Due to the fact that offshore aquaculture occurs in ecological contexts that differ significantly from nearshore contexts, different kinds of ecological risks may also arise as the sector grows. Here we present a synthesis of information on the ecological risks posed by offshore aquaculture and how to mitigate them.
There is growing interest in cultivating fish, shellfish, and seaweeds in offshore waters. Existing pilot and commercial facilities demonstrate that offshore aquaculture is technically feasible, and that it can improve growing conditions and farmed animal health under some conditions.
Many of the advances that have improved the ecological outcomes of nearshore aquaculture is likely to apply to offshore facilities as well. However, some ecological risks will likely to persist in the near term.
The vulnerability of offshore farm site infrastructure to weather events and vessel collisions could be similar to nearshore sites and result in escape events, and the farming of finfish will likely require feeds that include fishmeal and fish oil, ingredients derived from finite marine resources, and terrestrial-origin ingredients whose embodied carbon footprint may be high.
Moreover, because offshore aquaculture occurs in ecological contexts that differ significantly from nearshore contexts, different kinds of ecological risks may also arise as the sector grows.
Here is a synthesis of available information on the ecological risks posed by offshore aquaculture, its main potential ecological risk categories, and ways to reduce these risks.
Major types of offshore production infrastructure
As interest in offshore aquaculture has grown, several approaches to facility design have emerged. These approaches can be categorized as either open-pen design or closed containment, each with several sub-categories of design within them.
Open-pen designs are those that use only mesh-style barriers to contain the farmed population, allowing free, three-dimensional exchange between the ‘internal’ farm environment and the ‘external’ ambient environment. Ocean currents continuously deliver water to and through the pens, and farm wastes, such as unconsumed feed, solid and soluble metabolites, are discharged to the environment without capture or treatment.
Similarly, pathogens and parasites may be both introduced to and discharged from the farm environment. The most common open-pen designs are surface structures, which suspend mesh fish containment barriers from floating infrastructure, are moored to the seafloor by a series of strategically placed anchors, and have tensioned lines to maintain the shape of the net pen as currents act upon it.
“Flexible surface structures, have typified nearshore finfish culture for decades and utilize high density polyethylene (HDPE) to construct the circular pen collars, including a walkway for farm workers. These structures are designed to flex in response to wave action.”
Rigid surface structures, which typically employ steel to provide the pen superstructure, resist wave action but are otherwise similar to flexible surface structures in that the farmed animals have constant access to the water’s surface. In contrast to open-pen designs, closed containment designs physically separate the ‘internal’ farm environment from the ‘external’ ambient environment.
Physical separation allows the isolation of the farm population from potentially harmful ambient risks like pathogens, parasites, and algae blooms, as well as the opportunity to remove or treat farm waste products. However, this eliminates natural subsidies afforded by open pen designs, introducing the need to intake, circulate, and discharge water, as well as maintain its quality to support fish health and growth.
The deployment of closed containment designs currently lags behind open pen designs. In terms of diversity in species and scale, aside from finfish, farms can be designed to produce a variety of seaweed and shellfish species (Tables 1 and 2).
Seaweeds can be grown on a diversity of structures, including long-lines, gridlines, rope attachments, nets, lattice mooring, and other marine infrastructure (Tables 1 and 2) hellfish and seaweed often require the same kind of infrastructure, which often includes buoyancy control devices.
Shellfish and seaweed can also be grown together, potentially lowering the environmental footprint for growing and harvesting the crops. Offshore aquaculture farms also vary significantly in scale (Table 1).
They range from small systems, like Santa Barbara Mariculture Company or Aquafort, with 20 MT annual production intended for local distribution, to those like Nippon and Salmar’s Ocean Farm 1 that are operating on an industrial commercial scale with 50 times more capacity in deep waters (Table 2) and to commercial salmon farms that commonly produce greater than 1000 tons of fish per production cycle.
International offshore aquaculture status
It was found that information describing 33 offshore aquaculture operations, 15 of which are research or demonstration projects; 18 are commercial operations. These operations use different types of infrastructure to grow at least 15 species of fish and shellfish (Tables 1 and 2).
The diversity of species grown in offshore aquaculture facilities is only likely to grow in the future due to active innovation. Ecological impacts vary with the type of species cultivated (shellfish, seaweed, or finfish), the type of infrastructure used, farm operation practices, siting and site aggregation, and oceanographic conditions, among other factors.
Offshore aquaculture in U.S. waters
There has been increasing interest in growing finfish, shellfish, and seaweed in U.S. offshore waters during the past decade, resulting in greater investment and the development of support and research programs.
Offshore operations can be complicated to regulate under existing legal frameworks because of stakeholder conflict, maritime jurisdictional issues, and understudied environmental impacts. U.S. policymakers have pursued a variety of approaches to regulate offshore aquaculture, including
(1) asserting that the National Oceanic and Atmospheric Administration (NOAA) has the authority to govern aquaculture in the EEZ under the Magnuson-Stevens Fishery Conservation and Management Act;
(2) issuing Executive Order 13921, which seeks to streamline permitting with a prominent role for NOAA; and
(3) introducing new legislation to authorize NOAA to permit aquaculture, including finfish farming (S. 3100, H.R. 6258, the Advancing the Quality and Understanding of American Aquaculture Act – AQUAA).
The lack of aquaculture-specific federal policy for offshore facilities in the U.S. has driven most offshore aquaculture initiatives to operate in state waters (or in other countries) that offer similar conditions and degrees of exposure to what is expected from sites in federal waters (Table 2).
Ecological risks, mitigation strategies, and knowledge gaps
Many of the practices that have improved nearshore aquaculture outcomes over the past 20 years will likely to be employed in offshore aquaculture.
To develop a sound policy that can facilitate the development of the offshore aquaculture industry in U.S. federal waters without compromising the health of ocean ecosystems, it will be necessary to understand the main risks that offshore aquaculture poses to marine ecosystems and wildlife, which risks can be mitigated using current approaches, and which risks will require new or precautionary approaches.
In the following sections, we describe the main ecological risks associated with offshore aquaculture related to siting, infrastructure, stocking, feed, metabolic waste, parasites, disease, and escapes. We also describe mitigation strategies and knowledge gaps for each risk category.
Selecting appropriate sites for offshore aquaculture presents several challenges, which together create a risk of cumulative impacts. The U.S. offshore marine environment is more crowded than it would appear to be at first glance.
As a result, offshore aquaculture facilities have limited possible sites due to a high number of preestablished claims on space and other factors.
For example, NOAA’s choices for siting Ocean Era’s proposed offshore aquaculture operation in the Gulf of Mexico were limited because a number of criteria had to be met simultaneously: proximity to a commercial port; water depths of at least 130 feet to allow net pen submersion and maximize mooring scope; areas consisting of unconsolidated sediments for positioning the anchors; and avoidance of hard bottom habitats, artificial reefs, marine protected areas (MPAs), reserves, and Habitats of Particular Concern (HAPCs).
Additional considerations for site selection included, but were not limited, vessel traffic routes, oil and gas zones, military zones, fisheries and tourism zones, dredging sites, and the presence of habitat for endangered, threatened, and protected (ETP) species. Some knowledge Gaps:
✓ What is the long-term risk that commercial offshore aquaculture operations would aggregate given the need for favorable growing and operating conditions and existing claims on marine space?
✓ Are the existing regulatory tools appropriate for regulating offshore industry development, or should sector-wide development plans, underpinned by carrying capacity modeling, be used to prevent adverse cumulative impacts?
✓ How can technology best be leveraged to improve siting and monitoring?
✓ How can negative impacts on other sectors and stakeholders be prevented or minimized?
✓ How are the limitations of the ETP species distribution models, like building cost effective and targeted survey efforts, best addressed?
Infrastructure and interactions with marine wildlife
The production and installation of infrastructure (e.g., cages, pens, moorings) capable of containing farmed species in high-energy offshore environments pose a great technical challenge. Offshore aquaculture infrastructure and equipment must withstand or be resilient to strong offshore waves, winds, and currents as well as resist corrosion and fouling.
Plans to use pre-existing infrastructure like decommissioned oil rigs or marine wind farms have been considered globally, including examples such as Belgium’s Wier and Wind project and Germany’s offshore mussel and wind turbine sites.
Some knowledge Gaps:
✓ How can the risk that aquaculture infrastructure will be lost or damaged during transport and deployment be mitigated?
✓ While fishing pressure could be addressed with existing management frameworks, what are the most effective strategies for mitigating other risks associated with fish and wildlife interactions with offshore aquaculture infrastructure, like disease transmission or entanglement?
✓ What are risks associated with decommissioning an offshore aquaculture facility?
With few exceptions (e.g. tuna ranching operations), stock for growing at offshore finfish farms is expected to come from hatchery production. The environmental impacts of land-based brooding and rearing systems include habitat modification for facility construction, operational energy and water consumption, and metabolic waste disposal. Some knowledge Gaps:
✓ What data are necessary to collect to determine optimal stocking densities for a specific species in a given locale? How should the optimum be defined?
✓ What are the most efficient strategies to produce sufficient stock for each of the species being considered for offshore production and how can the constraints on their production be overcome?
✓ How can the carbon footprint of hatcheries and pre-stocking rearing operations for offshore farms be further reduced?
Feed generally represents 50–80% of the production costs of fed aquaculture and is typically the most significant contributor to fed aquaculture’s greenhouse gas emissions. A major ecological and production risk of feed use in offshore aquaculture is the reliance on marine ingredients.
While progress has been made in decreasing feed conversion ratios and substituting marine ingredients with other sources of protein and fatty acids, the nutritional qualities of marine ingredients make them difficult to substitute, so feeds still include considerable amounts of fishmeal and fish oil.
The primary mitigation strategies being explored for feed impacts are optimization of feed use and the use of alternative ingredients. Some knowledge Gaps:
✓ What are the most feasible ways to continue to incentivize the efficient use of fishmeal and fish oil content in aquaculture feeds?
✓ How can robust and consistent understanding of the ecological and social implications of feed use and manufacturing be achieved?
✓ How can technological, policy, and market developments accelerate the process of improving feed conversion ratios and feed formulations?
Parasites, pathogens, & escapes
Aquaculture facilities pose some risks associated with concentrating and amplifying pathogens and parasites, which can then escape and impact wild populations and ecosystems.
The farming of non-native species may create some risk of invasivity, in which the escaped species dominate the local ecosystem, resulting in biodiversity loss and other adverse ecological impacts. However, the extent to which offshore aquaculture would reduce or increase ecological risks associated with parasites, pathogens, and escapes remains understudied and largely unknown.
Although offshore aquaculture is a nascent industry globally, many pilots and several commercial operations have demonstrated that it is technically and perhaps economically feasible, and many lessons can be learned from them.
Interest in developing U.S. offshore aquaculture is increasing among industry leaders, investors, and policymakers. However, many of the risks described, such as those related to the performance of feed in fed aquaculture, are not yet clearly addressed by current or proposed regulation.
U.S. aquaculture facilities operating within a strong and well enforced regulatory system could potentially produce seafood with relatively fewer environmental impacts compared to seafood grown and harvested in contexts with weaker regulation, or in areas that are farther from major markets.
This is a summarized version developed by the editorial team of Aquaculture Magazine based on the review article titled “TOWARD AN ENVIRONMENTALLY RESPONSIBLE OFFSHORE AQUACULTURE INDUSTRY IN THE UNITED STATES: ECOLOGICAL RISKS, REMEDIES, AND KNOWLEDGE GAPS” developed by: Rod Fujita – Environmental Defense Fund, Poppy Brittingham – Stanford University, Ling Cao – Shanghai Jiao Tong University, Halley Froehlich – University of California, Matt Thompson – Anderson Cabot Center for Ocean Life, Taylor Voorhees – Monterey Bay Aquarium and Cargill Aqua Nutrition.
The original article was published on NOVEMBER, 2022, through MARINE POLICY.
The full version, including tables and figures, can be accessed online through this link: