Wild Stocks

Fish Aquaculture @TNC
Coastal aquaculture cages have the potential to cause impacts to wild stocks, due to being located in shared marine waters and containing fed species in a confined space. If proper planning and management are not in place, potential impacts to wild stocks that may occur include: removal of wild species for use as fry, farmed fish escapes, entanglement of wild species, and feed impacts. We discuss these potential impacts of finfish aquaculture and protocols to minimize these impacts to coastal ecosystems below.

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.


  • When possible, source fry and juveniles from hatcheries to relieve pressure on local populations
  • If no hatchery is available, all attempts must be made to source fry and juveniles from wild stocks that are not currently or at risk of being overfished



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.


  • Use cages or pens that are designed to reduce escapes and develop a escape reduction plan for the farm
  • Engage in regular monitoring of farm infrastructure to maintain the lines and equipment and check for escapements
  • Ensure particular care is taken when stock are transferred or harvested from cages or pens
  • Proactively consider potential vandalism of or theft from the farm and put protective measures in place
  • Ensure appropriate farm siting to minimize storm exposure and resulting escape incidents
  • Farm local species and discourage any farming of non-native or non-naturalized species
  • Consider whether it is appropriate to stock sterile fish, only if it locally appropriate and recommended by aquaculture specialists knowledgeable about species.
  • Conduct risk assessments of genetic impacts, and ideally, modeling of potential escape events on wild populations
  • Conduct risk assessments of potential competition for food and habitat between wild stocks and potential escaped stock



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.


  • Site farms away from known areas that are frequently utilized by protected species, such as nurseries, feeding grounds, and migration routes
  • Create a monitoring plan that includes regular monitoring for marine animals near the farm and checking ropes and nets for entanglement. If possible, utilize both human and technological means of monitoring (e.g., cameras)
  • Regularly maintain farm ropes, lines, and other gear to ensure ropes remain tensioned
  • Do not dispose of excess feed or waste near the farm as this can attract marine animals and increase the risk of entanglement
  • Consider utilizing marine mammal deterring devices, only as appropriate and recommended by scientists knowledgeable about local conditions and ecosystems




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



  • Use high quality specialized feeds for farmed species, not whole fish or waste from fish or other animals
  • Closely monitor feeding to minimize wasted feed and maximize feeding efficiency
  • Use feeds with the lowest fishmeal and fish oil inclusion rate, as appropriate for culture species
  • If feeds contain fishmeal and oil, ensure they are sourced from sustainably managed fish stocks
  • Ensure that other ingredients in feed are sustainable sourced (e.g., plant-based products such as soy)
  • Check feed composition to ensure that there are no toxins in the feed and that feed ingredients have enough omega-3 fatty acids for the culture species

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.

Translate »