Clearance rate (CR) was measured in blue mussels Mytilus edulis L. from Aiguillon Bay and the Oosterschelde using 3 different methods: the flow-through method, the bio-deposition method and the indirect or clearance method. CR differed significantly as a function of the method used and of the origin of the mussels. CR measured with the bio-deposition method were significantly lower than rates measured with the other methods. Results for the flow-through method depended, however, on how CR was calculated. CR using the flow-through and indirect methods was on average 10.0 l g -1 h -1 in mussels from Aiguillon Bay and 5.3 l g -1 h -1 in mussels from the Oosterschelde. The significantly lower CR of mussels from Oosterschelde was related to condition index and gill area, but could not entirely be explained by these factors.KEY WORDS: Clearance rate · Intercalibration · Method · Mussels · Gill area · ConditionResale or republication not permitted without written consent of the publisher † This paper is written in memory of Serge Bougrier, who died tragically during the project.

it might take decades before significant improvements are observed, and in many areas, land-based mitigation might not be sufficient to ensure good water quality as required by, for example, the EU Water Framework Directive (WFD). Furthermore,

There are plans to increase offshore aquaculture production of rainbow trout in the inner Danish waters, with an expected annual release into the environment of 100 t N and 12.8 t P per farm. We applied 3D modelling to estimate the spatial and temporal effects of 2 hypothetical offshore fish farms on water quality, sediment fluxes and nutrient transports in the Samsø Belt area. The model results showed that water quality (mainly dissolved inorganic N and primary production) was affected in open waters during the production period, leaving no traces 1 mo after the production period. Changes in bottom water oxygen levels and Secchi depths were not considered important for the system. Sediment organic content and nutrient fluxes increased from reference conditions locally below the fish cages. The sediment did not recover from the impact between the production periods, especially at the site with lowest resuspension. There was a gross transport of nutrients into the coastal water areas corresponding to 21% N and 16% P of the farm inputs, but this only resulted in minor changes in water quality. However, the additional nutrients from the fish farms counteract implemented nutrient reductions from other sources which aim to improve the ecological status, and the excessive nutrients were shown to reach several protected Natura 2000 sites. Future implementation of fish farms in the area requires detailed spatial planning, optimisation of farm design and nutrient compensation by marine measures, e.g. blue mussel farming.

Mussel dredging causes resuspension of sediment particles that reduce water clarity and potentially leads to reduced eelgrass growth. In order to study the impact of resuspension from mussel dredging on light conditions in the water column, field experiments were conducted at two sites in the Limfjorden. Light loggers were placed in two circular arrays around the dredge area. Vertical profiles of current velocity were measured by an ADCP and the sediment particle size composition was obtained from sediment core samples. The field data was used to force, calibrate and validate a sediment transport model developed in the FlexSem model system. Changes in sediment concentrations during and after mussel dredging were modeled for the two sites and for seven scenarios. We found that the distance and direction of the plume in the model was in good agreement with light logger data. The plume duration was less than 1 h, and the impact range was between 260-540 m. The scenarios showed that fishing intensity and current speeds were most important for shaping the sediment plumes. Changes in suspended sediment concentrations were 0.62-1.79 mg l −1 on median average and 1.22-11.61 mg l −1 for the upper quantile of the plume, which were on the same order of magnitude as background values in the Limfjorden. The amount of fishing days during the eelgrass growth season was 6-8% in Lovns Bredning and 16-35% in Løgstør Bredning and less than 1-2% of the total area was dredged per season. Even though there are substantial changes in the light conditions from the sediment plumes, the overall spatio-temporal impact in the study area is considered low. We recommend that management plans in other areas could sustain a shellfish fishery by limiting fishing intensity and frequency near eelgrass beds. The presented approach combines observational data, sediment transport modeling and reported fishing activity. It is a step forward within sediment transport modeling and could be incorporated into environmental impact assessments. The results have recently been used as scientific background for recommendations to improve the management plans according to the Danish Mussel Policy and relevant EU Directives.

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