Effects of Seed Mussel Fishery are More Subtle

19-04-2013   Some incorrect conclusions have been drawn on the basis of reports, released earlier this week, on the influence of seed mussel fishery in the Wadden Sea. NIOZ and IMARES Wageningen UR, having carried out much of this research project, state that the conclusions are more subtle.

The effects of seed mussel fishery and mussel farming on the natural values of underwater mussel beds have been studied in the past six years in the PRODUS project. The research institute IMARES supervised this project. IMARES issued a press release on 15 April after the final report had been published. Elements of this press release were phrased in such a way that some readers may have drawn conclusions that do not accord with the results of the research project.

The scientific research results are solid, but what is crucial is the interpretation of these results. The interpretation should perhaps have been phrased differently in the original press release.

Part of the research project was a comparison of sites where seed mussels were fished and sites where they were not. As many sites were in so-called unstable areas, mussels as well as other organisms also disappeared from the non-fished sites. The number of sites where the mussels remained was small (only on 3 out of 36 testing sites), but this does show that mussel beds can develop themselves, especially when the seed mussels are not fished.

In addition, farmed mussel beds were compared to wild mussel beds. The conclusion that mussels thrive equally well on farmed beds as on wild mussel beds creates the impression that farmed mussel beds are beneficial to the environment. They might be, but the reason that mussels thrive on farmed beds is mainly that these beds are in areas with higher levels of salinity, where more species can be found naturally. The wild mussel beds that were analysed are in areas with lower levels of salinity, where fewer species can be found naturally.

Finally, what we actually need to know is what would happen if parts of the Wadden Sea, and mussel beds in particular, were completely left in peace. This would mean that no human activities that may affect mussel beds would be allowed and that these beds should be monitored for years. This has not been done and these conditions are difficult to realize.

The conclusions NIOZ and IMARES have drawn are that on the basis of these research results it is not possible to claim that seed mussel fishery does not harm mussel beds in the Wadden Sea.

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More information:
Nienke Bloksma, NIOZ Communication: +31 (0)222 369 460 or +31 (0)6 53 49 47 14
Hans Bothe, IMARES Communication, +31 (0)6 10 67 40 06

PRODUS website

Some explanatory remarks on these conclusions
The PRODUS research project consisted of two parts. The first part was a long-term experiment on 36 research sites, selected from the wild mussel beds in the western Wadden Sea. Each site had been divided into two halves: one of these halves could be fished, the other one could not. These sites have been monitored for a couple of years: it was not only the mussels that were monitored, but also the sites that could be fished and the sites that could not be fished were also monitored and compared with regard to other fauna, including fish and other benthic animals.

Some of these research sites were on the so-called unstable sandbanks, while others were on stable sandbanks. The crucial difference is that unstable sandbanks are in areas where mussels are unlikely to survive the first winter, as the initial press release said. These banks are therefore fished in autumn.

Six months are the first fishing activities in the autumn, no significant difference could be seen between the sites where fishing was allowed and where it was not. On stable sandbanks, the differences persisted for a longer period: after two years the differences were still visible.

The fact that there are no significant differences were found between the averages in both groups is not telling in itself. On both the unstable sandbanks and on the stable ones, there turned out to be a total of three sites (out of the 36 sites analysed) where - as the final report put it  - after several years there were still 'exceptionally large numbers of mussels'. The report shows that this was especially on the sites that had not been fished. This suggests that long-term underwater mussel beds may be able to develop when seed mussel fishery is stopped. This may be an important issue in discussions on the ‘Mussel Covenant’.

The second part of the PRODUS research project focused on a comparison of biodiversity in farmed mussel beds and wild mussel beds. The latter are mainly near the Afsluitdijk, where there is more fresh water. Many animals, such as starfish, avoid going there because of the fresh water. The absence of starfish, which like seed mussels, may actually be one of the reasons why the mussel beds developed in that particular area. The biodiversity on farmed mussel beds, which are mainly in areas with salter water, turned out to be roughly equal to that of the wild beds.

The original press release claimed that relaying young mussels on farmed beds has a positive effect on the biodiversity of the Wadden Sea under the current conservation programme. This may be true. In 33 out of 36 sites, the mussels would most probably have disappeared if left to their own devices. Under the current programme, they are all transported to the beds and then indeed contribute to the biodiversity. But what if the current conservation programme is be replaced by a programme where the mussels are left in peace? Then in 3 out of 36 cases, mussel beds would develop where the biodiversity is considerably higher than in the farmed beds, especially if these beds were in areas with salter water than where the beds are now.

It is precisely because the settlement of these long-term wild mussel beds is relatively rare, that PRODUS could not provide a sound answer to the questions where and how often this might happen, as the results are based on less than 40 small sites that were free from seed mussel fishery. However, the result that long-term beds can develop and that the biodiversity of wild beds is likely to be higher in areas with higher levels of salinity than on the farmed beds is sufficiently important to be incorporated into further discussions concerning the ‘Mussel Covenant’.