Bivalve molluscs such as mussels (Mytilus edulis) and oysters (Ostrea edulis) are key species in coastal ecosystems, and contribute to increased biodiversity by creating habitats for other organ-isms and to several other important ecosystem services. In recent years, both Mytilus- and Os-trea-beds have been reported to be in decline in Europe, and there are strong indications of a similar situation also for Mytilus in Sweden.
For Ostrea, there is a general lack of knowledge about population sizes, which makes assessments of population development problematic. Conservation measures such as stock enhancement and restoration are therefore increasing as biodiversity is lost, and there is a growing interest in restoring habitats created by mussels and oysters in many parts of the world.
The threat analysis carried out by OSPAR for Mytilus- and Ostrea-beds in Europe indicates that the primary cause of loss of bivalves in Europe is overexploitation and habitat loss, as well as disease outbreaks caused by pathogens. The Swedish bivalve populations, however, are more or less spared from these impact factors. Nevertheless, other factors can also affect population de-velopment. For example, there is concern among management authorities that the invasive Pacific oyster may impact the populations of Mytilus and Ostrea in Sweden negatively.
All in all, there are good reasons to explore the possible alternatives for supporting the development of the Mytilus- and Ostrea-populations, and also methods that could be used if the Pacific oyster prove to impact the native bivalve populations negatively. The purpose of this project was therefore to produce a knowledge base for the establishment of management models for the pro-tection of Sweden's populations of Mytilus and Ostrea. Part of this work include knowledge building, both theoretical and practical, about possible restoration measures of bivalve beds, e.g. removal of Pacific oysters or stock enhancement.
To summarise, a number of performed activities are discussed in this report, including a removal experiment in which Pacific oysters were removed from both Mytilus- and Ostrea-beds, a stock enhancement experiment with Ostrea in which juveniles of Ostrea were placed on different sites, and several different studies on stock enhancement of Mytilus, including studies of biological conditions and methodology for population enhancement.
The removal experiments demonstrated that it was possible to clear Pacific oysters from bivalve beds but that this is time consuming and complicated, especially in Mytilus beds. In terms of numbers of oysters the effect of the removal was short-lived. Already one year after the removal, the number of Pacific oysters was equal, or higher, to the number before the experiment. In terms of biomass, however, many of the cleared sites did not show complete recovery och the biomass of the invasive species. How long the recolonization time was before the original biomass was re-established is unknown, but calculations indicate that this time is longer than for the abundance of oysters. Better technologies must be developed for more resource-efficient efforts if this type of activity is to be carried out regularly. Additionally, ways to utilize the harvested biomass of Pacific oysters must be established.
The stock enhancement experiments also showed that it was possible to re-establish bivalve beds on a pilot scale. Even in sites with good conditions, however, the target species will decrease after relaying. In the experiments performed, the survival of Ostrea (about 30-50 mm in length when re-laid) on the best sites was only about 16% after 2 years, and for Mytilus the cover of mussels (a mixture of large and small mussels) decreased by almost 50% in one year and the survival was about 20% after this period of time. In general, predation was observed to be a problem for both species. Small and medium-sized Mytilus were greatly affected by eider duck, while shore crabs caused high mortality for Ostrea. Site-specific conditions were also of great importance for the success of the experiments.
Examples of factors discussed in this report are, in addition to the presence of predators, also ice-cover, substrate, the possibility of moving in organisms for resto-ration experiments/activities, depth and exposure. It is obvious that much can be gained from solid preparatory work before stock enhancement activities are initiated. The experiences from the project also illustrate the importance of protection against exploitation of the restored populations and a good relationship with authorities and landowners.
A unique activity in the project that, to our knowledge, has not been tested before is the use of residual mussels from commercial mussel production for stock enhancement of Mytilus. By using waste mussels, increased circularity and improved resource utilization in aquaculture produc-tion can be achieved, with both ecological and economic gains as important natural environments are recreated and costs for waste management from production is reduced for the aquaculture companies.
However, it remains to be explored how extensive this type of activity can be as the goal in mussel farming is, of course, to produce a product without fouling.
In addition, the regulatory conditions for this type of use must be established.Finally, the report discusses a number of development needs that should be studied further be-fore future stock enhancement experiments. Among these aspects, monitoring of ongoing activi-ties and success rate of restoration should be implemented, guidelines for site selection should be established, optimized techniques for restoration adapted to local conditions should be devel-oped, and the ecosystem services that the bivalve beds can support should be documented in future work.
An important part of this is to learn from existing knowledge but also to put this knowledge into a local context with the goal of identifying important knowledge gaps for context relevant activities. Combined, these activities can be a first step towards establishing a manual for the restoration of mussels and oysters in Sweden.