Fish farm environmental impacts

Mathematical models

Marine Scotland Science (MSS) uses a range of mathematical models to determine the environmental effect that fish farming will have either on a whole loch system or on an individual site basis.

The loch models are primarily used for the Locational Guidelines for the Authorisation of Marine Fish Farms in Scottish Waters ( Locational Guidelines) as one of the tools used to determine the environmental sensitivity of an area to aquaculture development with regard to nutrient input and benthic impact.

Fish farming, or aquaculture, companies are required to apply for a licence to discharge wastes into the sea (under the Control of Pollution Act 1974), also known as discharge consents. SEPA processes these CoPA licences and the loch models are used in conjunction with other site-specific models to aid FRS in the statutory consultancy process for these licences.

The Loch Models

The loch nutrient model is a very simple box model that estimates the enhancement of dissolved nitrogen above background levels within a loch system; this is known as the Equilibrium Concentration Enhancement (ECE).
The model uses the simple equation:

ECE = S.M/Q

where:

• S is the rate at which nutrient nitrogen is discharged (kg/tonne production/year). This varies according to the species being stocked.

• M is the total maximum consented biomass of all the farms in the loch (tonnes)
  Q is the flushing rate of the Loch, calculated from the hydrographic characteristics of the loch (m3year).

The ECE value is then converted from kg m -3 to µmol l -1.

The loch benthic impact model uses a combination of equations, based on the Gowen model, to calculate the size and dimensions of the area covered by solid waste fall-out from the fish farm (waste food and faeces). The results are presented as a percentage of the whole surface area of the loch that are either impacted or degraded by carbon deposition.

Both the above models are run within a database that contains the hydrographic information on all the lochs covered by the Locational Guidelines and site specific fish farm information, such as the maximum consented biomass and the species of fish produced. The models are run for all the lochs simultaneously, for each new CoPA discharge consent and for each three month review of the Locational Guidelines.

Open Water Models

Sites that are not within sea lochs use a different suite of models to calculate the possible effects a new site or a modification to an old site will have on the environment.

As part of the consultancy process, CoPA licence applications from sites in open water use a site-specific benthic impact model. Information from the application form, hydrographic report and the FRS fish farm database are input into a data file. These details are then used to run a particle tracking ('random-walk') model which simulates the dispersion of particulate waste from the farm and its subsequent settlement onto the seabed. The results fromthe model are presented as the total area that is either impacted (>0.02KgC/m 2/yr) or degraded (>0.7KgC/m 2/yr) by waste fall-out from the farm.

For sites in open water, a Gaussian dispersion model is used. This tracks the patches of chemical over a gridded area representing the loch. The model is re-run, using different released quantities of medicine, until the predicted concentration fields after the stipulated time do not breach the Maximum Allowable Concentration (MAC) and Allowable Zone of Effect (AZE).

Medicine Dispersion Models

Environmental Quality Standards (EQSs) are set to ensure that concentrations of certain chemicals released in to the environment are below those likely to cause environmental effects.

For sites within lochs the acceptable levels of chemo-theraputant a farm is allowed to use in any 24 hour period, and also the total number of treatments allowed in one year, without subsequent concentrations exceeding the specified EQS are calculated within the FRS Database, mentioned under Open Water Models, above.

[Schematic diagram of the idealised sea loch assumed by the simple box models. The sea loch is represented by a rectangular basin with identical physical characteristics to the real system. The tidal current amplitude decreases from Uo at the mouth to zero at the head, with values at each site, Us, calculated accordingly. Fish farms are represented by the filled rectangular block, with the areas of impacted seabed denoted by the filled ellipses.]

For sites in open water a Gaussian dispersion model is used to track the patches of chemical over a gridded area representing the loch. This model is re-run using different released quantities of medicine until the area where maximum concentration is exceeded, and the maximum concentration after the specified time do not exceed the EQS. A mathematical model of a predicted nutrient enhancement as a function of total consented biomass is also available.

The Reports

The information from all the models run for a new CoPA application are collated into a report, which forms part of the environmental assessment that FRS provide to The Scottish Environment Protection Agency ( SEPA) as a statutory consultee. The results of the loch models are also used to designate the Category of the loch for the Locational Guidelines for Fish Farms, whiich is reviewed every three months.