Water quality around fish cages is a very important factor for overall ecosystem health and the operational success of the farm. For fed species like finfish, excess feed can turn into dissolved nitrogen and phosphorus and cause impacts to benthic communities. Sensitive habitats such as coral reefs, seagrasses, and mangroves can be also damaged by excess nutrients in the water, which can stimulate algae blooms.
Larger numbers/densities of fish cages have a greater potential to result in water degradation. While some areas may be able to support a smaller number of cages without negative water impacts, increasing the number of cages or stocking higher densities of fish can create excess nutrients that the nearby environment cannot absorb sustainably. When in excess, these nutrients can potentially cause damaging effects, in the form of algal overgrowth and eutrophication, which now affects a large share of coastal waterbodies worldwide. As a general principle, it is important to limit the number of cages in small areas where the released nitrogen and phosphorus could be detrimental to the local ecosystem.
It is also important to note that except in localized examples, aquaculture is not generally the principal source of nutrients or cause of eutrophication in coastal waterways. Agriculture and runoff from populated areas are generally the largest contributors to eutrophication. However, in some circumstances, aquaculture has played a significant role and has shown to contribute as much as 10% of nitrogen loading and 26% of phosphorus loading in individual sites. ref
A generally accepted depth for marine finfish cages is at least twice the depth of the bottom of the cage to have minimal impacts on water quality, benthic environment, and sensitive habitats. This recommended depth though is dependent on local habitats and other factors. With lower current flow, a greater depth will allow for more effluent to be transported downstream and dissipate into the environment. Depending on the benthic environment, different anchoring systems will need to be evaluated to allow for the appropriate cage installation. ref Proper planning during site and cage type selection are essential in determining areas with appropriate seafloor depth.
Proximity to Sensitive Habitats
A generally acceptable distance from corals is 200 meters to have minimal impacts on water quality, benthic environment, and sensitive habitats. This recommended distance is dependent on local habitats and other factors and is considered a conservative estimate. If cages are directly on top of coral reefs or seagrasses and in shallow areas, cages might obstruct sunlight reaching the coral or seagrass impacting photosynthesis. Even if reefs and seagrass are downstream of the cages, it is imperative to evaluate the speed of currents to determine if effluent will reach and negatively impact these environments. Mangroves are also important habitats for reef animals as they provide shelter and nursery grounds. Cages should not be placed in mangrove areas as nutrient accumulation might negatively affect the ecosystem. Similarly, proactive planning and regular monitoring needs to take place to evaluate whether there is current flow from cages into mangrove areas and, if so, the mangroves are able to absorb the additional nutrients. ref
The concept that different aquatic environments can sustainably support a certain threshold of total fish weight is known as carrying capacity. If that carrying capacity threshold is passed, negative effects can occur that can jeopardize water quality and nearby habitats. There are many different methods and complex models that can explain and predict an environment's carrying capacity and thus total number of cages and fish that the environment can support. It is important to understand that carrying capacities differ between locations, depending on many factors, such as currents, natural flushing, depth, etc.
While conducting a carrying capacity study / creating a location-specific model is one of the most accurate ways to assess carrying capacity, these models are often expensive and require complex data sets that might not be readily available. As such, there are some countries that have employed alternate ways of setting limits on how much aquaculture can occur in the waterbody, such as setting a maximum percentage of water body that can be used for fed aquaculture or placing conditions on minimum distance between farms. Depth, currents, tides, feed type, feed quantity and selected species are factors that will affect the carrying capacity of an area. ref
Water Currents and Circulation
Tidal flows and currents are an important aspect in siting proposed cages. Inward tides can transport cage nutrients closer to the coast and into mangroves, estuaries, and areas with denser populations, while outgoing tides can transport effluent towards the open ocean. Currents remove nutrients from the cage area and allow for oxygen-rich seawater to pass through the cage and provide the needed oxygen for the growing fish. Alternatively, finfish cage areas with no currents or sufficient tides will be stagnant and will not provide proper flushing. It is important to observe tide and current history to be able to predict how well proposed areas will be able to sustain finfish aquaculture production. ref
Environmental monitoring should occur to determine whether the farm is impacting water quality. This monitoring should ideally include total suspended solids, water temperature, dissolved oxygen, salinity, nitrogen (ammonia, nitrate, nitrite), phosphorus, silicates, chlorophyll, and pH. At a minimum, monitoring should include measuring dissolved oxygen and ammonia. ref It is important to monitor these water quality parameters in various locations around the cage to determine how much the cage is impacting the local waters.