Quantifying settling rates of particulate wastes from aquaculture cages

Ongoing expansion of aquaculture in Scottish coastal waters, by unlocking additional salmon farming capacity and supporting sustainable development within the Scottish salmon sector, is balanced by requirements from the Scottish Environment Protection Agency (SEPA) to minimise environmental risk from this expansion. In this context, the fish farm waste deposition model NewDEPOMOD (SAMS, 2021), developed by the Scottish Association for Marine Science (SAMS), is the preferred regulatory tool to assess both ongoing compliance of existing fish farms and for the appraisal and licencing of new aquaculture applications. The predictive capabilities of NewDEPOMOD clearly rely on accurate physical representation and parameterisation of the release and movement of waste materials from aquaculture cages, as well as the spatial extent to which the seabed beneath and adjacent to the cages is adversely affected by deposited particulate wastes, such as uneaten food and faecal materials. At present, the settling velocities for waste feed and faeces are represented in NewDEPOMOD by distributions around a mean settling rate for each type of particulate wastes, which have only limited verification against measured settling data.
A series of laboratory experiments utilising a sophisticated grid-stirred settling column facility is conducted to better define the expected range of settling velocities for both types of particulate wastes and include, for the first time, the additional influence of turbulent flow conditions. Careful control of the turbulence intensity and shear rate generated in the column by varying the oscillation stroke and frequency of the grid array produces turbulent flow fields that can mimic both sheltered and exposed coastal marine environments in which aquaculture sites are deployed. Within the laboratory study, settling velocity data of both types of particulate wastes are captured through illumination and visualisation with digital CMOS cameras and application of a particle tracking technique using ImageJ software. A large and statistically-significant number of particles will be captured and measured, under different grid-generated turbulent flow conditions, to develop new improved distributions of settling rates for both particulate types (i.e. feed and faeces), as well as new relationships between settling velocity and turbulent shear rates.
The parameterisations from this laboratory work will be evaluated in NewDEPOMOD to ensure it is optimised for current aquaculture management practices and environmental protection. The settling velocity results can also inform the evaluation of current field-based methodologies to understand, and quantify, the percentage of feed that remains uneaten at any given farm site. Accurate quantification of this value is of significant interest to the wider aquaculture sector, and particularly in relation to emerging and innovative farming techniques such as semi-enclosed fish farm systems.