Scales and boundary conditions for aquaculture

Scales

In 1994, the UK's Comprehensive Studies Task Team defined 3 zones or scales relating to point source discharges. The idea of scales has proven useful in relation to managing a variety of environmental pressures and impacts. These zones are shown in the diagram, as they relate to aquacultural sites marked by black dots. The essential differences between the scales are in the residence times of particles and water. These defining time scales link to corresponding spatial scales

zone A - local to the farm; water residence times of a few hours; in regions of low or moderate dispersion, particles fall to the sea-bed within this zone, and their footprint effectively defines zone A; in mesotidal and microtidal waters a larger zone A+ will be defined by the ellipse within which tidal currents distribute soluble wastes during one tidal period; zone B - the scale of small water bodies, such as a fjordic basin that can be treated as laterally homogenous; water residence times of a few days to several weeks, a period sufficiently long for nutrient enrichment to lead to enhanced growth of phytoplankton; in offshore waters, the zone's extent can be estimated from information about persistent currents and dispersion, including dispersion due to tidal flows in mesotidal and macrotidal waters; zone C - regional scale; water residence times of weeks to months, sufficently long for complex ecosystem processes to take place; likely to be hydrographically complex; provides the 'background' conditions with which waters in potentially effected zone Bs exchange;

CSTT (1994). Comprehensive studies for the purposes of Article 6 of DIR 91/271 EEC, the Urban Waste Water Treatment Directive. Published for the Comprehensive Studies Task Team of Group Coordinating Sea Disposal Monitoring by the Forth River Purification Board, Edinburgh.

Scale and management

There are differences between the scales in the type of environmental management required:

• zone A - farm scale - this is where obvious impacts from aquaculture occurs; in some jurisdictions farmers are required to manage and monitor impacts themselves, in their own interests and to present themselves well to environmentally aware members of the public; public environment management can in these cases confine itself to a licensing and auditing role.
• zone B - water body scale - pressures may result from several sources and impacts may not be obvious until ecosystem resistance has been overwhelmed; public environmental management needs to play a more active role in estimating assimilative and carrying capacities and policing their use by the multipler users; there may be possibilities for synergy, with shell-fish or seaweed aquaculture benefitting from nutrients input by finfish farms;
• zone C - regional scale - issues are those of planning for multiple types of use, of monitoring conditions for their own sake and to provide reference values for zone B water bodies, and making regulations for the management of the smaller scales.

Scales, feedback loops, and boundary conditions

The simplest models for organisms describe the behaviour of a single fish or bivalve, and then multiply that by the total number of animals in a farm in order to estimate the total effect of the animals on the environment. The animals are effected by conditions in the water, and excrete material to the water, but output is assumed not to influence input.
This assumption is only realistic when farmed animals are few in relation to water exchange. Combining an organism model with an zone A scale pelagic model allows for feedback: for example, as the numbers of farmed animals increase, it becomes more likely that they will substantially increase ammonia, or decrease oxygen, or compete for phytoplanktonic food, within the farm. Outside the farm is the water body, and in the case of farm-scale models this provides the 'boundary conditions' for the farm scale model. It is a fundamental assumption in modelling that the boundary conditions are not influenced by changes simulated by the model.
Zone B scale models deal with the more complicated feedbacks that can occur within water bodies, such as when nutrients excreted by fish begin, after a few days, to stimulate the growth of phytoplankton. The 'boundary conditions' for zone B models are those in the larger zone C with which the water body exchanges. The conditions can also include inputs from land and exchanges with the atmosphere.