Modelling the environmental impacts of suspended mussel (Mytilus edulis L.) farming
The potential impacts of marine aquaculture operations on the environment are reviewed. The reported effects of suspended mussel farms on the benthic environment are examined and the potential impacts discussed. A framework to assess the impacts of suspended mussel farms is presented. The use of simulation models to predict the impact of fish farm wastes on the benthic environment is discussed and the mathematical theory supporting such models is presented. The applicability of these models to mussel farming is discussed and the data required to undertake such modelling identified. The effect of increased sedimentation on the macrobenthic community, physical structure and biogeochemistry of the surficial sediment around three suspended mussel farms are examined. At one site, the benthic community was subjected to bulk sedimentation, organic enrichment and reduced macrobenthic infaunal diversity. Elevated levels of organic carbon were recorded close to the farm. At the remaining two sites, benthic impacts were less clear and not demonstrably due to the mussel farms. The settling velocity of mussel faeces and pseudofaeces was required to enable modelling of particles ejected from the farm sites. An experiment was devised to measure this parameter.- The settling velocity of mussel faeces (~0.5 cms-I ) was less than pseudofaeces (~1 cms-I ). Differences in these settling velocities were attributed to the organic content and particle size of the excreted matter. The particle tracking model DEPOMOD (Cromey et at., 2000a) was used as a platform from which to develop a simulation model predicting the benthic impact of suspended mussel farms. Parameters within the model were modified to be represent a mussel farming scenario. Data from the three sites surveyed were applied to the model. Although the model results compared favourably with the field data, the model tended to overestimate the benthic impact as measured by the Infaunal Trophic Index. The results of the model are discussed and improvements and further experiments are identified.