world, providing them with suitable habitat is very important. Through managed realignments or tidal restoration, intertidal salt marshes are created or restored. Which fish use these measures however, is an understudied subject. In this study, it was investigated which fish use two natural and two constructed salt marshes in the Scheldt estuary (The Netherlands), to understand the functions artificial salt marshes provide for different fish species. Insight in the functions marshes could provide for fish is of importance for coastal managers, to optimize restoration or creation of salt marshes, to better serve fish in their needs.
To assess the fish populations using the marshes and with what purpose, fish was caught, using fyke nets and seine nets. For each species, it was determined which function the natural and constructed salt marshes could provide them, based on their length (age) and life cycle. Sampling of the fish populations took place in two natural marshes: Sint-Annaland and Het Verdronken Land van Saeftinghe, and in two constructed marshes: Rammegors and Perkpolder. All locations were sampled in autumn 2020, by placing six fyke nets, that were retrieved after 24 hours, and fishing a minimum of two transects with the seine net. In spring 2021, additional sampling was executed in Rammegors using the same method. For this location, a seasonal comparison was made as well.
An extensive use by fish was observed for all four marshes. Thirteen species of fish were found to use the salt marshes studied in autumn 2020. These species used the marshes either as feeding ground, nursery ground, or permanent habitat. Locally, specific species like goby (Pomatoschistus spec.), eel (Anguilla anguilla), and juvenile mullet (Mugilidae spec.), were found to reach high numbers. In April 2021, seven species were caught in Rammegors. In May 2021, six species were caught. Seasonal differences observed in Rammegors were the presence or absence of certain species. An example is the high abundance of juvenile European flounder (Platichthys flesus) in spring 2021.
The constructed marshes seemed to largely serve fish the same way the natural marshes do, as all species present in the natural marshes were caught in the constructed marshes as well. An exception is the constructed marshes serving juvenile mullet (Mugilidae spec.), as this species was much more abundant in the natural marshes. This difference is most likely caused by the level of development of the constructed marshes, causing absence of favourable feeding conditions for mullet. In general, Rammegors seems to serve as a nursery ground for several fish species, whereas Perkpolder functions more as a foraging ground for juvenile fish and estuarine resident species. It was assumed this difference is caused by the differing abiotic conditions and lack of vegetation in Perkpolder.
Additional research is recommended, to get a better understanding of the ‘demands’ fish have to use a salt marsh for a specific purpose. This research could exist of additional sampling, combined with the mapping of vegetation, inundated area and abiotic conditions in the marsh. Another interesting subject that should be studied more extensively are the eel in Rammegors, as this species seems to use this location in high numbers.