4.3. Broad carrying capacity estimation for aquaculture zones

For purposes of aquaculture zoning, carrying capacity sets an upper limit for the number of farms and their intensity of production that retains environmental and social impacts at manageable and/or acceptable levels, which then implies overall sustainability. At the zone level, carrying capacity will typically be expressed as a level of production (in tonnes) produced through a number of farms located in geographic space, or production in tonnes per hectare or km2.

Within aquaculture zones, carrying capacity has two primary dimensions:
• ecological carrying capacity: the maximum production that does not cause unacceptable impacts on the environment; and
• social carrying capacity: the social licence for the level of farm development that does not disenfranchise people or result in net economic losses to local communities.
At a large zone level, preliminary limits to the number of farms and intensity of production are set based on a large-scale understanding of the area or waterbody proposed to be or already allocated to aquaculture.
This contrasts with setting more detailed carrying capacity estimates for AMAs and for individual sites in which more specific assessment is made of local conditions. There are circumstances where an aquaculture zone could become an aquaculture management area if a suitable management plan is developed and implemented. Typically, however, aquaculture zones are broader scale areas that may contain one or more AMAs and numerous sites.

 

4.3.1 Ecological carrying capacity

To estimate carrying capacity in the context of fish aquaculture, models are usually used to estimate a maximum allowable production, limited primarily by modelling changes to environmental conditions.
Nutrient input or extraction and oxygen changes (depending on the species to be cultivated) can be assessed, for example, on a specific catchment area or waterbody for a given number of aquaculture units. For extractive production, such as shellfish, food depletion is the major consideration along with effects on wild species and food availability for them.
The assessment of ecological carrying capacity is based on the capacity of the ecosystem to continue to function through the application of environmental quality standards that cannot be exceeded when aquaculture is included into the system. It is sometimes referred to as assimilative capacity, implying the system is able to assimilate a certain level of nutrients or oxygen uptake without causing detrimental effects such as eutrophication. Aquaculture produces or uses dissolved and particulate matter that enter the environment, uses oxygen and other resources, and adds residues from diseases or parasites and other treatment chemicals. It is the consequences of these on the ecosystem that are used in estimating ecological carrying capacity. The capacity of a particular area also depends on water depth, flushing rates/current velocity, temperature and biological activity in the water column and bottom sediments, and attempting to define the level of ecological resilience. The multifactor nature of ecological capacity is one of the reasons why models are often applied, as models can attempt to integrate the multiplicative and cumulative nature of these factors.
It may also be important to take into account background wastes entering a shared waterbody, coming from other sources such as sewage discharges and diffuse inputs from agriculture, domestic waste and forestry. The basic reasoning is that the collective consequences of all aquaculture farms and background inputs can be compared with the ecological capacity of the ecosystem, which can then determine how much aquaculture can sustainably be conducted within a certain physical space. In reality, diffuse inputs (as opposed to point sources) are difficult to assess and measure, which makes estimating the existing consequences of these background wastes difficult. It may also be that activities such as forestry or agriculture have occurred for millennia already, and therefore current water quality and conditions may already reflect the impacts of such activity.
The negative impacts of exceeding ecological carrying capacity include eutrophication, increases in primary productivity and potential phytoplankton blooms fueled by nutrients discharged from farms, accumulation of noxious sediments in the form of fish faeces and feed wastes, and loss of biodiversity due to declining habitat quality. The consequences for aquaculture farmers can be dramatic, including loss of fish stocks on the farms because of blooms, oxygen stress and disease; and exceeding ecological carrying capacity often aggravates fish health problems and social conflicts. Environmental impacts of aquaculture vary with location, the production system and species being grown. Fish cage culture is an open system that extracts oxygen from water, and discharges faecal and feed and other wastes into the surrounding water and sediments. Pond culture is a closed system, and releases nutrient-rich water and effluents during water exchange and/or pond draining during harvest.
Bivalves depend upon natural productivity for their food, but compete with other organisms for food (organic matter, microalgae, etc.) and dissolved oxygen in the water column, and seaweed production can reduce light penetration affecting environmental conditions and species below. The fact that there is no “consequence free” aquaculture means that there is a basic need to determine ecological carrying capacity.
One of the earliest applications of mass-balance modelling in aquaculture was the use of Dillon and Rigler’s (1974) modification of a model originally proposed by Vollenweider (1968), which used phosphorus (P) concentration to estimate the ecological carrying capacity of freshwater lakes, assuming that P limits phytoplankton growth and therefore eutrophication (Beveridge, 1984). Inputs to the environment from fish culture are evaluated to determine likely changes in overall water quality.
This model has been used widely to estimate carrying capacity of lakes to support fish farming, as in Chile.
Further modifications of this model have also been used assuming nitrogen as the limiting factor (Soto, Salazar and Alfaro, 2007).
Ecological carrying capacity models integrate hydrodynamic, biogeochemical and ecological processes in the environment with oxygen consumption, sources, and sinks of organic matter and nutrients derived from farm activity linked to the ecosystem state. There are currently few models that assess carrying capacity fully at the zonal scale; EcoWin (Ferreira, 1995) is one example that combines hydrodynamic models with changes to water biogeochemistry to look at large-scale, multiyear changes under non-aquaculture and aquaculture conditions (Ferreira, 2008a; Sequeira et al., 2008).
On a slightly smaller zonal scale, models such as the Loch Ecosystem State Vector model (Tett et al., 2011) resolve seasonal variations in oxygen and chlorophyll in defined sea areas; and the Modelling—Ongrowing fish farms—Monitoring (MOM) model used for farm level assessment also contains a module for wider scale evaluation of water quality and oxygen concentration (Stigebrandt, 2011).
In Chesapeake Bay and the Puget Sound, United States of America, the EcoWin model has been combined with a farm-level model (FARM) and with other tools into a production, ecological, and social capacity assessment that builds together ecological carrying capacity modelling with a stakeholder engagement process that seeks to reduce social conflicts (see Bricker et al., 2013; Saurel et al., 2014). Other similar projects have occurred in Portugal (Ferreira et al., 2014) and Ireland (Nunes et al., 2011). Availability of models to assess freshwater systems is more limited.
Until more precise modelling can be undertaken at the zonal level, it is possible to apply simplistic approaches to limit production to acceptable levels. Examples include the Philippines where a maximum of 5 percent of an aquatic body can be used for aquaculture, although this does not estimate carrying capacity per se. In Norway from 1996 to 2005, feed purchases were used to monitor aquaculture development.
This worked initially as a quota that limited the amount of feed that could be delivered to farms.
As well as serving as an indicator of production (rather than capacity), this system had the benefit of rapidly reducing feed conversion ratio (FCR), as farmers tried to optimize the use of the feed allocated to them while maximizing production, which in turn reduced environmental consequences. This was combined with a limit on the cage volume of 12 000 m3 per licence together with a maximum fish density in cages. This number of licences with volume limit, along with rules for biomass and feed quota, was the framework used to control production development. Norway’s approach has since been updated to now assess carrying capacity directly at site and/or small area scales.
Indices have also been used to assign the status of waterbodies into discrete categories that define typically a specific water status with regards to aquaculture development, or whether or not aquaculture is liable to have an effect (e.g. in the latter case, of eutrophication potential using the TRIX index in Turkey, see Annex 5); or to define areas considered to be the most environmentally sensitive to further fish farming development due to the high predicted levels of nutrient enhancement and/or benthic impact (Gillibrand et al., 2002). Gillibrand et al. (2002) scaled model outputs from 0 to 5, and the two scaled values (nutrients and benthic impact) were added together to provide a single combined index. On the basis of this combined index, areas were designated as Category 1 (sensitive to more production, and therefore no more production allowed); Category 2 (production potential, with caution); or Category 3 (least sensitive, and opportunities to increase production).
Overall, the larger the area or zone being evaluated, the more complex and more difficult it is to make reliable estimations of carrying capacity owing to the multiple interacting dynamic factors that affect it and acceptable limits in environmental change.

4.3.2 Social carrying capacity

Social carrying capacity is less tangible than other carrying capacities, but is the amount of aquaculture that can be developed without adverse social impacts (Angel and Freeman, 2009; Byron and Costa-Pierce, 2013).
Social licence for aquaculture is affected by cultural norms, and can be affected by social mobility and wealth of people and by the species grown and aquaculture practices undertaken, seen as either polluting (e.g. fed fish) or non-polluting (e.g. non-fed fish or extractive species) whether or not this is explicitly correct. Social capacity for aquaculture is also affected by perceived or actual ecological degradation, the extent to which aquaculture impacts other livelihoods, exclusion of legitimate stakeholders from decision-making, and incompatibility of aquaculture with alternative uses, which are all key sources of social conflict.
Social conflicts can be minimized through good engagement in the development and management of aquaculture zones, adverse impacts on the ecosystem and use of space. Fair business practices and the creation of opportunities for local communities along the aquaculture value chain from manufacture and supply of inputs through to processing, transport and marketing will build alliances among the local population. Proper stakeholder engagement, sharing of information and timely communication in the planning process can help investors avoid social conflicts.