The most important effects of the mussel seed fishery on the natural values of mussel banks.
Mussel seed fishing has short-term effects on the natural values of wild banks. The observed differences between areas that were closed and open to fishing were temporary due to the natural dynamics. Moreover, the research has revealed that not only wild mussel banks, but also parcels with cultured mussels, are rich in benthic animals and fish: these are “hot spots” for biodiversity in the western Wadden Sea.
At least as many species were observed on parcels with cultured mussels as on the wild banks. This indicates that transferring mussels from the mussel banks to the cultured parcels does not harm biodiversity. However, most of the cultured parcels are located closer to the North Sea than the wild banks. In these areas, the salinity of the water is higher. This is beneficial for the biodiversity of the parcels.
The research into the effects of mussel seed fishing was conducted by comparing the sections of the study sites that were closed and open to fishing. As part of this research, a distinction was made between fishing on unstable banks in the autumn and fishing on the remaining, relatively stable banks in the spring. Unstable means that these banks will probably disappear before spring due to predation by starfish and/or winter storms. Harvesting mussel seed from banks in the autumn results in lower mussel stocks on these banks. However, the study revealed that the mussel stocks declined sharply during the winter not only in the open sections, but also in the closed ones. This reflects the decision to fish on the relatively unstable banks in the autumn, where the likelihood of survival is smaller. Mussel seed fishing in the spring, which takes place in relatively stable areas, leads to significantly lower mussel stocks in the open sections relative to sections that are closed to fishing.
The consequences of mussel seed fishing for natural values are directly related to the effects on the mussels themselves, and thus to the differences between the autumn and spring fisheries. After the autumn fishery, for virtually all variables, no differences could be demonstrated between open and closed sections. This also applied to the mussel stocks. However, clear effects occurred immediately after the spring fishery: fewer benthic animals and fish were observed on the sections that were open to fishing, and mussel stocks declined as well. This effect continued for up to two years. Subsequently, the differences between open and closed sections became insignificant. This is because few mussels ultimately survive in the closed sections as well. Mussel seed fishing in the spring therefore affects benthic animals and mussel stocks adversely, but only for the short term.
Natura 2000 policy contains the following improvement objective: “...the presence of various stages of development on part of the mussel beds”. The PRODUS research has shown that closing areas for mussel seed fishing does not mean that mussel banks will develop by themselves. Other factors, such as predation by starfish, also affect this development. The knowledge that has resulted from this research can be used to develop measures to benefit nature management and sustainable mussel culture.