Marine Cage Culture and the Environment: Twenty-first Century Science Informing a Sustainable Industry

Lucy Towers
13 January 2014, at 12:00am

Technological innovation has made it possible to grow marine finfish in the coastal and open ocean. Along with this opportunity comes environmental risk. As a federal agency charged with stewardship of the nations marine resources, the National Oceanic and Atmospheric Administration (NOAA) requires tools to evaluate the benefits and risks that aquaculture poses in the marine environment, to implement policies and regulations which safeguard our marine and coastal ecosystems, and to inform production designs and operational procedures compatible with marine stewardship.

Technological innovation has made it possible to grow marine finfish in the coastal and open ocean. Along with this opportunity comes environmental risk. As a federal agency charged with stewardship of the nation’s marine resources, the National Oceanic and Atmospheric Administration (NOAA) requires tools to evaluate the benefits and risks that aquaculture poses in the marine environment, to implement policies and regulations which safeguard our marine and coastal ecosystems, and to inform production designs and operational procedures compatible with marine stewardship. 

There is an opportunity to apply the best available science and globally proven best management practices to regulate and guide a sustainable United States (U.S.) marine finfish farming aquaculture industry. There are strong economic incentives to develop this industry, and doing so in an environmentally responsible way is possible if stakeholders, the public and regulatory agencies have a clear understanding of the relative risks to the environment and the feasible solutions to minimize, manage or eliminate those risks. This report spans many of the environmental challenges that marine finfish aquaculture faces. We believe that it will serve as a useful tool to those interested in and responsible for the industry and safeguarding the health, productivity and resilience of our marine ecosystems.

This report aims to provide a comprehensive review of some predominant environmental risks that marine fish cage culture aquaculture, as it is currently conducted, poses in the marine environment and designs and practices now in use to address these environmental risks in the U.S. and elsewhere. Today’s finfish aquaculture industry has learned, adapted and improved to lessen or eliminate impacts to the marine habitats in which it operates. What progress has been made? What has been learned? How have practices changed and what are the results in terms of water quality, benthic, and other environmental effects? To answer these questions we conducted a critical review of the large body of scientific work published since 2000 on the environmental impacts of marine finfish aquaculture around the world. Our report includes results, findings and recommendations from over 420 papers, primarily from peer-reviewed professional journals.

This report provides a broad overview of the twenty-first century marine finfish aquaculture industry, with a targeted focus on potential impacts to water quality, sediment chemistry, benthic communities, marine life and sensitive habitats. Other environmental issues including fish health, genetic issues, and feed formulation were beyond the scope of this report and are being addressed in other initiatives and reports. Also absent is detailed information about complex computer simulations that are used to model discharge, assimilation and accumulation of nutrient waste from farms. These tools are instrumental for siting and managing farms, and a comparative analysis of these models is underway by NOAA.

We anticipate this report will be useful to both industry and coastal and ocean managers. Farm owners and operators can learn about the current state of knowledge regarding environmental effects of cage culture and apply it to guide siting and other farm management practices. Coastal managers and community planners can use this information to make decisions about the environmentally responsible economic opportunities that aquaculture offers. Federal, state and local regulatory agencies can apply the analysis as they develop and implement permitting and monitoring processes for coastal marine finfish farming and the emerging U.S. offshore aquaculture industry. The scientific research community can use this report to guide future studies which will improve our knowledge of how fish cages function within the marine ecosystem, help improve farm efficiency and further decrease environmental effects. This report also provides a scientific basis for national and international outreach and education.

We hope this analysis will prove useful for integrating current scientific knowledge about the real, potential and perceived effects of marine finfish cage culture and support continued development of an industry which is both economically and environmentally sustainable. Below is a synopsis of observations and trends observed for various environmental effects discussed in this report. 

Water Quality

The primary potential effects to water quality associated with marine cage culture include dissolved nitrogen and phophorus, turbidity, lipids and dissolved oxygen fluxes. Usually there are no measurable effects 30 meters beyond the cages when farms are sited in well-flushed waters.

Nutrient spikes and declines in dissolved oxygen sometimes are seen following feeding events, but there are few reports of long-term risk to water quality from marine aquaculture. The trend of many studies over the last 20 years indicates that improvements in feed formulation and feeding efficiency are the major reasons for decreased nutrient loading and acceptable water quality in and near farms, and explains why significant enrichment to the water column at offshore farms is generally not detected. 

Impaired water quality may be observed around farms in nearshore or intertidal habitats where flushing is minimal and at farms using feeds that include unprocessed raw fish rather than formulated feeds. Protection of water quality will be best achieved by siting farms in well-flushed waters. 

Benthic Effects

There is a great deal of scientific information about the biogeochemical processes in sediments near fish farms and how those processes may be driven under nutrient enrichment. Excess feed and fish waste are discharged from the farms and, if they accumulate, may alter the chemical processes of decomposition and nutrient assimilation. Well-managed farms may exhibit little perturbation and, where chemical changes are measured, impacts are typically confined to within 100 meters of the cages. Benthic chemical recovery is often rapid following harvest. In contrast, heavily impacted sites may have anaerobic conditions persisting in the sediment and extending hundreds of meters beyond the farm perimeter. 

Organic matter can accumulate on the bottom and push the benthos to an anaerobic and ultimately azoic state. In heavily farmed or depositional areas, remediation of highly enriched sediments may take several years.

Impacts can be avoided by placing farms in deep, well flushed areas over erosional seafloor. This results in a net movement of organic matter away from the farm, dispersing nutrients over a broader area for decomposition and assimilation. While this approach protects the immediate farm perimeter, care must be taken to monitor areas downstream of the farms to detect far-field effects, especially in habitats sensitive to nutrient enrichment and sedimentation. The accumulation of some organic matter below farms is to be expected, especially toward the end of a grow-out period when farm biomass is at its peak. Visual observations of benthic conditions below farms are a valuable tool throughout a crop cycle for assessing whether operations are within the capacity of the ecosystem. 

Farms located in deep water with continuous or episodic benthic scouring of organic waste will be less likely to exhibit sediment degradation. As with water quality, benthic geochemical impacts are most pronounced at enclosed, nearshore or coastal farm sites with insufficient depth and flow to disperse organic wastes.

In some areas, nutrients from farm discharge may be absorbed as food for wild marine organisms. In other locations, deposition beneath cages will have minimal, ephemeral or acceptable effects. Still other farms may need fallowing periods ranging from a few months to one or two years to recover the benthos. Site specific characteristics such as hydrodynamics, trophic status of the water column benthic shear, sediment composition, water depth and nutrient loading will interact to determine which of these will be the scenario for any given farm site.

Effective sediment monitoring protocols should include key site-specific indicator parameters like sulfide, redox potential and total organic carbon to allow for early detection of impacts. Within an adaptive management framework, a good monitoring program can be used to adjust farm management to avert serious and persistent impacts to the benthos. Monitoring and research to quantify downstream, far-field and long-term effects of fish farms beyond the immediate cage perimeter will continue to be important. The use of stable isotopes as tracers of farm waste output at larger spatial and temporal scales is a promising tool to help in this area. Continued efforts comparing different monitoring technologies and protocols to provide reliable, accurate and cost effective methods of assessing enrichment and biogeochemical impacts will be beneficial to both the industry and regulatory entities. Image analysis and acoustic methods are being successfully tested in the field, thus offering cheap and quick alternatives to traditional geochemical analysis.

Marine Life

The broader ecological role of aquaculture operations within the marine environment must be considered since fish farms in the open ocean must co-exist with a host of wild organisms including phytoplankton, benthic fauna, wild fish, marine mammals and corals. If farm nutrients accumulate and persist in the water column or sediment, marine organisms can be impacted. At appropriately-sited and well-managed farms, natural processes can be sufficient to assimilate nutrients. In nutrient limited marine environments these inputs may even fertilize marine food webs, boosting overall productivity.

At some farm sites, a phytoplankton response to nutrient loading was reported, but generally this is a low risk and causal linkages to algal blooms are not evident. Because a change in primary productivity linked to fish farm effluents would have to be detected against the background of natural variability, it is difficult to discern effects unless they are of great magnitude and duration.

At larger scales, the occurrence of many anthropogenically derived nutrients in coastal marine waters, also make it difficult to attribute increased primary productivity directly to aquaculture. Hydrology of farms located near shore or in semi-enclosed water bodies which may be poor farm sites must be carefully examined to prevent eutrophication and increased primary productivity in coastal areas and habitats. A knowledge gap continues to be how dissolved nutrients are dispersed and assimilated over large marine areas, and how ecosystem productivity may be affected under increasing production from multiple farms.

Changes in the benthic community are evident when sediments become enriched with organic farm waste nutrients. At well flushed sites in deep water and with efficient feed management, ecological impacts tend to be minimal and confined to the area just beneath the cages. Under light organic enrichment an increase in benthic species abundance and biodiversity may be observed and can be a net benefit to the community. At moderately impacted farms, effects may extend to 100 meters beyond the farm edge.

In enriched sediments, the benthic species composition and diversity shift toward tolerant generalists like capitellid polychaetes. The far-field effects of aquaculture to the ecological functionality of food webs and secondary production have not been studied, are difficult to ascertain and should be an area of future monitoring and research efforts.

The excess food and waste released from fish cages may be food for wild fish, especially benthic feeders. Cages may also provide shelter and foraging habitat for wild fish. These characteristics may be beneficial to the local and regional environment. Wild fish and other marine life often aggregate around fish cages and this may be considered a beneficial impact to marine life at some locations. As fish are attracted to farms, the potential for negative and positive interactions with human fishers may increase and farm management or regulatory steps should be considered to minimize conflicts. Likewise, marine fish and mammalian predators may also be attracted to farms.

Little research has documented the extent to which marine predators target wild fish around farms, but this would be useful for understanding ecological interactions between farming and marine life.

At modern fish farms, impacts to predatory sharks and marine mammals are being minimized with improved net technologies and removal of dead fish from cages to prevent predation on cultured fish. Siting away from known aggregation sites and installing rigid predator exclusion nets are effective at preventing negative impacts to cultured fish, farm structures and marine predators.

Acoustic deterrent devices are not consistently useful against sea lions and seals and may have deleterious impacts to non-target marine mammals. In the U.S., nonlethal interventions to prevent marine mammal predation are preferred. At marine fish farms, entanglement in the farm structures may pose a slight threat to sea turtles, dolphins, whales and seabirds. Keeping lines taut and the water free of debris are effective at minimizing or eliminating conflict with marine mammals and turtles.

The potential effect of marine cage culture to corals, seagrass and mangrove forests are of
concern to resource managers and scientists. These ecosystems may be sensitive to nutrient enrichment and sedimentation, making them potentially vulnerable to farm effluent. If farms are located upstream of sensitive habitats, careful monitoring should be implemented for early detection of any perturbation.


The use of antibiotics, therapeutants and antifoulants at marine fish farms has declined greatly (up to 95%) in the last 20 years, resulting in decreased potential for secondary harmful effects of these chemicals on the marine environment. 

Vaccination, improvements in fish husbandry and best management practices are proven alternatives for achieving and maintaining fish health. Antifoulant chemicals are being replaced largely with onshore de-fouling or mechanical methods for controlling biofouling. Heavy metals from feed and antifoulants are known to accumulate beneath cages, but are often in low concentrations and sequestered in the sediment.

Management Tools

Beyond good site selection, fallowing and integrated multi-trophic aquaculture (IMTA) are two management tools that can be used to further reduce the potential environmental effects of marine fish farms. Fallowing is the practice of relocating or not re-stocking marine fish cages to allow the sediment below to undergo natural recovery, both geochemically and ecologically, from the impacts of nutrient loading. Under ideal conditions, farms should not require a fallowing period for the purpose of sediment recovery. Currently, this practice is widely and successfully implemented around the world as a method for preventing longlasting damage to the benthic environment.

IMTA is the practice of culturing species from multiple trophic levels in systems that allow for the assimilation of fish waste particulates and dissolved nutrients into additional valuable crops, thereby reducing environmental discharge and expanding the economic base of a farming operation.

The species most commonly selected for IMTA with marine fish are seaweeds, oysters and mussels, but lobsters, sea urchins, sea cucumbers and other invertebrates are also being used. Though largely experimental, the culture of additional marketable seafood products may provide dual benefits of economic profitability and reduction in nutrient enrichment.

The correlation of latitude, geographic area and trophic status of the receiving waters with the degree of biological and geochemical response to farm discharge is a critical area for further investigation. Comparative meta-analyses of environmental impacts are needed. The question of environmental impacts of any farm should be considered within a holistic context taking into account the array of oceanographic, hydrological and ecological characteristics of the site and the structural, technological and production aspects of the farm. One pattern that does emerge is that decreasing environmental risk from aquaculture appears to be driven by prudent siting of operations outside of shallow, enclosed, coastal and nearshore waters lacking dispersive current regimes, coupled with modern feed, aquatic health and farm management. This observation is important as it suggests that farming with minimal or acceptable environmental effects is possible in many ecosystems as long as proper safeguards are in place to minimize nutrient and chemical discharge and to manage its immediate and cumulative impacts.

These safeguards may be in the form of regulatory oversight or industry-developed best management practices. Ideally, a combination of the two approaches would be most beneficial.

This report provides a broad perspective on a range of potential environmental impacts and their relative intensity, which should be coupled with detailed, site-specific information to make good management decisions about a proposed or operational farm site during its lifetime. As farming expands, cumulative impacts may become more apparent, and thus, robust monitoring protocols are necessary and should be proactively designed to discern both near and far-field environmental impacts.

Further Reading

You can view the full report by clicking here.

January 2014