The main objective of this thesis was to study some of the most important factors that influence filtration and bioenergetics of bivalves and to identify some of the future challenges and possibilities in Danish long-line mussel production. In the laboratory, I studied the effects of current velocity and Diarrhetic Shellfish Toxins (DST) containing algae on the filtration rate of blue mussels (Mytilus edulis). In the field at natural conditions, I studied mussel growth and food depletion. Furthermore, the nutrient removing potential and cost-effectiveness of mitigation mussel production was investigated.
The effect of current velocity (0.05-1.4 m s-1) on the clearance rate of M. edulis was studied using an automatic setup, which controlled and monitored the algal concentration continuously. The effect of current velocity was studied on either three or twenty mussels. Current velocities of up to 1.4 m s-1 did not affect the clearance rates of M. edulis when twenty mussels were allowed to form self-organized clumps. However, when only three M. edulis were present, mussels became inactive at current velocities >0.6 m s-1. The tolerance to high current velocities provides opportunities for offshore production of blue mussels in Denmark since current velocities in the Danish coastal waters in general are below 1.4 m s-1.
The effect of DST on clearance and respiration rate of M. edulis was studied in terms of exposures to equivalent bio-volume cell concentrations of either the non-toxic algal Rhodomonas salina or the DST-containing algal Dinophysis acuta. Our results showed that the clearance rate were reduced for mussels exposed to DST-containing D. acuta compared to mussels exposed to equivalent bio-volume cell concentrations of the non-toxic R. salina. Furthermore, closure of shell-valves was observed to occur earlier as well as the reduction in clearance rate became more pronounced with increasing concentrations of D. acuta. In addition, the total amount of DST accumulated in the mussels exceeded the regulatory limit for human consumption. However, DST-containing D. acuta did not have a severe effect on respiration rate. We concluded that DST most likely caused the reduction in clearance rate. Therefore DST-containing mussels can be a latent problem for mussel growth in future Danish offshore mussel production, as it is in other offshore mussel productions in Europe.
Mussel growth and food depletion was studied within a mussel farm, located in the highly eutrophic Skive Fjord, Denmark. Using a variety of methods we obtained information on spatial (organism to farm scales) and temporal (seasonal) variations in food depletion. The results obtained by the different methods were integrated into a depletion model for the mussel farm. The results showed that high mussel growth rate was present during most of the year and that food depletion only occurred at different spatial scales within the farm. Furthermore, we showed that the mussel farm studied was under-utilized in terms of production of mussel biomass and a surplus of food was available to support the growth of more mussels than already produced.
Finally, we investigated the potential of using mussels as a mitigation tool for removal of excess nutrients in eutrophic coastal waters. A full-scale long-line mussel farm optimized for cost efficient nutrient removal was established in Skive Fjord where biological and economic parameters related to nutrient removal were monitored throughout a full production cycle. We concluded that mitigation mussel cultures are a cost-effective measure for nutrient removal in coastal areas and can be used as an alternative or supplement to land-based mitigation measures.