• Habitats and Species

Fish & Invertebrates

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What You Should Know

  • Climate change is causing oceans to warm. This warming can lead to sea level rise and prompt some fish species to move to different areas. Such changes have widespread impacts on ecosystems, fish, and marine invertebrate species.
  • Higher concentrations of carbon dioxide (CO2) in the atmosphere increase ocean acidity, making it difficult for shellfish to build strong shells and threatening various forms of marine life.
  • Offshore wind activities during different phases of project development can have different impacts on fish and species of marine invertebrates (e.g., zooplankton, shellfish, squid). Although the impact is temporary, seabed disturbances during construction can affect species that live on or just below the ocean floor. Localized changes to habitat surrounding offshore wind turbines or subsea power cables used in offshore wind farm operations (i.e., export or inter-array cables) can also have variable impacts.
  • Electromagnetic fields (EMFs) from unprotected subsea power cables can cause short-term changes in behavior in some species. However, these cables are typically buried or protected by rock, which reduces the intensity and localized effects of EMFs.
  • Species that communicate using sound can be affected by noise generated during construction (i.e., vessel traffic and pile-driving activities). Technology applications and installation methods can reduce the intensity of noise and mitigate these effects.
  • All offshore wind projects are required to develop approved scientific monitoring programs to assess the effects of construction and operation of offshore wind infrastructure. The results of these monitoring programs have provided greater clarity of short-term effects and the response of fish and invertebrates to construction and operations.
  • Construction and development best practices are used to mitigate negative effects, and new techniques are being developed to better address potential issues.
  • Restorative measures can also be taken to return habitats to their original state, while the artificial reef effects of turbine structures often improve localized biodiversity and ecosystem health.

Interest in EMFs has grown in recent years due to mounting concerns that anthropogenic (i.e., human-induced) EMFs from subsea power cables may have effects on marine life, particularly fish and invertebrates that depend on natural magnetic fields for navigation and other essential life functions. However, there is no conclusive evidence that EMFs generated by subsea power cables, such as those used in offshore wind farm operations, negatively affect individual organisms or populations. While a small number of studies have shown that some marine animals can detect and respond to EMFs, recent field experiments, monitoring efforts, and accurate modeling indicate that potential impacts are highly localized. Because the observed effects are minor and the spatial footprint of EMF sources is small, impacts on individuals and populations are considered unlikely (U.S. Offshore Wind Synthesis of Environmental Effects Research [SEER], 2022a; Bureau of Ocean Energy Management [BOEM], 2024a; Garavelli et al., 2024).

EMFs are a type of low-frequency radiation present in the environment, generated by both natural and anthropogenic sources, such as Earth’s geomagnetic field, thunderstorms, and more recently, subsea power cables. EMFs generated by subsea power cables during offshore wind farm operations can have variable impacts on marine life that occupy habitats along a cable route. These impacts vary based on the type and amount of electrical current a cable carries, the cable design, and the proximity of an organism to a cable. Alternating-current (AC) and direct-current (DC) power cables that may be used in offshore wind projects produce EMFs at different magnitudes and frequencies (Normandeau Associates et al., 2011). Responses to EMFs may occur due to exposure; however, an organism must have the sensory ability to detect the EMF produced by these cable types and be close enough to the source. Bottom-dwelling species, such as benthic or demersal organisms (i.e., those that live and/or feed on or near the seafloor), are more likely to be susceptible to EMF effects than those further up in the water column (BOEM, 2019). Therefore, subsea power cables are buried, when practicable, to reduce the potential impacts of EMFs by increasing the distance between the source and nearby organisms (Figure 1). To learn more about the seafloor and water column, visit Coastal and Marine Habitats.

Figure 1. Depiction of an EMF from a power cable (left) and relative field strength (right) from a snapshot in time. The electric field (orange) is contained by the cable shielding. The magnetic field (blue) is produced by both AC and DC cables. A motion-induced electric field (green) is created as a conductive object moves through the static DC magnetic field of the Earth or the magnetic field from a subsea cable. The figure does not show an induced electric field that would be created around an AC cable due to the rotating magnetic field (AC only) (SEER, 2022a).

Some fish and invertebrate species use either electric (electrosensitive) or magnetic (magnetosensitive) signals, along with other senses, to locate food, habitats, and spawning areas. These include species such as salmon, eel, sturgeon, tuna, sharks, skates, rays, and lobsters (BOEM, 2019). For an organism to sense the EMF emitted by subsea inter-array or export power cables used in offshore wind projects, the emitted intensity and frequency must fall within the organism’s electro- or magnetosensitive detection range (SEER, 2022a). Some species of fish and invertebrates have been found to detect electric fields up to 25 Hertz (Hz), which makes detection of EMFs from DC cables, generally operating at frequency of 10 Hz, possible. Detection of EMFs from AC cables, which typically operate at a frequency of 60 Hz, is much less likely (SEER, 2022a).

Potential effects of EMFs on benthic and demersal fish and invertebrate species can include behavioral responses, altered movement patterns, and physiological effects (Taormina et al., 2018). Temporary changes in behavior and movement patterns in response to EMFs from DC power cables have been observed in species such as sturgeon, skates, and lobsters (Wyman et al., 2023; Hutchison et al., 2018). Wyman et al. (2023) noted varied evidence of green sturgeon responding behaviorally to EMFs from a subsea DC power cable, but found no strong negative effects on migratory behavior or success. Similarly, Hutchison et al. (2018) found that while movement patterns of the American lobster and little skate were significantly altered within the cable EMF zone, the cable did not act as a barrier to their movement. For marine invertebrates, a synthesis by Albert et al. (2020) reported that temporary behavioral and physiological responses to both AC and DC EMF exposure can occur in crustaceans, echinoderms, mollusks, and polychaetes. However, the implications of these responses at the population level remain unidentified.

Although EMF data related to offshore wind farm operations are sparse, laboratory and field studies suggest that the EMF risk associated with subsea power cables is minimal at typical power levels. For instance, Gill and Desender (2020) concluded that biological or ecological effects associated with subsea power cables range from weak to moderate at the EMF intensities associated with marine renewable energy. In a review of potential EMF impacts from subsea power cables on commercial and recreational fish species, BOEM (2019) found that bottom-dwelling fish were more likely to encounter EMFs, with skates having the greatest potential for exposure. However, no evidence of negative impacts from EMFs was found for any of the fishery species studied.

Although research on EMFs in the marine environment continues to progress, EMF detection ranges are not well known for many species (SEER, 2022a). As such, EMF detection or exposure thresholds for marine organisms cannot be established by regulatory agencies. However, consensus based on the available research generally concludes that any potential effects from EMFs generated by offshore wind farms would be minor to negligible and would not register at the population level. To minimize potential effects of EMFs on marine organisms, offshore wind developers implement best management practices, approved by federal agencies, during the installation of export and inter-array cables. These practices include burying cables, using protective coverings (rock or concrete blankets) when burial is insufficient, and applying industry-standard cable shielding. These measures reduce EMF emissions into the surrounding environment to levels that may be detectable by individuals, but are unlikely to affect their health or impact populations.

Various types of fish and invertebrate species occupy the offshore areas where wind energy projects may be sited. Benthic and demersal species are those strongly associated with the seafloor, utilizing various seafloor habitats for feeding, spawning, or protection (e.g., skates, flounder, sea scallops, lobsters). Pelagic species are those mostly found in the water column, that either migrate long distances or drift with currents (e.g., tuna, sharks, squid, plankton). Plankton (microscopic organisms typically less than 4 mm in size that drift with currents) often include the larval stages of many fish and invertebrate species that have been spawned in, or transported to, the offshore pelagic environment.

Key species found in the Northeast U.S. Outer Continental Shelf (OCS), where a majority of U.S. offshore wind development is planned, include fish and invertebrates of commercial, ecological, and cultural importance, particularly those relevant to Tribal Nations. Commercially harvested species such as the Atlantic cod and Atlantic sea scallop, protected species such as Atlantic salmon and Atlantic sturgeon, and ecologically important forage species such as sand lances, occur within the Northeast OCS marine ecosystem (Hare et al., 2016). This indicates a potential overlap between these species’ distributions with planned wind energy areas off the East Coast. The U.S. West Coast and Gulf of Mexico regions, while still in the early phases of offshore wind development, also face similar overlap between planned wind energy areas and key fish and invertebrate species. For instance, recent investigations identified both high-value fisheries (Wang et al., 2022), including groundfish, squid, and Dungeness crab, and essential fish habitats (BOEM, 2024b) intersecting with the Humboldt and Morro Bay wind energy areas off the West Coast. A similar environmental assessment of planned wind energy areas in the Gulf of Mexico identified high overlap with the commercial penaeid shrimp fishery, however sensitive environments and critical habitats were found outside these planned areas (Randall et al., 2022).

Two important statutes that focus on the protection and management of species and habitat in the marine environment are the Endangered Species Act (ESA) of 1973 and the Magnuson-Stevens Fishery Conservation and Management Act. The ESA provides a framework to conserve and protect endangered and threatened species and their habitats. NOAA Fisheries (a division of the National Oceanic and Atmospheric Administration [NOAA]) is responsible for the protection, conservation, and recovery of more than 160 endangered and threatened marine and anadromous (i.e., fish that migrate up rivers from the sea to spawn) species under the ESA. The Magnuson-Stevens Fishery Conservation and Management Act is the primary law that governs marine fisheries management in U.S. federal waters. Because offshore wind farm infrastructure is located in coastal areas, state agencies also play a role in review and management via the Coastal Zone Management Act and other regulations. For a summary of the role of federal and state jurisdictions with respect to offshore wind, review this Congressional Research Service summary report (CRS, 2023).

Some of the most extreme cases of ocean temperature rise due to climate change are projected to occur in waters off the Northeast U.S. (Saba et al., 2016), which have already experienced a consistent warming trend in both long-term data and recent surface and bottom water temperature measurements (Northeast Fisheries Science Center, 2024). A major impact of climate change on the world’s oceans has been the redistribution of marine organisms. Thermal habitat modeling conducted by Morley et al. (2018) on over 600 marine species, including fish and invertebrates, found that many species are projected to experience future shifts in distribution, moving poleward to cooler waters. Geographic shifts in species distributions can have economic implications, such as regional changes in fisheries catch composition (Cheung et al., 2013; Free et al., 2019; Cheung et al., 2021), as well as ecological implications when food web dynamics are altered through the introduction or removal of key species (Pörtner & Peck, 2010; Pinsky et al., 2020).

Hare et al. (2016) conducted an assessment of the vulnerability of 82 marine fish and invertebrate species to climate change on the Northeast OCS. Fish species that were found to have very high vulnerability to the effects of climate change included Atlantic salmon, American shad, blueback herring, hickory shad, shortnose sturgeon, alewife, rainbow smelt, Atlantic sturgeon, and winter flounder. Invertebrates with very high climate vulnerability included ocean quahog, bay scallop, and Eastern oyster. Results from Hare et al. (2016) indicate that these species have the highest risk of altered abundance or productivity from the projected climate change impacts over the next three decades. Among ocean ecosystems studied in the region, the Gulf of Maine has experienced some of the fastest rates of warming over the last two decades. This warming has led the Gulf of Maine to lose some of its subarctic characteristics, causing a decline in stocks of important prey species (e.g., copepods and euphausiid shrimp) and commercially important species near the southern limit of their distribution range (e.g., Northern shrimp, Atlantic cod, and Southern New England American lobster) (Pershing et al., 2021).

Additional negative impacts of climate change include reduced shellfish fishery yields and aquaculture production due to ocean acidification (Hare et al., 2016; Stewart-Sinclair et al., 2020). Increased concentrations of CO2 in the ocean cause it to become more acidic, leading to reduced availability of calcium for shell formation in marine organisms. Ocean acidification also erodes the minerals used by oysters, clams, lobsters, shrimp, coral reefs, and other marine life to build their shells and skeletons, making these structures thinner or harder to maintain and reducing energy for other life functions like finding food or reproducing (Fabry et al., 2008; Cooley et al., 2015; Leung et al., 2022). For more on the effects of ocean acidification on marine life, see the National Oceanic Atmospheric Administration’s (NOAA) webpage on ocean acidification.

Any activity that affects the ocean surface, water column, currents, or seafloor has the potential to impact fish and invertebrate species. Offshore wind installation and operation may cause different types of disturbances, including sound, suspended sediment, seabed disturbance, and potential changes to oceanographic conditions. These changes can result from seabed preparation, cable installation, the presence of structures in the water, intakes and discharges, artificial light, and EMFs, as discussed in this section’s Spotlight Question.

Sound

Underwater sound can cause impacts by producing both sound pressure and particle movement. Sounds can be impulsive (typically louder/higher energy, intermittent, and short-term) or continuous (typically softer/lower energy, ongoing, and long-term) (Popper et al., 2022a). These types of sound can vary widely in intensity and affect organisms in different ways. Sound sources related to offshore wind (Figure 2) include:

  • Acoustic site surveys and sediment coring used to investigate the seafloor during site assessment;
  • Installation sound from foundation pile driving;
  • Operational sound from rotation and vibration of the wind turbines; and
  • Sound from the dismantling of wind farm components during decommissioning (Mooney et , 2020; SEER, 2022b).

Figure 2. Sources of underwater sound during key phases of offshore wind development (Mooney et al., 2020).

The most intense underwater sound from offshore wind development occurs from impact pile driving. The effects of this intense, impulsive noise on fish and invertebrates can include behavioral changes, physiological injury, and, depending on a species’ noise tolerance threshold, mortality (Mooney et al., 2020). Impacts of sound on fish vary depending on a fish’s ability to detect sound pressure, the acoustic sensory organ used, and the distance from the sound source (Popper et al., 2014; Popper et al., 2022b). Impacts from underwater sound diminish with distance from the sound source, as seen in Figure 3 (Mooney et al., 2020). Fishes that have swim bladders (i.e., most bony fishes) are more susceptible to injury than those without, as these fishes generally have lower sound pressure thresholds and can hear a wider range of frequencies (Popper et al., 2014; Mooney et al., 2020). Physiological injuries in fish caused by intense, impulsive sound include auditory hair cell loss (Mooney et al., 2020) and damage to hearing tissues and other organs (Popper & Hastings, 2009).

For invertebrates, intense underwater sound may cause anatomical damage, physiological stress, and behavioral responses (Carroll et al., 2017; Solé et al., 2017).

Figure 3. Potential effects of underwater sound with distance from sound source (Mooney et al., 2020).

The low-amplitude sound generated during the operation of wind turbines is not expected to cause physiological injury to aquatic organisms due to its lower sound pressure levels (ICF, 2021). However, continuous sounds, such as vessel noise and turbine operation noise, have the potential to mask auditory cues used by some fish (e.g., biological cues used by soniferous fish such as Atlantic cod during spawning). Masking communication at specific frequencies could disrupt activities such as foraging or breeding (Popper & Hawkins, 2019; Mooney et al., 2020). To minimize the effects of sound on marine organisms, offshore wind developers use multiple mitigation techniques, including:

  • Technologies to muffle sound during pile driving, such as bubble curtains, isolation casings, and hydro sound dampeners;
  • Soft starts for pile driving, where the gradual increase in hammer blow energy allows mobile species to leave the area; and
  • Time-of-year restrictions that prohibit sound generating activities, such as pile driving, when sensitive marine life is present in the project area (SEER, 2022b).

Up-and-coming mitigation technologies for sound effects are discussed in the Mitigation Innovations section below.

Seabed Preparation and Cable Installation

Subsea export power cables deliver energy from substations within the offshore wind farm to substations that connect to the local power grid, while inter-array power cables connect turbines to each other. Each wind farm has a designated export cable corridor, typically a few hundred meters wide, where cables can be installed (Figure 4).

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Figure 4. Components of a typical offshore wind energy project (BOEM, 2019).

Within these corridors, developers conduct detailed seafloor mapping to understand the sediments and species present. Seabed preparation and subsea power cable installation activities, such as dredging, boulder clearance, trenching, cable burial, and the use of cable protection, can disturb habitats. Impacts may include habitat conversion (e.g., from soft-bottom to hard-bottom or vice versa), displacement of organisms, sediment transport and deposition that can bury slow-moving invertebrates, and potential mortality of fish and invertebrate larvae due to entrainment (i.e., the unintentional removal of organisms by the suction field created during hydraulic dredging). While direct impacts to fish and invertebrates may occur during seabed preparation and cable installation, benthic habitat recovery is expected to occur post-construction. Such recovery has been documented in soft-sediment habitat disturbance studies (Dernie et al., 2003), specifically after dredging (Desprez, 2000; Coates et al., 2015), mining operations (Murray et al., 2024), cable burial (Kraus & Carter, 2018), bottom trawling (de Marignac et al., 2008, Hiddink et al., 2017, Rabaoui et al., 2019), and in the installation and operation of the Block Island Wind Farm in Rhode Island waters (BOEM, 2020; Fonseca et al., 2024).

Offshore wind developers use best management practices to minimize and monitor impacts to the seafloor during preparation and cable installation, such as:

  • Cable micro-siting (small siting adjustments) to avoid complex and sensitive habitats;
  • Relocating boulders to similar boulder habitats and depositing dredged material in areas with similar sediment composition to promote species recolonization;
  • Employing detailed modeling to assess sediment transport and turbidity impacts (i.e., the concentration of suspended sediment particles in the water); and
  • Conducting benthic habitat and fisheries monitoring surveys to track recovery after construction and compare to baseline conditions.

Sand waves (large, ridge-like features that form on the ocean floor) and other soft-bottom benthic features on the OCS of the Middle Atlantic Bight are naturally dynamic structures formed by the interaction of currents and sediment (Dalyander et al., 2013). As such, post-construction recovery of soft-sediment habitats typically occurs on relatively short timescales due to the dynamic nature of sediments (BOEM, 2020). Since the sediment is mobile and the habitat is ever-changing, the benthic communities present are generally early colonizers adapted to disturbance. Where disturbing complex habitats cannot be avoided, the recovery of more stable, hard-structure-oriented communities may take many years to return to their former composition. Where new hard surfaces are introduced (e.g., cable protection), soft-bottom habitats will be functionally converted to hard-bottom for the life of the project, and monitoring of new marine organisms inhabiting these structures will take place instead of assessing recovery (Wilber et al., 2022a; Wilber et al., 2022b; Methratta, 2024).

Presence of Structures in the Water

The introduction of hard surfaces and static structures into the marine environment can influence seafloor and water column communities by altering seabed habitats, creating artificial reefs, modifying local current flows, shifting species distributions, and facilitating the spread of invasive species. To learn more about these impacts visit Coastal and Marine Habitats.

To prevent erosion from moving water, turbine foundations are often surrounded by large rock piles, known as scour protection. These additions convert previously soft-bottom habitats (composed of fine sandy or muddy sediments) into hard-bottom substrates (characterized by pebbles, cobbles, or boulders). This habitat transformation provides structural complexity that attracts various marine species (Wilber et al., 2022a) and likely alters local benthic communities (Figure 5). Over time, organisms such as mussels, barnacles, anemones, and algae colonize these new substrates, forming novel habitats, food webs, and species interactions, a phenomenon known as the “reef effect” (De Mesel et al., 2015; Coolen et al., 2020; Degraer et al., 2020).

A synthesis study on the reef effect of offshore wind farms by Degraer et al. (2020) found increased species densities, biological diversity, and biomass in the communities nearest the turbine foundation. A meta-analysis on finfish abundance at offshore wind farms by Methratta and Dardick (2019) observed almost universal increases in the abundance of benthic and demersal fish species.

At the Block Island Wind Farm researchers observed minimal direct environmental impacts from construction and noted a significant rise in habitat complexity and biological activity over time. Four years post-construction, the site exhibited a super-abundance of blue mussels colonizing turbine foundations, enriched sediments from organic matter, and growing populations of structure-oriented fish like black sea bass (BOEM, 2020). Similarly, South Fork Wind (SFW), located off the coasts of Block Island and Montauk Point, has shown promising early results through a comprehensive benthic monitoring program led by INSPIRE Environmental and Marine Imaging Technologies. Surveys conducted before, during, and after construction (2022–2024) found no significant disturbance to soft sediments or infaunal communities (i.e., benthic organisms that live within the sediment) and documented robust epifaunal (i.e., benthic organisms that live on the substrate) colonization and increased presence of key marine species near the new structures. These consistent findings from two separate locations reinforce the idea that offshore wind infrastructure can not only coexist with marine ecosystems but may also enhance them. Monitoring at SFW will continue through at least 2029 (INSPIRE Environmental, 2025).

While the reef effect is often considered a net positive, there is the potential for negative impacts on certain species. If the wind farm area becomes too much of a fishing hot spot or acts as an ecological trap (where a population occupies a suboptimal habitat), it could lead to a decrease in the targeted fishery population (Vandendreisecche et al., 2013; Reubens et al., 2013; Gill et al., 2020). Additionally, hard substrate introduced by turbine foundations could serve as a “stepping-stone” for invasive species, potentially displacing commercially or ecologically important native species (Adams et al., 2014; Kerckhof et al., 2016; Coolen et al., 2020).

To learn more about the reef effect see the Mitigation Innovations section below or visit Coastal and Marine Habitats.

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Figure 5. Offshore wind farm structures create habitats for invertebrate organisms that colonize the foundations at various depths, attracting predatory fish, seabirds, and marine mammals (Degraer et al., 2020).

Changes in water movement around offshore wind turbines may affect the local incidence of larval fish and invertebrate species that settle out of the water column onto benthic substrates. Alterations in these settlement patterns could impact food availability for species higher up the food chain (ICF, 2021). Research funded by BOEM has explored the effects of offshore wind development on larval distribution and settlement due to changes in hydrodynamics (Johnson et al., 2021), and additional research on the topic is forthcoming.

Typically, the added structure from offshore wind development is considered to have a net neutral or positive effect on the surrounding ecosystem, primarily from the artificial reef effect (English et al., 2017; Gill et al., 2020). However, the degree of benefit or impact may vary by species and location (ICF, 2021). Each study area should be evaluated individually to fully understand the potential short- and long-term impacts of structures introduced into the water column. To learn more on offshore wind impacts on ocean hydrodynamics, see Deeper Dive: Ocean Hydrodynamics, Offshore Wind Farms, and the Mid-Atlantic Bight Cold Pool in the Resources section or see the Spotlight Question in Coastal and Marine Habitats.

Intakes and Discharges

Offshore substations located in and near lease areas collect electricity from wind turbines and transmit it to onshore facilities via export cables. While most early offshore wind projects are not expected to use high-voltage direct-current (HVDC) converter stations, some current and future substations may incorporate them. These converter stations require the use of cooling water intake systems (CWIS), which can pose risks to nearby fish and invertebrate species through impingement and entrainment.

Impingement occurs when organisms are trapped against intake screens by the force of water drawn into the CWIS, while entrainment occurs when organisms are pulled through the intake system and into the cooling mechanism. Eggs and larval stages are particularly vulnerable to entrainment. CWIS intake flows at offshore wind converter stations could range from 7 to 10 million gallons per day, with a typical intake velocity of 0.5 feet per second (in compliance with EPA velocity-based guidelines).

Depending on substation design, potential impacts can be minimized by limiting intake velocities, running a single pump during operations, and/or using variable frequency drives. A modeling study by White et al. (2010) which examined entrainment impacts on larval dispersal and population dynamics at coastal power plants, found minimal effects on the population densities of benthic marine organisms, except in cases where populations had already been heavily depleted by other stressors. That study modeled CWIS intake volumes of approximately 2 billion gallons per day, which is several orders of magnitude greater than the 7 to 10 million gallons per day anticipated at offshore wind converter stations, suggesting that entrainment impacts from offshore wind operations may be minimal. However, further research is needed to assess potential differences between coastal and offshore environments.

Similarly, a synthesis review by Barnthouse (2013) on CWIS-related impingement and entrainment impacts in marine and freshwater ecosystems concluded that these impacts were generally minor compared to more significant drivers of fish population declines and ecosystem degradation, such as overfishing, habitat destruction, pollution, and invasive species.

In addition to impingement and entrainment, the CWIS cooling process discharges heated effluent (liquid waste) back into the surrounding environment at a maximum temperature of 90°F (32.2°C). This thermal discharge may have adverse effects on nearby fish and invertebrates. The size of the affected area depends on local hydrodynamics and the CWIS design, but is generally confined to the immediate vicinity of the HVDC converter station. Offshore wind developers planning to use HVDC converter stations (e.g., Sunrise Wind, LLC and SouthCoast Wind, LLC) have conducted modeling studies indicating that the warmer outflow is likely to have minimal effects on surrounding habitats and species (Middleton & Barnhart, 2022). Potential impacts on surrounding seawater from CWIS use are subject to permitting under the EPA’s National Pollutant Discharge Elimination System (NPDES).

Since previous studies have primarily focused on nearshore, estuarine, and riverine ecosystems, it will be important to identify and monitor the environmental impacts of offshore converter stations once these facilities are commissioned.

Artificial Light

In accordance with Federal Aviation Administration (FAA) and U.S. Coast Guard (USCG) lighting standards, wind turbine generators and offshore substations must be outfitted with appropriate markings and lighting to prevent collisions with vessels and aircraft. Work vessels traveling to and from offshore wind farm project areas also use artificial lighting outside of daylight hours. At night, this artificial lighting may affect pelagic fish and invertebrates by altering localized movement patterns due to light attraction or avoidance, shifting distributions in response to altered movement patterns, and inducing behavioral changes (Marangoni et al., 2022).

The extent of these effects, and the degree at which they occur, largely depends on the duration of light exposure and the depth to which the light penetrates the ocean surface. Unlike continuous light sources, which have been shown to influence behavior and movement (Longcore & Rich, 2004; Bassi et al., 2022), lighting on wind turbines and work vessels is intermittent, and therefore unlikely to cause sustained light-related impacts on fish and marine invertebrates.

While some effects may be unavoidable, all potential impacts from an offshore wind project are evaluated within a mitigation framework aimed at avoiding, minimizing, or mitigating adverse effects to the extent feasible.

Sound

As the introduction of sound to the underwater environment can be one of the most harmful effects, many mitigation efforts focus on reducing sound impacts, especially those from pile driving activities. Recent innovations in sound reduction include isolation casings such as the IQIP Integrated Monopile Installer and the AdBm Technologies Noise Mitigation System.

The IQIP Integrated Monopile Installer uses a double-wall steel casing with a bubble curtain between the casing and the pile. This design has been shown to reduce pile driving noise by 13 to 16 decibels (Koschinski & Lüdemann, 2020).

The AdBm Technologies Noise Mitigation System surrounds the pile with large arrays of Helmholtz resonators, in the form of custom injection-molded blocks, that trap air bubbles to capture and reduce noise (Wochner, 2019). On its own, this system can reduce pile driving sound by 7 to 8 decibels, and by 14 to 15 decibels when combined with a bubble curtain (Elzinga et al., 2019). These reductions represent approximately an 8% reduction in pile driving sound, based on a reference sound pressure level of 200 decibels (Reinhall & Dahl, 2011).

Since underwater sound levels produced by pile driving can vary depending on factors such as substrate characteristics, depth, pile diameter, and size of the impact hammer, multiple Noise Abatement Systems (NAS) may be used in combination to ensure that sound thresholds are not exceeded.

Seabed

Innovations in subsea power cable protection and stabilization materials can help offset some of the seabed and habitat disturbances caused by installation. For example, ECOncrete ECO Mats have been developed to promote colonization of benthic marine organisms. Made with ECOncrete Admix, these interlocking mats feature complex surface textures designed to facilitate colonization. Compared to smooth-surface concrete blocks, ECOncrete ECO Mats have been shown to support greater species richness, diversity, and abundance among both non-mobile and mobile benthic species (Sella et al., 2021). Taormina et al. (2020) found that concrete mattresses used to stabilize subsea cables off the coast of France have a similar effect and provided suitable habitat for five groups of large fish and invertebrates.

Reef Effect

Add-on structures that attach to offshore wind turbine foundations have been developed to promote reef growth, provide shelter for benthic organisms, and provide ecosystem support to specific species through nature-inclusive designs. Nature-inclusive designs integrate ecological features into offshore wind infrastructure, such as turbine foundations and scour protection, to create suitable habitat for native species or communities (Hermans et al., 2020). Recent add-on structure innovations include the Reef Ball Foundation Layer Cake and the Witteven + Bos Cod Hotel.

In a study of fish colonization of artificial reef structures in the Caribbean, Hylkema et al. (2020) found that fish abundance, biomass, and species richness were significantly higher on artificial reef structures compared to sandy bottom control sites. Among the structures tested (reef balls, rock piles, and Layer Cakes), the Layer Cake supported the highest fish abundance and biomass. Additionally, Kingma et al. (2024) found that modifying scour protection to be more nature-inclusive in North Sea wind farms was associated with increased benthic species richness and functional traits.

The Cod Hotel (Hermans et al., 2020) was specifically designed to accommodate Atlantic cod in North Sea offshore wind farms. It consists of a steel gabion basket filled with perforated steel tubes and monitoring funnels. These add-on structures are expected to increase the biomass of Atlantic cod and other fish species around wind turbine foundations by offering shelter for various life stages and enhancing the availability of prey items, such as small crustaceans.

A valuable resource for exploring emerging mitigation and monitoring technologies is the Wind Energy Monitoring and Mitigation Technologies Tool, developed by Working Together to Resolve Environmental Effects of Wind Energy (WREN) and accessible via the Pacific Northwest National Laboratory’s (PNNL) TETHYS website: Wind Energy Monitoring and Mitigation Technologies Tool | Tethys (pnnl.gov).

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