Test of a model that predicts benthic impact of salmon netpen aquaculture

Robert H. Findlay (USA)

ABSTRACT

An iterative process of model construction and empirical measurements leading to a testable, predictive model was conducted. The effort began with a conceptual model that was used to evaluate previously published studies, predict critical missing data and structure initial experimental designs. This model predicted that benthic carbon flux and aerobic respiration rates would be critical to predicting benthic impact. The endpoint of the formation of Beggiatoa mats was deemed the appropriate endpoint indicating unacceptable degradation of the benthic environment for Maine coastal waters. A range benthic variables (geochemical, microbiological and macroinvertebrate) measured over time beneath fish cages (4 separate sites) and at an adjacent (+100m) ambient sites (equal number of observations at each site type) showed that benthic O₂ consumption and CO₂ production were the only variables with a strong correlation to organic carbon sedimentation. The formation of Beggiatoa mats was linked to lack of oxygen when current speed was reduced for 2 h or longer during a tidal cycle. A non-linear model relating minimal current speed to oxygen and carbon delivery was developed from these findings. This model predicted that the formation of Beggiatoa mats only occurs when oxygen supply is reduced below the threshold level required to oxidize sedimented organic matter. This prediction was tested and found to be accurate at 3 additional net-pen site in coastal Maine. The work demonstrates that current plays a major role not only in determining deposition and resuspension, but also in delivering O₂ to oxidize benthic carbon. Minimization of local benthic impact requires that benthic carbon flux not exceed the ability of the benthos to aerobically degrade the sedimented carbon. The success of this approach is based on model constructions that formulate all assumptions as testable hypotheses. This work also demonstrates the importance of an integrative process where both models and empirical measurement are used as management tools for understanding the role of aquaculture in the marine environment.