Shellfish-DEPOMOD: Modelling the biodeposition from suspended shellfish aquaculture and assessing benthic effects

Abstract

By predicting the dispersal of particulate aquaculture wastes around farm sites, numerical modelling can provide an effective tool to assess the spatial extent of environmental effects. The present paper describes how the aquaculture waste model DEPOMOD (Cromey, C.J., Nickell, T.D., Black, K.D. 2002a. DEPOMOD — modelling the deposition and biological effects of waste solids from marine cage farms. Aquaculture 214, 211–239.), originally developed for finfish aquaculture sites, was adapted and validated for suspended shellfish aquaculture. Field data were collected for species-specific model input parameters (mussel biodeposition rates and particle settling velocities) and several finfish model parameters (farm representation and calculation of aquaculture wastes) were adjusted for the shellfish scenario. Shellfish-DEPOMOD was tested at three coastal mussel Mytilus edulis farms with differing hydrodynamic regimes in Quebec, Canada. For each site, model predictions were compared to observed deposition measured in situ with sediment traps. Sedimentation rates under the three mussel culture sites were ca. two to five times those observed at corresponding reference sites. Mussel biodeposits were predicted to accumulate within 30 m of the farms in the shallow depositional sites while being dispersed more than 90 m in the deeper dispersive site. At the farm site in Great-Entry Lagoon, model predictions agreed well with field data for the 0+ and 1+ mussel cohorts when the maximum biodeposit production parameter was used. At the farm site in House-Harbour Lagoon, model predictions did not agree with observed sedimentation rates, due most likely to the resuspension and advection of non farm-derived material and complex hydrodynamics. The model correctly predicted the pattern of waste dispersal at the third farm site in Cascapedia Bay, although it underestimated biodeposition. Predicted fluxes may have been underestimated at this site because biodeposits from biofouling communities were not included in the calculation of aquaculture wastes. The relationship between modelled long-term biodeposition and benthic descriptors was assessed for the three farms. Alterations to the benthic community were observed at high biodeposition rates (>15 g m−2 d−1). At the most disturbed site, predicted fluxes were best correlated with the Infaunal Trophic Index (ITI) (R=−0.79, P<0.001), followed by AZTI's marine disturbance index (AMBI) (R=0.64, P<0.001). The potential application of Shellfish-DEPOMOD in terms of the management of shellfish aquaculture sites is discussed.