If properly managed and impacts are minimized, cages in coastal waters can become fish aggregating devices (FADs) and potentially provide habitat value. Algae will grow on the structure of the cages which will attract small zooplankton which in turn will attract small fish and crustaceans. These small organisms will eventually attract larger predators to the structure, thereby creating a small ecosystem around the FAD. Properly managed coastal finfish cages have the potential to attract fish to an area that would otherwise not have fish. ref
Source of Fry and Broodstock
Broodstock are often produced in a land based hatchery, where fish and other species are bred to produce larvae and grown to a large enough size to be transferred to grow-out facilities. However, in some locations and for some species, aquaculture production systems carry out what is sometimes to referred to as “ranching”. This form of aquaculture relies on the capture of young wild stocks as fry or sub-adults that will be relocated for an extended period to ocean cages and fed until they are brought to market. Species which commonly rely upon this method are milkfish, tuna, yellowtail species (seriola), and crustaceans.
From an ecological perspective, hatchery-based methods for aquaculture are generally strongly preferable to the ranching techniques. Extracting large quantities of juvenile species from the wild can negatively impact local populations - in particular, stock reproduction and overall abundance, if conducted on a large scale. Ranching can impact the food web and have trickle down effects across the whole marine ecosystem. Additionally, stocking wild species into an intensive culture system can create a biosecurity risk and potentially introduce diseases.
Many of these possible environmental issues can be mitigated by sourcing fry and juveniles from a reliable closed hatchery system where the staff have control over the life cycle of the adults and larvae. ref In hatchery systems, captive held adults are used to produce juveniles, which in turn are transferred to marine cages to grow. By using hatchery-reared species, the farm manager does not rely on wild populations to stock cages, with the exception of sourcing of selected adult broodstock.
As aquaculture occurs in the marine and nearshore environment, if a cage or netting breaks or care is not taken during harvesting or stock transfer operations, cultured stock can escape into the wild. These escape events are also sometimes known as a “spill.” There are several ecological impacts that an escape event can cause: altering of food web dynamics, transfer of disease to wild populations, and genetic impacts on wild populations through interbreeding with wild species. If escaped stock establish a population in the wild, it is possible they can compete with other wild species or transmit diseases to wild populations. Escaped species can also interbreed with wild stocks and, depending on the status of the farmed species, can potentially genetically change or weaken wild stocks. ref
These environmental and genetic issues can be mitigated by proper cage management, including regularly assessing the condition of the cages and ensuring repairs are completed when needed. If the nets are not maintained and allowed to weaken, escapements will occur. However, if the manager is proactive in safeguarding the overall health and condition of the nets, escapements will be minimized.
Entanglements of Protected Species
Impacts of aquaculture operations on species of concern, such as marine mammals (dugongs, dolphins, whales), sea turtles, and seabirds should also be considered. Given that most aquaculture operations are characterized by fixed sites and have tensioned mooring lines, most marine finfish operations are generally characterized as having a low risk of entanglement compared to other activities with drifting or untensioned lines, such as pot fisheries or gillnets.
Aquaculture feed is one of the most important drivers of sustainability in finfish farming. Feed also tends to be the most expensive component of farm operations and often still relies on fish meal and oil sourced from wild stocks of fish. In many countries and small-scale farms, whole fish, fish trimmings, and/or animal slaughter waste may be used as feed as opposed to commercial pellets. Utilizing whole non-specialized feeds can reduce water quality as these materials can easily dissolve and decompose in the water column or on the seafloor, leading to organic debris buildup that can impact the surrounding ecosystem.
Using these feeds instead of commercial pellets is ecologically and economically inefficient as lower growth rates may occur because they do not provide the minimum nutritional requirements for the cultured species. Also, by using organic material that has not undergone some level of processing or sterilization as feed, the farm operator may introduce pathogens and parasites. ref Proper management procedures should include seeking out commercial pellets to reduce the environmental impact of the farm. Management must make sure that as much feed as possible is consumed by the fish, as uneaten pellets can sink to the seafloor or float away with the current, both of which can have environmental impacts on the local area.
A parameter that is very important to keep in mind is FIFO, or Fish In-Fish Out. This parameter indicates how much wild forage fish is needed to produce a certain amount of farmed fish. In the case of salmon, 0.82 kg of forage fish are needed to produce 1 kg of farmed salmon and 0.53 kg are needed to produce 1 kg of marine fish in general. ref
Additionally, species such as bivalves and algae do not require feed and can actually improve water quality through filter feeding and nutrient uptake. These species remove nutrients (including nitrogen and phosphorous) via uptake in tissue and shell, which is then removed from the water body during harvest. Bivalves contribute to water clarity by filtering organic and particulate matter from the water column. These processes can help mitigate anthropogenic impacts on water quality and lower the likelihood of eutrophication that may be caused from unsustainable finfish feeding practices.
Ecological Society of America, Effects of Aquaculture on World Fish Supplies
Incidence and Impacts of Escaped Farmed Atlantic Salmon Salmo salar in Nature
Protected Species and Marine Aquaculture Interactions
Transition from Low-Value Fish to Compound Feeds in Marine Cage Farming in Asia
Best Management Practices for Marine Cage Culture Operations in the U.S. Caribbean
Coastal Fish Farms as Fish Aggregation Devices (FADs)
Fish in: Fish Out (FIFO) Ratios for the Conversion of Wild Feed to Farmed Fish, Including Salmon