New tools to assess the environmental effects of mussel dredging

Autor: Saurel, Camille, Andersen, Kasper, Barreau, Pascal, Maar, Marie, Pastor Rollan, Ane, Larsen, Janus, Mohn, Christian, Murawski, Jens, She, Jun, Kjerulf Petersen, Jens
Jazyk: dánština
Rok vydání: 2021
Předmět:
Zdroj: Saurel, C, Andersen, K, Barreau, P, Maar, M, Pastor Rollan, A, Larsen, J, Mohn, C, Murawski, J, She, J & Kjerulf Petersen, J 2021, New tools to assess the environmental effects of mussel dredging . DTU Aqua Report, nr. 390-2021, DTU-Aqua . < https://www.aqua.dtu.dk/om_dtu_aqua/publikationer/rapporter/rapporter_siden_2008 >
Saurel, C, Andersen, K L, Barreau, P D A, Maar, M, Pastor, A, Larsen, J, Mohn, C, Murawski, J, She, J & Petersen, J K 2021, New tools to assess the environmental effects of mussel dredging . DTU Aqua-rapport, no. 390-2021, DTU Aqua, Nykøbing Mors, Denmark . < https://www.aqua.dtu.dk/english/-/media/institutter/aqua/publikationer/rapporter-352-400/390-2021_new-tools-to-assess-the-environmental-effects-of-mussel-dredging.pdf >
Popis: In order to improve the scientific basis for public management of mussel and oyster fisheries, it is the purpose of this investigation to develop new knowledge about the indirect effects on the marine environment of mussel and oyster dredging activities, and in particular to develop new tools and methods to assess the indirect fishery effects at the level of entire basins. Three main topics are studied: The extension and duration of sediment plumes caused by resuspension of sediments due to mussel dredging, the effects of transplantation of mussels from areas frequently exposed to oxygen depletion and links between fishery and recruitment of new spat to mussel beds.Sediment plumes created by dredging was studied as a combination of in situ experiments in Lovns Bredning and Løgstør Bredning in the Limfjorden, Denmark. In the field experiments, mussel fishermen dredged along specified tracks or in predefined areas, and water properties like light extinction, oxygen consumption, hydrodynamic conditions and turbidity were measured in relation to fishing intensity. Further, sediment characteristics of the test area were determined. Data from the field experiments were fed into a new sediment resuspension and plume dispersion model included in a newly established FlexSem framework model of the Limfjorden with a spatial resolution of 10 m in the studied areas. The FlexSem framework was coupled to a new hydrodynamic version of DMI’s ocean model HBM that had been expanded with a Limfjorden domain, and new modules were added. This new hydrodynamic setup enhances the forecasting capabilities from sea level predictions to forecasts of all physical ocean parameters with reasonably good quality predictions of temperatures, salinity and currents in high temporal and spatial resolution. Results showed that the plume has an extension of 260-540 m from the dredge track and with a duration of approximately one h. Results showed that the plume has an extension of 260-540 m from the dredge track and with a duration of approximately one h. The plume's extension depends on the sediment characteristics of the fished area defined by grain size distribution, hydrodynamic conditions and fishing intensity. In the scenario setting, it was decided to apply conservative estimates so the model results can be assumed to be the worst case.The effects of mussel transplantations from areas often exposed to oxygen depletion were studied as a combination of laboratory experiments determining oxygen consumption of decaying mussels and 3D coupled hydrodynamic-biochemical model, including hypoxia development of Lovns Bredning using the FlexSem framework. Based on data from the industries current relay praxis, scenarios of removal of 3600 and 6000 t mussel respectively were chosen as realistic. Modelled results showed that the transplantations on a short-term scale increased oxygen concentrations and reduced nutrient and Chl a concentration in the water column on a long-term scale. There was an immediate negative effect on mussel biomass but post the oxygen depletion, remaining mussels grew faster and thus compensated for the removed mussels. Field experiments measuring mussel recruitment on the bottom and in the water column on five different locations with fished vs un-fished plots could not demonstrate a direct link between mussel recruitment and fisheries. However, the field experiment suggested that the thinning of the mussel beds promoted growth measured as shell length of the remaining mussels. We suggest that this result combined with the analysis of fishery data could explain that fishery promotes growth in remaining mussel, making the beds sustain fisheries for 2-3 years under the current fishing pressure. A modelling approach was used to study the mussel larvae dispersal, and the connectivity between the different broads of the Limfjorden using an Agent Based Model and the Limfjorden physical model. Modelled results indicated that mussel larvae were present in most of the broads, and this was validated by in situ measurement of mussel recruitment in the water column. Field measurements results indicated that many mussel larvae were recruited in the water column, but failed to recruit at the bottom. This confirms the complexity of the bottom mussel recruitment dynamic where other factors such as predators, intra and inter-competition for space and food reduce the chances of spat survival.Following to the Danish mussel policy direction on improving mussel management, the presented results were used as background for new management recommendations. Modelling and field measurements showed that the sediment plume produced by mussel fishing activity extended 260-540m away from the dredge track and could last up to 1h in the water column. The modelled cumulative sedimentation from fishery events was below natural sedimentation. Together with the given low fishing activities (number of boats and fishing days per year) near the eelgrass boxes in Lovns Bredning, sedimentation was not considered as an indirect negative effect on eelgrass growth and development. Therefore, it was recommended to reduce the eelgrass boxes buffer from 300 m to 100 m in the management plans of Lovns bredning. Hence, a buffer between these zones is defined as protection against physical damage rather than indirect effects from resuspended sediment. Accordingly, for the fishing season 2020-2021, the buffer zone was reduced to 100m in the study area. It is further recommended that management include positive effects when considering permits for mussel transplantations from eutrophic ecosystem with frequent hypoxia, where removal of mussel biomass by mussel fishery could be used as hypoxia mitigation tool to improve water quality.
Databáze: OpenAIRE
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