Ecasa Toolbox

Public Environment Management: Dealing with expansion

If a farmer seeks permission to expand their farm, it is likely that their site has, so far, provided favourable conditions for growing fin-fish or bivalve shellfish. However:

    • a site that is produces X tonnes of shellfish may not produce 2X tonnes if doubled in area and stock. This may be because of insufficient capacity to flush away wastes or supply oxygen, or because the supply of planktonic food is insufficent to feed more bivalves.
    • a site that is suitable for X tonnes of fish may not support the growing of 2X tonnes because of insufficient capacity to flush away wastes or supply oxygen. Impact on the sea-bed may become too great. In addition, the capacity of the water body to assimilate waste may become an issue.

Both indicators and models can help in assessing the potential of a site or water body to assimilate the extra wastes that will result from fin-fish or shell-fish farm expansion, or to provide the extra food needed to support greater production of bivalve shellfish. In general, sustainability is ensured by controlling aquaculture so that assimilative capacities and carrying capacities are not exceeded at the scale of the farm or on a water body scale.

Environmental indicators

Consider first, indicators of sensitivity. All other things being equal, more sensitive sites and water bodies need more careful consideration before expansion is allowed.

Next, consider pressure and state or impact indicators. In the case of farm-scale indicators, compare present values (at X tonnes of production) with values at the site before the farm was in place, or values within the same water-body but at a distance from the farm site. How much have they changed? Are they approaching environment thresholds set when permission was given to start the farm? If there is already a large change in indicator values, or if they are close to thresholds, then it is likely to be unwise to expand the local site. If the farm-scale indicators show little change, and remain well within safe limits, then the next step might be to use water-body scale models to study potential impacts on the larger scale. This may be especially important if several farms are proposing expansion. Consider especially the following water-body scale impacts: eutrophication in the case of fin-fish farms; and excessive removal of phytoplankton in the case of bivalve shell-fish farms.

Environment/farm type/species Indicator Category Scale Indicators
tentative; links to be added
All pelagic environments; all species Sensitivity A: farm Current speed higher is better
All pelagic environments; all species Pressure A: farm Ammonia and ammonium
Dissolved oxygen
Inside farm, compared with external reference conditions
All benthic environments; all species Pressure and state/impact A: farm ITI, AMBI, Redox ...
At/under farm, compared with external reference conditions
All pelagic environments; all species Sensitivity B: water body Water residence time faster turnover is better
All pelagic environments; finfish State B: water body chlorophyll avoid marked increase
transparency avoid decrease
basin water minimum oxygen avoid decrease
All pelagic environments; bivalve shell-fish State B: water body chlorophyll marked decrease might be due to excessive consumption by farmed bivalves


Models can be used to estimate the changes in ecosystem state that might result from the farming of more fin-fish. Potential changes can be compared with EcoQOs to ensure that a site or water body's assimilative capacity is not exceeded, as explained in Management for Sustainability. The most relevant models are those listed for fin-fish aquaculture on the Public Environment Management page dealing with Assessing Impacts. The shellfish models studied during ECASA were mostly concerned with optimizing production rather than with the environmental impact of bivalve farming: these models are listed on the Shellfish farming: assessing potential page. Managing the effect of bivalve consumption on phytoplankton can be seen as part of the matter of keeping shellfish production within a water-body's carrying capacity, as considered on the Environmental Planning page.