Development of mussel stocks
After the spring fishery, the mussel stocks differ between the sections that are closed and open to fishing, but not after the autumn fishery. The research also shows that mussel seed fishing does not have any negative effects on the formation of new mussel banks.
A comparison was made between the development of the mussel stocks on 40 sections that were closed to mussel seed fishing and on the adjoining 40 sections that were open.
Relative to the closed sections, significantly fewer mussels were observed after the spring fishery on the sections that were open to fishing. The difference between these two sections was still observable up to two years after the spring fishery. Subsequently, no difference was observed because mussels on the closed sections also disappeared. In the autumn, mussel seed fishing takes place on unstable banks; at these locations, no difference in mussel stocks was demonstrated between sections that were open and closed to fishing. Most of these unstable banks also disappear without fishing, because the mussels are washed away and/or eaten by starfish.
New mussel seed banks are created by spatfall (planktonic mussel larvae that fasten themselves to solid substrates). One of the questions was whether the mussel seed fishery affects this process, for example because the mussel spatfall is better on existing banks since the new spat can fasten themselves to the existing mussels. The research has shown that large-scale spatfall, which takes place every two years on average, is not dependent on existing mussel banks, and is consequently not affected by the mussel seed fishery. Although low density spatfall, known as the background spatfall, is higher in existing mussel banks, in densely populated banks the spatfall does not increase any further. No effects of the mussel seed fishery on this type of spatfall have been demonstrated either; this is understandable because some mussels always remain behind after mussel seed is harvested. The conclusion of this part of the research is that mussel seed fishing does not have any negative effects on the establishment of new mussel seed banks.