Predictive depositional modelling (DEPOMOD) of the interactive effect of current flow and resuspension on ecological impacts beneath salmon farms

Nb Keeley,Eo Goodwin,Cj Cromey,Cm Macleod,Mt Gibbs

doi:10.3354/aei00068

Abstract

ABSTRACT: Sediment resuspension is an important factor in controlling the impact of any localised point source impacts such as salmon farms; at high-flow (dispersive) sites, resuspension can significantly reduce potential effects. Depositional modelling (DEPOMOD) is widely used to predict localised seabed impacts and includes an optional flow-related resuspension module. This study examined the observed impacts at 5 farms with contrasting flow regimes to evaluate the role of modelled resuspension dynamics in determining impacts. When resuspension was included in the model, net particle export (i.e. no significant net downward flux of organic material) was predicted at the most dispersive sites. However, significant seabed effects were observed, suggesting that although the model outputs were theoretically plausible, they were inconsistent with the observational data. When the model was run without resuspension, the results were consistent with the field survey data. This retrospective validation allows a more realistic estimation of the depositional flux required, suggesting that approximately twice the flux was needed to induce an effect level at the dispersive sites equivalent to that at the non-dispersive sites. Moderate enrichment was associated with a flux of ~0.4 and ~1 kg m−2 yr−1, whilst highly enriched conditions occurred in response to 6 and 13 kg m−2 yr−1, for low and dispersive sites, respectively. This study shows that the association between current flow, sediment resuspension and ecological impacts is more complex than presently encapsulated within DEPOMOD. Consequently, where depositional models are employed at dispersive sites, validation data should be obtained to ensure that the impacts are accurately predicted.

Highlights

Aquaculture, and in particular sea-cage fish farming, is a significant primary industry that is undergoing rapid expansion worldwide
DEPOMOD (Cromey et al 2002a) is probably the most established and widely used depositional model for the purposes of predicting salmon farm effects, largely because it has been proven in a wide range of environments and is considered to be robust and credible (SEPA 2005, ASC 2012)
DEPOMOD was developed for salmon farming in cool temperate systems, it has been applied successfully to cod farming (CODMOD, Cromey et al 2009), and to both warm-temperate culture of sea bream and bass (i.e. MERAMOD, Cromey et al 2012) and more recently tropical fish-culture

Summary

Introduction

Aquaculture, and in particular sea-cage fish farming, is a significant primary industry that is undergoing rapid expansion worldwide. Some of the key input parameters that are required, such as observations of current dynamics, bathymetry and basic farming practice information (e.g. cage layout, feed characteristics and input rates), are relatively easy to obtain, whilst others can be more difficult to quantify (e.g. feed wastage, critical erosion thresholds). In these latter cases, default data can be employed as long as the model is not overly sensitive to these parameters. The validation process for these new applications was relatively straightforward and only required site-specific data and the inclusion of a few new processes (e.g. wild fish feeding), indicating that the physical components were on the whole comparable and transferable

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Results