Increasing commercial interest in seabed mineral resources has been driven in part by a perceived need for metals to enable the ‘green transition’ (Miller et al., 2021). A legal framework governs human activities in the marine environment, including rights to marine minerals (see Miller et al., 2018 and Thompson et al., 2018). Broadly, coastal states have rights to resources that fall within the exclusive economic zone (EEZ) within 200 nautical miles (370 km) of their coastline. Some commercial shallow seabed mining has already taken place within EEZs, such as offshore diamond mining in Namibia by Diamond Fields International Ltd. Mineral deposits on the deep seabed in the area beyond national jurisdiction (ABNJ) are regulated by the International Seabed Authority (ISA), a United Nations body. In this Perspective we focus on deep seabed mining in the ABNJ, commonly known as the Area.

The mineral deposits of greatest commercial interest are: (i) polymetallic nodules on the abyssal plains; (ii) seafloor massive (polymetallic) sulfides at hydrothermal vents; and (iii) ferromanganese crusts on seamounts. Of the three deposit types, nodules are most likely to be exploited first because of the even greater technical challenges associated with mining vents and seamounts ( Figure 1A ) (Miller et al., 2018). Deep seabed mining operations have the potential to pose significant risks to ocean ecosystems and not enough is known about the deep seabed mining industry. Although substantial uncertainties and unknowns remain, it is likely that commercial extraction of marine minerals at any scale will have a negative impact on the local ecosystem and potentially further afield. Potential impacts on cetaceans are not well studied to date.

No commercial-scale deep seabed mining operations have yet occurred. In June 2021, however, Narau informed the ISA of its intention to trigger the so-called 'two-year rule', meaning that seabed mining could be permitted in 2023 with whatever rules are in place at that time. The ISA is (as of February 2023) continuing to work on draft regulations for exploitation of minerals in the Area. In spite of significant financial and logistical setbacks – and concerns voiced by marine scientists and conservation groups – seabed mining companies continue efforts to develop commercial operations (Miller et al., 2018).

The general consensus of marine ecologists that have published on this subject is that commercial extraction of minerals from the deep seabed would cause lasting and irreversible damage to fragile ecosystems (Jones et al., 2017; Niner et al., 2018; Levin et al., 2020). Anticipated impacts from mining include noise and light pollution and dispersal of sediment plumes, in addition to direct effects on species and habitats (Miller et al., 2018). Arguably the greatest impact will be habitat loss because mining removes mineral substrate, though predicting the scale and extent of the damage to fragile marine habitats is extremely difficult. Although technological advances have led to improved ocean mapping and scientific understanding of deep-sea biodiversity, ecosystem dynamics and connectivity, much remains unknown. A ten-year global census concluding in 2010 estimated that only approximately 20% of marine species had been described by science (Costello et al., 2010).

Cetaceans, including baleen (mysticetes) and toothed whales (odontocetes), inhabit areas of the oceans where mineral deposits of commercial interest are located. For example, the Clarion Clipperton Zone (CCZ) in the Eastern North Pacific is a key target for manganese nodule extraction and contains complex bathymetry including abyssal plains and seamounts (Cuvelier et al., 2020; Leitner et al., 2021). The CCZ is within the distribution range for many cetacean populations, some of which are globally threatened according to the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species. Seamounts are areas of high productivity and provide important nutrition and navigation cues for cetaceans (Garrigue et al., 2015). In other areas of the ocean, cetaceans have long been negatively impacted by marine industrial activities – many species have been reported as sensitive to chemical pollution, habitat degradation and anthropogenic noise associated with industries such as oil and gas exploration and with commercial shipping (Kavanagh et al., 2019; Pires et al., 2021; Sahri et al., 2021). To date, assessments of impacts from deep seabed mining on biodiversity have focused on those species associated with the seabed and not mobile marine megafauna, many of which have poorly described distributions and ecology in the open ocean areas where mining is likely to occur.

This Perspective aims to provide a preliminary evaluation of the potential impact of deep seabed mining on cetaceans. In particular, we highlight the need for extensive research to address the lack of empirical data related to cetacean ecology in offshore areas and commercial-scale deep seabed mining. Given such knowledge gaps we hope that this Perspective will provide the basis for further discussion and research – and the application of the Precautionary Principle.

Oceanic cetaceans inhabit deep regions of the ocean from the shelf break to the abyssal plains, either as primary habitat (for example, the beaked whales and oceanic dolphins) or during migrations (for example, humpback whales, Megaptera novaeangliea). Gathering data on species offshore presents logistical challenges resulting in considerable knowledge gaps relating to their distributions and ecology. Nonetheless, cetacean distribution data available from online repositories, such as OBIS-Seamaps (https://seamap.env.duke.edu/), indicate significant spatial overlap between cetacean habitats and potential deep seabed mining sites.

The CCZ is in the eastern Pacific range, covering an area of approximately 11,650,000 km² at an average depth of 5,500 m (Wedding et al., 2015; Marsh et al., 2018), and provides habitat to an estimated 22–30 species of cetacean, including dolphins, sperm whales, kogiids and ziphiids (Carwardine, 2020). Population densities are largely unknown. Even though systematic surveys of the CCZ area are sparse, data from multiple research cruises reported in the OBIS-SEAMAP repository show multiple cetacean species within the CCZ ( Figures 1B–G ), including sperm whales (Physeter macrocephalus), multiple baleen and beaked whale species, Risso’s dolphins (Grampus griseus) and dwarf and pygmy sperm whales (Kogia spp.). Although more survey data are needed to determine temporal and spatial use of the CCZ by cetaceans, available data clearly confirm the presence of the aforementioned species in the region.

Seamounts are found in numerous locations around the globe and are also a potential target for deep seabed mining (particularly those whose summits are relatively shallow), as well as being important pelagic biodiversity hotspots. For example, seamounts in the deep waters of the Azores are feeding stations for dolphins (Cascão et al., 2020) and for humpback whales during migration (Ross-Marsh et al., 2022). Data from bottom-mounted hydrophones deployed on Vema seamount, southeast Atlantic, indicated that numerous baleen whale species were present around the seamount (Elwen et al., in press). Similar data collected at Antigonia seamount, South Pacific, indicated the constant presence of odontocetes, with these animals likely moving from deeper to shallower waters to forage at night (Barcock, 2022). Humpbacks tracked across the South Pacific have been recorded spending extended periods of time at seamounts and regrouping in great numbers there, either for feeding or socializing (Derville et al., 2020). Other studies suggest that oceanic seamounts are also important for humpback whale mating, calving and nursing (Derville et al., 2018).

One of the major potential impacts on cetaceans from deep seabed mining activity is expected to be from noise produced by commercial operations, and for that reason is the primary focus of this Perspective. Anthropogenic noise in the ocean travels, or propagates, differently depending on the frequency and amplitude of the source. Ocean sound can propagate far from a point source of production, especially in sound fixing and ranging (SOFAR) channels created through interactions between temperature and pressure, which cause additional internal refraction and horizontal propagation (Prideaux & Prideaux, 2016). Ocean noise is characterized into three frequency bands: low (10 to 500 Hz); medium (500 Hz to 25 kHz); and high (>25 kHz) (Hildebrand, 2009). Low-frequency noise arises predominantly from human sources (commercial shipping and seismic exploration) and has potential for long-range propagation – for example, seismic low frequency airgun noise has been detected over a distance of 3,000 km from source using an array of autonomous bottom-mounted hydrophones (Nieukirk et al., 2004). Medium-frequency sounds have the potential to travel tens of kilometers; high-frequency noise is attenuated within a few kilometers (Hildebrand, 2009).

The frequency, amplitude and duration of noise that could be generated by deep seabed mining will depend upon how minerals are extracted and/or how many companies might carry out extraction in a given area. A reasonable expectation is that commercial-scale mining would operate 24-hours a day, at varying depths at the seabed (Miller et al., 2018), and that noise emitted would depend upon the collection process, specific location and bathymetry. Some deposits could be mined using remotely operated vehicles (ROVs), with a resulting slurry of seabed material and seawater pumped to surface support vessels for primary processing, and with deposition of waste sediment back into the water column (Miller et al., 2018). Noise is likely to be concentrated at the seabed and surface, although midwater pumps, slurry flow and ROV equipment would distribute noise throughout the water column (Christiansen et al., 2020). Surface support vessels produce low frequency and broadband sound (over a broad range of frequencies) similar to that produced by commercial shipping (Hildebrand, 2009). At the seabed, ore extraction is likely to produce a broad range of frequencies, some of which will be at the lower frequencies and could travel for at least several hundred kilometers. Pumps on surface vessels and material transfer between surface vessels may be analogous to noise emitted by standard dredging and drilling although sound propagation could be quite different at greater depths (McQueen et al., 2019). According to Kyhn et al. (2014), drillships that operate at circa 500 m depth introduce broadband noise ranging from 184–190 dB re 1 µPa at 1 m. Acoustic telemetry, used to coordinate ROV equipment, uses loud, repetitive, high frequency sound that can be 40–60 dB louder than dolphins at frequencies around 17 kHz, which is within their audible range. Telemetry is beyond the upper audible range for many adult humans (around 15 kHz on average, Markandeya et al., 2018), but is well within the range that dolphins, porpoises and beaked whales use to communicate.

Indeed, cetaceans produce and detect sounds within a number of specific frequency bands, for conspecific communication (for example, to coordinate group behavior during foraging, social interactions and breeding, Bailey et al., 2009) and to navigate with echolocation, and with significant overlap with a variety of anthropogenic sources of noise ( Figure 2 ) (Allen et al., 2013).

Interference of ocean noise with the sound perception of animals is termed auditory masking and can cause a variety of behavioral changes (Erbe et al., 2016). Masking occurs when anthropogenic noise degrades the perception of biologically important signals, such as those used for communication or navigation (Terhune & Killorn, 2021). The degree of masking is determined by the extent to which the frequency and amplitude of the received noise overlaps with cetacean bioacoustics (Erbe et al., 2016). Depending on the frequency of the noise, individuals close to the source of the sound may be more affected by auditory masking because there is less sound attenuation across short distances than long distances (Erbe et al., 2016). Cetaceans reliant on low frequency sound are more likely to experience masking over long distances because low frequency sound travels the furthest (Hildebrand, 2009; Prideaux and Prideaux, 2016).

Research into the sounds made by whales suggests that increased anthropogenic noise, such as from commercial ships, military sonar or seismic surveys, can mask calls between mating fin whales (Balaenoptera physalus) (Croll et al., 2002). Another study concluded that anthropogenic noise increased the risk of humpback whale mother–calf separation because normal vocalizations between them are very quiet (Videsen et al., 2017). Toothed whales such as sperm whales that rely on higher frequencies of 15 kHz can be negatively affected by sonar (von Benda-Beckmann et al., 2021). Anthropogenic noise, particularly from mid- and low-frequency sonar, seems to elicit either a startle response (foraging beaked whales surface quickly, increasing risk of gas emboli, sometimes followed by stranding) or a cessation in foraging (Frantzis, 1998; Cox et al., 2006; Hooker et al., 2019). Blackwell et al. (2015) found that bowhead whales (Balaena mysticetus) can initially shift calling frequencies to outcompete, but ultimately stop calling in response to increasingly loud airgun noise.

The density, abundance and ecology of most beaked whale species are poorly understood but the spatial overlap between deep seabed mining regions and beaked whale habitat is a concern. Beaked whales such as northern bottlenose whales (Hyperoodon ampullatus) and Cuvier’s beaked whales have been shown to respond to mid-frequency active sonar (1–10 kHz, between 117 – 126 dB re 1 µPa at 1 m) (DeRuiter et al., 2013; Bernaldo de Quirós et al., 2019; Wensveen et al., 2019). Not all cetaceans respond in the same way to a given sound. For example, noise from dredging and shipping prompted avoidance responses in minke whales (Balaenoptera bonaerensis), which are sensitive to low-frequency sound, but did not deter bottlenose dolphins (Tursiops truncatus), perhaps because odontocetes are less sensitive at low frequencies (Anderwald et al., 2013). There is, nonetheless, a clear need for further research into the potential for sound from commercial mining activities to alter the behavior of marine mammals and the implications of this at a population level.

The current fragmented nature of ocean governance in relation to mining activities hinders conservation effort because the seabed is partly under ISA jurisdiction and partly under the jurisdiction of coastal states. The governance of deep seabed mining exploration (and exploitation) in areas beyond national jurisdiction falls under the purview of the ISA, which has responsibility to manage anthropogenic activities in the Area. The ISA has, as of February 2023, issued 31 exploration contracts in the ABNJ for the three main mineral types. In spite of it being an international UN organization, the transparency of the ISA’s work has been called into question (Ardron et al., 2018; Gerber and Grogan, 2020).

More broadly, the lack of coherent ocean protection mechanisms means that marine flora and fauna are vulnerable to increasing anthropogenic threats. The UN negotiations to develop a legally binding instrument for conservation and sustainable use of marine biological diversity in areas beyond national jurisdiction provide an opportunity to consolidate commitments to marine conservation, improve governance of human activities in the high seas and implement dynamic management approaches (Crespo et al., 2020). How such developments may influence the exploration for and exploitation of seabed mineral resources remain to be seen.

Oceans globally are facing increasing pressure from anthropogenic activities. Deep seabed mining would add to those stressors. There are concerns that exploiting non-renewable resources from the deep sea runs counter to United Nations (UN) Sustainable Development Goal 14, which aims “to protect the health of the ocean”, and also brings into question Article 140 of the United Nations Convention on the Law of the Sea (UNCLOS), which stipulates that activities within the ABNJ are to be carried out for “the benefit of mankind as a whole” (UNCLOS, 1982). If deep seabed mining companies begin commercial-scale exploitation in 2023, the industry could damage the oceans in ways we do not fully understand before germane regulations are even developed, and perhaps at the expense of species that have been the focus of conservation effort for many years. Far-from-sight impacts on cetaceans could go largely unquantified and unnoticed, along with those on other pelagic predator species that rely on deep ocean areas, including sharks.

The demand for minerals to support green energy and transport is often cited as justification for seabed mining. This link is questionable and highlights the importance of deeper scrutiny into the drivers underlying the pursuit of seabed minerals and the need to address them as far as possible through more sustainable means (Miller et al., 2021). The IUCN Cetacean Specialist Group has classified five cetacean species as ‘critically endangered’, including the North Atlantic Right whale (Eubalaena glacialis) and the newly described Rice’s whale (Balaenoptera ricei), as well as an additional 19 sub-species or populations. Currently 12 species are listed as ‘endangered’, including the sei whale (Balaenoptera borealis) and blue whale (Balaenoptera musculus) (IUCN, 2022). Insufficient data are available to assess the population status of a significant number of species, though the distributions of at least some of these species include areas being explored for commercial seabed mining. In this Perspective we aim to catalyze focused discussions between cetacean and deep ocean specialists to act with urgency to assess the impacts that deep seabed mining could have on cetacean populations, including through collaboration to cement new research partnerships. An integrated research partnership or consortium could be developed through existing networks, perhaps building on the example of the Southern Ocean Research Partnership that aims to maximize conservation-oriented outcomes for Southern Ocean cetaceans. Given the imminent threat that the two-year rule presents to ocean conservation, we suggest there is no time to waste.

KT, JW and KM conceived the perspective. KT, KM, JW wrote the manuscript. SD, CL, DS, PJ critically reviewed the paper. All authors contributed to the article and approved the submitted version.