New wave
Model chooses best sites in Scotland for salmon farming
THE Scottish salmon farming industry’s expansion has been limited by concerns over its potential environmental impact – on wild fish stocks and on the seabed.
The other limiting factor is that many of the easiest to operate, sheltered locations are reasonably well filled, said Thomas Adams of SAMS (Scottish Association for Marine Science).
He has been evaluating the environmental conditions required for the development of offshore aquaculture, as part of the Off-Aqua project.
A particular environmental challenge in salmon aquaculture is posed by sea lice, he said. Enhanced availability of host fish at farms can allow lice to reach much greater numbers than they would naturally.
Dispersive environments in an offshore setting will reduce the pressure of sea lice from salmon farms, with reduced interaction with wild fish as well as increased dispersal.
The hope, too, is that more exposed environments will be more dispersive of waste and chemical treatments so they don’t have so much of an impact on the seabed.
And thirdly, the hope is that these environments will be less impacted by harmful algal blooms (HABs) which tend to proliferate in enclosed sea lochs.
There are a range of issues associated with moving salmon production sites to more exposed locations in Scottish waters, including physical, ecological, economic and fish welfare issues, and the Off-Aqua project includes four work packages:
• Physical oceanography – detailed physical observations at three contrasting sites, representing a spectrum of conditions and potential sites for development;
• Wave modelling and risk analysis – a long term (25-year) high resolution hindcast wave model will simulate conditions on the west coast with specific focus on sites of interest;
• Hydrodynamic and biophysical modelling (which is Adams’s work) – to evaluate sea lice connectivity and HAB development in contrasting environments;
• Fish health and welfare – open water aquaculture offers a less predictable environment than fjordic systems, but the impact on farmed animals has been little studied.
The research team has been physically modelling the west coast consistently for six and a half years with a meteorological-hydrodynamic model, with information on currents,
temperature, and salinity.
In 2019, the domain was expanded to incorporate more exposed environments. The aim is to show how outputs can be used by industry, regulators and other stakeholders to help guide management of new and existing sites.
A biological particle tracking model was used to simulate the spread of ‘sea lice’ larvae from three different farm sites Rum existing exposed farm ; Gorsten sheltered farm ; and a more intermediate site.
The sites also differ in the number of neighbours they have; the Rum site has very few neighbours, for instance, which in uences the spread of lice and retention whether they are able to re infect the site .
While the Rum site behaves relatively independently, sheltered sites are typically more connected to other sites and have higher ‘self infection’ rates, said Adams.
n the relationship of farms with their local environment, whether farms are net receivers of lice or dispersers of lice, and whether they suffer from large amounts of self infection, affects how they are best managed.
The next steps in the project will look to develop forecasting capability with the hydrodynamic model.
ore exposed sites offer an opportunity to reduce environmental impacts in terms of sea lice connectivity, with an associated reduction in outbreak frequency and risk to wild fish, said Adams.
They may also offer increased dispersion of excess organic material. But while physical conditions at more exposed sites generally lie within the range suitable for fish, they can pose operational di culties for site managers.
A range of factors must be taken into account when selecting the most sustainable approach to industry expansion, and choosing sites upon which to focus.