Sediment transport and dispersal during mussel fishery

Mussel fishery in Denmark is heavily regulated and controlled in Natura2000 areas under the Danish mussel policy. Restrictions such as gear size and weight, quotas and limit in the amount of area impacted have been established in order to protect key ecosystem components such as macroalgae and benthic organisms.

For eelgrass, no tolerance is accepted, fishery is restricted below 5m depth in the eelgrass zone, and a buffer zone of 300m is established around the eelgrass where no fishery is allowed to take place. In order to assess the potential indirect effects of dredging operations such as: i) reduction of light availability to the eelgrass beds due to sediment resuspension during and after dredging ii) smothering and burial of eelgrass seedling and beds from transport and sedimentation coming from sediment plumes, two sets of experiments were conducted in the Limfjorden in two different broads and with different fishery intensity.

A range of water quality parameters was measured such as: light intensity, suspended particulate matter concentration, sedimentation, turbidity, oxygen and chlorophyll concentrations were measured under natural condition versus various fishing intensity at different distance from the fished areas. Field data showed that the resuspended material signal from mussel seed fishery activity is highly dependent on the current direction and the fishing practice.

Collected data was used to calibrate a sediment transport module in the Flexsem model developed for the Limfjorden. The model describes the transport of different size fractions of sediment particles under different fishing scenarios and will be used as a new tool for managing fisheries and help to decide on an appropriate buffer zone to protect the existing eelgrass beds from indirect impacts from mussel seed fisheries.