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1. nature
2. npj ocean sustainability
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4. article

Climate change to drive increasing overlap between Pacific tuna fisheries and
emerging deep-sea mining industry
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* Open Access
* Published: 11 July 2023

CLIMATE CHANGE TO DRIVE INCREASING OVERLAP BETWEEN PACIFIC TUNA FISHERIES AND
EMERGING DEEP-SEA MINING INDUSTRY

* Diva J. Amon ORCID: orcid.org/0000-0003-3044-107X1,2,
* Juliano Palacios-Abrantes ORCID: orcid.org/0000-0001-8969-54163,
* Jeffrey C. Drazen ORCID: orcid.org/0000-0001-9613-38334,
* Hannah Lily ORCID: orcid.org/0000-0002-5888-74125,
* Neil Nathan ORCID: orcid.org/0000-0003-3945-84841,
* Jesse M. A. van der Grient ORCID: orcid.org/0000-0002-9115-38756 &
* …
* Douglas McCauley ORCID: orcid.org/0000-0002-8100-653X1

Show authors

npj Ocean Sustainability volume 2, Article number: 9 (2023) Cite this article

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ABSTRACT

In ocean areas beyond national jurisdiction, various legal regimes and
governance structures result in diffused responsibility and create challenges
for management. Here we show those challenges are set to expand with climate
change driving increasing overlap between eastern Pacific tuna fisheries and the
emerging industry of deep-sea mining. Climate models suggest that tuna
distributions will shift in the coming decades. Within the Clarion-Clipperton
Zone of the Pacific Ocean, a region containing 1.1 million km2 of deep-sea
mining exploration contracts, the total biomass for bigeye, skipjack, and
yellowfin tuna species are forecasted to increase relative to today under two
tested climate-change scenarios. Percentage increases are 10–11% for bigeye,
30–31% for skipjack, and 23% for yellowfin. The interactions between mining,
fish populations, and climate change are complex and unknown. However, these
projected increases in overlap indicate that the potential for conflict and
resultant environmental and economic repercussions will be exacerbated in a
climate-altered ocean. This has implications for the holistic and sustainable
management of this area, with pathways suggested for closing these critical
gaps.

INTRODUCTION

The ocean constitutes 71% of the Earth’s surface, and nearly two-thirds of this
are areas beyond national jurisdiction (ABNJ, or for the water column ‘the high
seas’). Within ABNJ, there are different legal regimes and governance structures
for the non-living resources (e.g., minerals) and the living resources (e.g.,
fishes). The United Nations Convention on the Law of the Sea (UNCLOS) created a
central intergovernmental agency, the International Seabed Authority (ISA), that
has responsibility to govern activities relating to minerals and the ABNJ
seafloor, as well as the resulting potential environmental impacts1. The same
Convention provides a legal regime for the use, conservation, and management of
high-seas fishery resources2. These legal rules are generally implemented via
regional fisheries management organisations (RFMOs), created to regulate
economically-important highly-migratory fish stocks in ABNJ3. RFMOs themselves
differ in their scope, approach, and mandate, with five RFMOs that address
highly migratory tuna stocks and tuna-like species such as marlin or swordfish.
While some stocks are considered healthy4,5, there are significant gaps in
coverage and ineffective management for others6. These multiple designations
have diffused responsibility for ABNJ, creating challenges for the management of
marine resources in the high seas, especially when the separate management of
the living and non-living resources may result in conflicting strategies.

The Clarion-Clipperton Zone (CCZ) is a large area (4.5 million km2) located
between Hawai‘i and Mexico in the eastern Pacific Ocean7. There is high
environmental heterogeneity across this area, demonstrated by primary
productivity increasing west to east and north to south, dissolved oxygen
concentrations influenced by the presence of a mobile oxygen minimum zone in the
water column, and high densities of polymetallic nodules on the seafloor8,9.
These nodules have attracted commercial interest in deep-sea mining, with 17
exploration contracts currently issued across the region by the ISA, covering a
quarter of the area of the CCZ seabed (1.125 million km2)
(https://www.isa.org.jm/exploration-contracts/exploration-areas/).

There are also commercial fish stocks of tuna in the CCZ, with this region
falling under the jurisdiction of two tuna RFMOs: the Inter-American Tropical
Tuna Commission (IATTC) and the Western and Central Pacific Fisheries Commission
(WCPFC)10. These two RFMOs oversee three of the main Pacific tuna species:
bigeye (Thunnus obesus), skipjack (Katsuwonus pelamis), and yellowfin (T.
albacares) tuna. Together, fisheries targeting these species captured 3.5
million tonnes (IATTC 687,000 tonnes and WCPFC 2.9 million tonnes) in 2022,
which was 66% of global tuna catches reported that year5,11. Within the CCZ and
surrounding waters, catches for these species average 35,000 to 78,000 tonnes
per year10. These are some of the most profitable fisheries in the world; market
price (i.e., dock price) fluctuates between $1,000 USD per tonne for skipjack
tuna to over $5,000 USD for bigeye tuna, reaching an end value of over $10,000
USD per tonne12. In total, the economic value of these three species within
these two RFMOs fluctuate around $5.5 billion USD per year (WCPFC $4.3 billion
USD, IATTC $1.2 billion USD)13 (www.seaaroundus.org).

If deep-sea mineral exploration projects, currently operational in the CCZ, are
permitted by the ISA to move to an exploitation phase, substantial environmental
impacts would likely be caused14,15,16,17. Additionally, conflict between
fisheries and deep-sea mining will likely occur given the existing spatial
overlap within and around the CCZ10. There are at least four different
mechanisms by which nodule mining in the CCZ could negatively impact fisheries
specifically. However, the extent of these impacts is largely unknown and/or
debated, especially given the significant scientific gaps in this region18,19.
First, there will be two plumes, one where sediment is stirred up by the mining
of the nodules at the seafloor, and a second where unwanted water and material
separated from the nodules is discharged into the ocean from the surface mining
vessel. The discharge plumes will raise the particle concentration in the water
column. This could interfere and harm filter feeding apparatuses and gills of
tuna and their prey (which include diurnal vertical migrators), reduce visual
communication, and increase stress hormone levels10,20. This could extend the
impacts of deep-sea mining horizontally for tens to hundreds of kilometres and
vertically for hundreds to thousands of meters10,21,22.

Second, the return-water discharge plume is expected to contain elevated
concentrations of metals. As minerals are collected, they will likely fragment
with some dissolving into seawater and some adhering to sediment or organic
particles. Such particles could be ingested and incorporated into deep-sea food
webs entering our seafood supply with bioaccumulation in tuna21. Even if there
were only localised effects or low risks from toxic accumulation or contaminant
presence, this could still have a high impact on tuna fisheries through a
negative consumer/market reaction19,23. Further, the tuna species fished in the
CCZ are highly migratory and any contamination of fish through the food chain
could percolate through the wider stock distribution19. Third, mining noise
could also be extensive and cause physiological impacts in tuna and their prey,
leading them to alter their feeding and/or reproductive migrations, and
potentially reducing catch rates21,24,25. Lastly, an increased density of mining
vessels restricted in their ability to manoeuvre could limit fishing vessel
operation, as well as result in changes to tuna behaviour (avoidance or
attraction). These impacts may extend to further ABNJ biodiversity e.g.,
seabirds that are dependent on sub-surface facilitated feeding by tuna may
themselves shift into higher degrees of overlap with mining activities with
uncertain consequences for their health26,27.

The ocean has absorbed 90% of the extra heat and 20–30% of the CO2 released from
anthropogenic activities over the last decades, resulting in profound
geochemical changes28,29. As temperature and pH continue to increase, and oxygen
concentration decreases, marine species have adopted different coping
strategies, shifting their historical distributions, sometimes toward higher
latitudes or deeper waters30. Specifically, yellowfin, bigeye and skipjack tunas
in the Pacific Ocean are expected to shift distribution and abundance towards
the equatorial eastern Pacific31 and poleward, with some studies suggesting
shifts are already occurring32,33. Such shifts are expected to continue within
the current century regardless of the climate-change scenario34,35, threatening
catches of targeted species36, jobs, and revenues37, and challenging
international fisheries management globally38,39.

This paper was instigated by recent findings that three commercially-important
species of tuna will experience climate redistribution, with the equatorial
eastern Pacific acting as a future climate refugia31. These changes could have
profound risks to the economies, livelihoods, and well-being of Pacific small
island developing States and coastal States31. Given the emergence of deep-sea
mining, a new ocean industry that has the potential to result in biodiversity
and habitat loss across large scales in the same area of the Pacific, we explore
here the future intersections between tuna fisheries and deep-sea mining under
climate change in the CCZ specifically. We also discuss the challenges of
effectively managing the living and non-living resources in this area, and end
with recommendations to achieve holistic and sustainable management of this area
in a rapidly changing ocean.

RESULTS AND DISCUSSION

POTENTIAL CONFLICTS BETWEEN FISHERIES, DEEP-SEA MINING, AND CLIMATE CHANGE

Results show increases in biomass for all three tuna species within the CCZ by
the mid-21st century relative to today under both climate-change scenarios
tested (Fig. 1). When considering the overall percentage change in biomass for
CCZ contract areas and the associated buffer zone, increases ranged from 10% for
bigeye tuna under Representative Concentration Pathway (RCP) 4.5 to 31% for
skipjack tuna under RCP 8.5, with an average of 21% for all three species for
both RCPs (Fig. 1). Overall percentage increases in biomass for the CCZ by the
mid-21st century are not projected to show much variation between RCP 4.5 and
8.5, suggesting that tuna will move to the CCZ regardless of the climate-change
scenario (Fig. 1). The small difference in projected tuna biomasses between
scenarios can be seen for most ABNJ of the Pacific region31 and may be
attributed to little variation between RCP pathways throughout the first half of
the 21st century40.

Fig. 1: Percentage change in the biomass of tuna for the Clarion-Clipperton Zone
by the mid-21st century (average of 2044 to 2053) relative to present (average
of 2009 to 2018).

Three species of tuna are included from left to right: bigeye tuna (Thunnus
obesus), skipjack tuna (Katsuwonus pelamis), and yellowfin tuna (T. albacares).
The black line around the CCZ denotes 200 kilometers from deep-sea mining
exploration contract-area boundaries. This buffer was used as several modelling
studies have suggested that midwater sediment plumes may spread over such
distances. The buffer was created using the geoprocessing tool Buffer in qGIS
v3.8. All maps are split into the two relevant RFMOs: the Western and Central
Pacific Fisheries Commission (WCPFC) (dark blue) and the Inter-American Tropical
Tuna Commission (IATTC) (light blue). Percentage values on the top right of the
CCZ in each panel represent the percentage changes in tuna biomass for the
entire CCZ. a–c are under Representative Concentration Pathways (RCP) 4.5 and
d–f represent RCP 8.5.

Full size image

The distribution of biomass change within the CCZ does, however, vary spatially
and between species (Fig. 1). Specifically, there is an increase in skipjack and
yellowfin tuna biomasses within most of the CCZ, regardless of the
climate-change scenario (Fig. 1). On the other hand, bigeye tuna presents a
reduction-to-no-change within most of the region, with the exception of the
eastern zone where increases will be substantial in both climate-change
scenarios (Fig. 1). Spatial patterns show a higher concentration of bigeye tuna
in the eastern part of the CCZ under both climate-change scenarios, while
skipjack shows a more central (RCP 8.5) or eastward (RCP 4.5) trend (Fig. 1).
Finally, yellowfin tuna shows a more homogenous distribution (Fig. 1). However,
such spatial variation should be approached cautiously given the high mobility
of tuna species and the resolution of these results.

Taking into account the historical expansion of industrial-scale tuna fisheries
and the shift in distribution driven by greenhouse-gas emissions by the mid-21st
century, these commercially-important species will migrate into an area where
deep-sea mining could be underway (Fig. 1)31,41. This could result in increasing
conflict between the two industries if more tuna catches are obtained from
mining areas10.

Tuna fisheries in the CCZ may be impacted by deep-sea mining in several ways:
the direct impacts of nodule mining are predicted to include noise, light, and
movement of ships at the sea surface, as well as the benthic and discharge
plumes. The level of interaction with and impact from the return-water discharge
mining plumes and tuna fisheries will be influenced by the depth of the release
of discharge. That is, the deeper the release depth, the smaller the spread of
the plumes will likely be and the less chance to impact tunas and the life they
depend on. Discharge depths below the mesopelagic zone (200–1000 m depth) will
be more likely to avoid deep-diving tuna, such as bigeye tuna that are usually
between 0 and 500 m and their prey which can live to 1500 m
depth9,21,42,43,44,45. However, this is likely to be more costly and technically
challenging for deep-sea mining operations. Additionally, deeper discharges of
plumes will likely still affect non-tuna species present at deeper depths,
including benthic species that may still be important components of the
functioning and food webs of this area.

Further, differences in ecology, including mobility, distribution, associations
with floating objects, diet, predatory behaviour, stock structures, and spawning
patterns, could result in varied impacts to each tuna species. Oceanographic
factors (such as current flow rates and direction) will also affect the
likelihood of impact. In addition to the above specific direct impacts that may
be caused to tuna from deep-sea mining, it is also possible that the cumulative
environmental impacts of deep-sea mining serve to make the CCZ generally less
hospitable for tuna, ultimately lessening their options for climate
refugia21,32.

Any impacts on tuna at a population or stock level could lead to effects on
tuna-dependent economies that fish within and around the CCZ, presenting
unresolved equity issues. The five countries that obtain the highest average
annual tuna catches (in tonnes) in the CCZ are Mexico, Venezuela, Nicaragua,
Panama, and Colombia, with Mexico and Venezuela obtaining 21% and 10% of their
tuna there respectively10. Between 2009 and 2018, ABNJ fisheries of yellowfin,
skipjack and bigeye tuna landed 250,000 tonnes, generating $512 million USD for
these countries, with yellowfin creating the highest revenue of the three with
109,000 tonnes generating $281 million USD13 (www.searoundus.org). While these
values represent overall ABNJ catches, it demonstrates the economic importance
of these species to these countries. As climate change shifts the distribution
of Pacific tunas eastwards from Pacific Island national jurisdictions to
high-seas areas31 such as the CCZ, new fishing opportunities will likely arise
for those countries fishing in the region. Additionally, nations that do not
fish the most in the licensed mining areas of the CCZ may still derive a
moderate proportion of their RFMO tuna catches there. For example, China obtains
17% of their RFMO-reported tuna catches in the mining areas and 200-km zone,
while Belize and Nicaragua obtain 11%10. Given the shifting of these stocks due
to climate change, and the potential overlap between fishing and mining
activities within and around the CCZ, these percentages are likely to increase,
which could represent a substantial economic risk for nations fishing in the
region.

There may also be further socioeconomic impacts (e.g., loss of income, decreases
in employment, lowered food security, and well-being) to nations surrounding the
CCZ if tunas are impacted by mining and their migration patterns into
neighbouring Exclusive Economic Zones are altered. Negative impacts to tuna
stocks, or even increased consumer concern about toxic accumulation or
contaminant presence, could have particularly devastating consequences for
several countries whose economies are described as ‘tuna-dependent’23,31.
Ultimately, deep-sea mining by more economically developed States could lead to
small island developing States and other developing coastal States being
disenfranchised, with a resulting increased risk of legal challenges and/or
conflict. While we focus specifically on the CCZ where deep-sea mining may be
more likely to commence, it should be noted that fishery overlap with mining
occurs in other regions and ocean basins, and future work is required to
identify how industry overlap may be affected by climate change10. In addition,
while tuna dominate catches obtained in the CCZ10, there are other
commercially-important species, such as billfishes, that occur in the CCZ, and
there are other areas of potential mining-fishery overlap such as at encrusted
seamounts for seamount-associated fish species.

SCIENTIFIC GAPS

Sustainable resource-management regimes generally rely upon a robust scientific
understanding of the environment and the impacts from development activities.
However, there is a lack of basic scientific knowledge about the CCZ, especially
regarding deep-sea ecosystems18. There is also, at this nascent stage of the
industry where no large-scale mining operations have yet commenced, little
applied knowledge about how deep-sea mining will impact CCZ ecosystems and
fisheries. This is linked to the lack of information on environmental baselines,
potential responses to mining impacts, and specific deep-sea mining technology
and mining processes (e.g., the depth of discharge)18. There is also little
known about these three focal tuna species with regard to their sensitivities to
deep-sea mining activities and climate change, in particular for juveniles and
the communities these tuna depend on31. More scientific data is needed to
understand the spatial and temporal dynamics of the mining discharge plume
(including dissolved and particulate metals), their consequences for tuna and
fisheries, and climate modelling to increase accuracy of future predictions. The
cumulative and/or synergistic impacts of deep-sea mining, fisheries, and climate
change are also unknown, but likely important18.

GOVERNANCE GAPS

Governance of the CCZ is challenging for a number of reasons including, but not
limited to, the significant scientific knowledge gaps discussed above and
fragmented multi-sectoral governance in this region. These challenges will
likely magnify as human activities increase and the impacts of climate change
are felt more acutely46. The number of legal regimes (e.g., UNCLOS, UN Fish
Stocks Agreement, UN Framework Convention on Climate Change, national laws) and
governance structures (e.g., ISA, Food and Agriculture Organization,
International Maritime Organization, WCPFC, IATTC, those of individual States)
applicable to the CCZ results in a fragmented patchwork that prevents a holistic
approach to managing human activities in this area. However, it is only three of
these organisations that are responsible for applying an ecosystem-based
approach – the ISA, the WCPFC, and the IATTC.

UNCLOS continues to evolve, and in March 2023, a text was agreed between States,
paving the way for a historic treaty to conserve and sustainably use marine
biodiversity in ABNJ (the BBNJ Agreement). This treaty has been designed to
avoid undermining existing governance regimes in ABNJ, so it remains to be seen
how the new legal regime and structures will interact with management of
fisheries and deep-sea mining, and components of the agreement related to
area-based management31,47,48,49. Notwithstanding, there are a number of UNCLOS
(and other international law) duties relevant to States and the ISA conserving
and/or not adversely affecting biodiversity in the water column (including fish
stocks), including as a result of deep-sea mining activities (e.g., UNCLOS
Articles 145, 192, 194, 206, 209, and UN Straddling Fish Stocks Agreement, the
International Convention for the Prevention of Pollution from Ships (MARPOL),
the Convention on Biological Diversity Articles 3, 8, 14, and customary
international law obligations to prevent transboundary harm).

Further uncertainty arises from the unsettled status of the ISA’s rules,
regulations, and procedures for exploitation of mineral resources in ABNJ (known
as ‘the Mining Code’). This complex matrix of rules and legal standards are
currently under multilateral negotiation50. It has been noted that climate
considerations were little featured in the draft text presented by the ISA’s
technical body to its member States for further negotiation51. UNCLOS requires
that deep-sea mining must operate with ‘reasonable regard for other activities
in the marine environment’ and vice versa, including fisheries, and specifies
that deep-sea mining installations may not be established in areas of intense
fishing activity (Article 147). The principle of ‘reasonable regard’ (often
equated with ‘due regard’ used elsewhere in UNCLOS) requires consideration of a
balance of interests on a case-by-case basis, often requiring dispute resolution
and interpretation by the Courts52. It is not clear how ISA contractors’ duty of
‘reasonable regard’ for fisheries will play out in practice, but would appear to
require more detailed regulations to have operational effect, which are not
currently in place at the ISA. Indeed, there has been surprisingly limited
consideration, engagement, or consultation thus far between the ISA and fishing
industries and/or RFMOs in relation to mining activities in the CCZ. The ISA’s
list of 69 observer organisations features no RFMOs or fishery groups
(https://www.isa.org.jm/observers/). The FAO, which has a focus on fisheries, is
an intergovernmental organisation observer to the ISA, but has neither attended
nor made any submission to ISA proceedings in at least the past five years
during which the Mining Code has been negotiated (Authors’ own research, from
ISA sessions’ delegation lists:
https://www.isa.org.jm/wp-content/uploads/2023/02/ISBA_27_A_INF_6-List-of-Delegations_Assembly_27th-Session-rev-11082022.pdf,
https://www.isa.org.jm/wp-content/uploads/2022/06/ISBA_26_A_INF_3.pdf,
https://www.isa.org.jm/wp-content/uploads/2022/06/isba25-a-crp5_0.pdf,
https://www.isa.org.jm/wp-content/uploads/2022/06/isba-24a-crp-4_0.pdf). The
Regional Environmental Management Plan (REMP) for the CCZ, one of the ISA’s
principal environmental management tools, does not include assessment of fish
stocks or fishing activity in the region, does not consider potential impacts on
fisheries, and does not require consultation by ISA contractors or organs with
the fishery sector53,54,55.

There have also been concerns raised about a lack of transparency at the ISA56,
and a lack of participatory approach in ISA decision-making56,57. As an example,
Venezuela, one of the nations identified as particularly reliant upon fisheries
in the CCZ area, is not a member of the ISA, and as such may need proactive
outreach from the ISA in order to be informed about the regime and to engage in
its deliberations (in a non-voting capacity). There is no evidence of such
outreach, and Venezuela does not currently attend or engage in any way, in the
ISA’s meetings (Authors’ own research, from ISA sessions’ delegation lists:
https://www.isa.org.jm/wp-content/uploads/2023/02/ISBA_27_A_INF_6-List-of-Delegations_Assembly_27th-Session-rev-11082022.pdf,
https://www.isa.org.jm/wp-content/uploads/2022/06/ISBA_26_A_INF_3.pdf,
https://www.isa.org.jm/wp-content/uploads/2022/06/isba25-a-crp5_0.pdf,
https://www.isa.org.jm/wp-content/uploads/2022/06/isba-24a-crp-4_0.pdf).
Additionally, at both the ISA and RFMOs, there are questions around abilities to
address critical environmental-management issues given the level of influence by
the interests and political will of their constituent members, rather than
reflecting truly whole-of-government mandates. However, 16 of 20 States (plus
the EU) in the IATTC and all except one in the WCPFC are members of the ISA.
Five member States are Sponsoring States in the IATTC and seven within the
WCPFC, while six in the IATTC and seven in the WCPFC have called for a ban,
pause, or moratorium on deep-sea mining, pointing to a need for dual and
holistic consideration.

The remoteness of the CCZ presents an additional challenge with regard to
monitoring, control, and surveillance. The ISA has yet to agree on an inspection
and monitoring regime for future activities in the CCZ58, but currently relies
upon self-reporting by its contractors without independent data verification59.
Concerns already exist about inadequacies to combat, and therefore persistence
of, illegal, unreported, and unregulated fishing globally60; deep-sea mining may
be responsible for similarly challenging environmental infractions. It is
unclear the extent to which fisheries activities could disrupt deep-sea mining
activities in the same region, and what measures may be taken to prevent or
remedy this. It is notable that the monitoring, surveillance, and control of
fisheries on the high seas (which may include vessel monitoring systems, data
collection and reporting, inspection schemes or observer programmes, and
sanctions for non-compliant vessels) is largely dependent on the ability and
willingness of individual flag States to exercise effective control over vessels
flying their flag, and States may have differing degrees of capability in that
regard61,62.

The potential for deep-sea mining to affect fisheries also requires
consideration of how the miners will be regulated. To ensure reasonable regard
for fisheries operating in the same region, the ISA will need powers not only to
monitor, but also to enforce compliance by miners with its Mining Code (once
adopted). The practicalities of this part of the regime have received little
attention to date at the ISA63. There are various legal and political
complexities at play. The ISA itself, as an intergovernmental organisation, does
not have jurisdiction to create or prosecute criminal offences, nor to conduct
inspections, arrests, asset-freezing, etc. within national jurisdictions. The
ISA must therefore rely on cooperation from individual States for such measures.
However, analysis suggests that some States currently sponsoring ISA exploration
contracts do not have relevant measures in place in their national legal
regimes, nor clear procedures in their court systems to deal with disputes that
may arise in relation to their contractors' actions64. An entity holding a
deep-sea mining contract with the ISA may itself be a State, which may be
unlikely to wish to impose such sanctions upon itself or its officers. An ISA
contractor may also be a private-sector entity sponsored by a State. This
sponsoring State is required to have its own measures in place to ensure
contractor compliance, which can offer an additional layer of regulatory control
within the overall ISA regime. There is however concern about a developing trend
of ‘sponsoring states of convenience’ whereby a private-sector ISA contractor
partners with a sponsoring State, which may have no meaningful relationship or
control over that contractor, and/or may have very limited regulatory capacity
in practice65,66.

To further complicate the enforcement regime for deep-sea mining in ABNJ, the
mining operations will occur from a vessel located in the high seas. According
to the UNCLOS regime, such a vessel is under the exclusive jurisdiction of the
flag State. There is no requirement in UNCLOS (or in the ISA rules currently)
for that flag State to be the same State as the ISA contractor / contract’s
sponsoring State. Nor even for the flag State to be a member of the ISA, and
thus bound by its regulations. It can therefore be seen that the compliance
regime for deep-sea mining in ABNJ is complex and may be susceptible to
governance gaps or abuse to evade monitoring and compliance measures.

PATHWAYS TO EFFECTIVE MANAGEMENT

The decades ahead will herald a new seascape for ocean management, with many
challenges to overcome for multiple marine sectors to be managed
synergistically, sustainably, and equitably. Below are recommendations for
pathways towards effective ecosystem-based and cooperative management of
multi-sectoral human impacts in the CCZ. Given the many outstanding critical
scientific gaps related to the impacts of deep-sea mining, fisheries, and
climate change, as well as their interactions, in the CCZ, that must be closed
through scientific research for effective management to be possible, it would be
prudent for the ISA not to permit mining unless and until the likely impacts are
properly understood, and manageable within agreed thresholds18,67,68, including
consideration of potential effects on tuna stocks. In this vein, there have been
increasing calls for a precautionary pause or moratorium on deep-sea mining,
from various sources, including downstream users of minerals, governments, ocean
experts, civil society and, most recently and of particular relevance, fishery
management organisations and downstream users of fisheries69,70
(https://seabedminingsciencestatement.org/; https://www.noseabedmining.org/).

There is also the need for the development and evolution of relevant rules,
regulations, and procedures of the ISA’s Mining Code, as well as the ISA’s
management tools including the standardisation of the ISA’s REMP process. These
regional management planning processes should be expanded to ensure all marine
life and uses (including fish stocks and fishery activities) are properly mapped
and assessed. The planning processes should include future forecast scenarios
that take into account climate modelling. All relevant stakeholders, including
RFMOs, the fishing industry, and Pacific tuna-dependent States, should be
brought into transparent, proactive, and consultative management processes.
Generally, the development of mechanisms for information exchange and a
formalised inclusive consultation process between the ISA and fishing industries
and/or RFMOs, as well as between the WCPFC and IATTC themselves specifically for
the CCZ, seems an urgent matter.

As suggested by Goodman, et al.71, the IATTC and the WCPFC could develop an
expanded framework for cooperation and collaboration that would allow them to
fulfil their conservation and management responsibilities under international
law. Specifically, these could include a formal mechanism for cooperation to
enable effective and efficient decision-making and action by the two RFMOs on
key issues, such as deep-sea mining and climate change71. Further cooperation
will be needed on scientific research and modelling to better understand the
biology and distributions of Pacific tuna stocks and how they will respond to
individual and cumulative human impacts71. More seamless work between the two
RFMOs will help to facilitate enhanced cooperation and collaboration with the
ISA and ultimately could be a key pathway for effective management. Such
dialogue could lead to targeted and collaborative scientific research, designed
to fill some of the current knowledge gaps that prevent robust science-based
rule-setting and decision-making. Better understanding of potential impacts upon
fisheries and fish-dependent economies and populations, will in turn assist the
individual member States of the ISA to take the difficult decisions facing them
about what level of adverse impact from deep-sea mining is considered acceptable
versus the forecasted benefits, and whether such impacts and benefits might be
distributed in an equitable manner. If fish stocks do appear to be adversely
impacted by deep-sea mining in ABNJ, then it seems likely that those affected
would wish to seek compensation. However, the legal framework for actioning such
a claim requires more robustness, with regards to where such a claim could be
brought, by whom, against whom, what damages could be claimed, and what degree
of fault and causation would need to be proven72. This suggests that litigation
in this area could be complex, lengthy, expensive, and multi-party.

The BBNJ Agreement may also prove to be an effective management tool for marine
ABNJ. Despite a commitment to “not undermine” existing agreements, without
consensus on how this will be implemented in practice, the outcomes of the BBNJ
Agreement have potential to influence both fisheries and deep-sea mining in a
changing ocean47. The Agreement might indirectly strengthen the performance of
the RFMOs and the ISA, due to the need to reduce the impact of fisheries and
deep-sea mining on marine biodiversity, respectively47. Additionally, the BBNJ
Agreement could broadly increase capacity building and technology transfer with
regard to the ocean, which may enhance the effectiveness of the decision-making
related to the use of marine genetic resources (MGRs), area-based management
tools (ABMTs), and environmental impact assessments (EIAs)47. The BBNJ Agreement
also has significant potential to increase cooperation among existing marine
governance organizations, including the sharing of scientific data and
information47.

Finally, for the overall benefit of all fisheries and for the planet as a whole,
all sustainable and equitable pathways for the reduction of greenhouse-gas
emissions to limit warming to 1.5 °C by the end of the century should be
considered and implemented34,73.

METHODS

STUDY AREA AND SPECIES

This study focused on ISA exploration contract areas for polymetallic nodules
within the CCZ. The CCZ region was denoted by a spatial shapefile with a buffer
zone of 200 kilometers from the border of outer contract areas as per van der
Grient and Drazen10, all falling between 110°W and 158°W and 5°N and 19°N (Fig.
1). The 200-km buffer was used as several modelling studies have suggested that
midwater sediment plumes may spread over such distances74,75,76 given that the
processes of flocculation, which could minimise plume extent, are unlikely to
occur in discharge plumes22,76. The buffer was created around exploration
contract areas using the geoprocessing tool Buffer in qGIS v3.877. Three Pacific
tuna species (bigeye, skipjack, and yellowfin), which are overseen in the west
Pacific Ocean by the WCPFC and in the east Pacific Ocean by the IATTC, were
assessed. Other species captured and covered by these RFMOs were not included
but are a small component of the catches.

DATA ON TUNA BIOMASS PROJECTIONS UNDER CLIMATE CHANGE

We used published projections of the effects of climate change on the
distribution and abundance of the three most commercially important Pacific tuna
species31 using the model SEAPODYM (Spatial Ecosystem and Population Dynamics
Model; www.seapodym.eu). Briefly, SEAPODYM is a 3D numerical model that
simulates changes in biomass over time based on environmental variables,
life-history stages, prey density, and their age dimensions78. The model relies
on underlying advection-diffusion-reaction equations, integrated on a 2°
latitude-longitude grid. Fish movements within the model are based on
relationships with environmental variables (e.g., temperature, oxygen
concentration, primary production) and dependent on life-history stage of the
fish species (e.g., larvae and small juveniles drift with currents while adults
have active movement based on habitat quality). The model accounts for both
natural and fishing mortality and follows a maximum likelihood estimation (MLE)
approach to resolve population dynamics, including the effects of fishing and
environmental variability. It is important to acknowledge that SEAPODYM is one
of the multiple ways that researchers have explored the impacts of climate
change on tunas and billfishes, including their associated fisheries5. Unlike
most existing models, SEAPODYM provides a mechanistic approach that includes
both population dynamics and trophic interactions thus providing biomass
estimates in the water column.

The environmental variables used to run SEAPODYM were temperature, dissolved
oxygen concentration, zonal/meridional currents and primary production, and
two-dimensional euphotic zone depth31. Environmental variables to simulate the
historical oceanic environment (1979 − 2011) were taken from the Nucleus for
European Modelling of the Ocean (NEMO) ocean framework79, while future ocean
projections (2011–2100) were taken from four Earth System Models (ESMs).
Specifically, the Institute Pierre Simon Laplace Climate Model 5 (IPSL-CM5A)80,
the Model for Interdisciplinary Research on Climate (MIROC)81, the Geophysical
Fluid Dynamics Laboratory Earth System Models (GFDL-ESM2G)82, and the Max Planck
Institute for Meteorology Earth System Model (MPI-MR)83. Results are presented
as the average of all four ESMs. The ESMs followed two Representative
Concentration Pathways (RCPs, a high emission scenario (8.5))84 and a medium
emission scenario (4.5)85. RCPs are greenhouse gas concentration trajectories
adopted by the Intergovernmental Panel on Climate Change. RCP 8.5 is considered
a worst-case climate-change scenario where emissions continue to rise throughout
the 21st century. RCP 4.5 is an intermediate scenario where emissions peak
around 2040, then decline.

ESTIMATING BIOMASS CHANGE WITHIN THE CCZ

The future percentage change in biomass within the CCZ relative to present was
estimated. First, the yearly biomass outputs within the CCZ for each species and
average for a present time (2009–2018) and mid of the 21st century (2044–2053)
were aggregated by grid-cell within the CCZ. Biomass percentage change
($\varDelta {B}_{i,j}$) was estimated for each tuna species j within a grid
cell i as follow:

$$\varDelta
{{\rm{B}}}_{i,j}=\frac{{{\rm{MB}}}_{j,i}-{\rm{P}}{{\rm{B}}}_{j,i}}{{\rm{P}}{{\rm{B}}}_{j,i}}*
100$$
(1)

where MB is the biomass in the grid cells at the middle of the 21st century and
PB is the biomass in that same grid cell at present time. In addition, we
estimate the percentage change within the whole area. For that, we first
aggregated all pixels within the CCZ per species, then averaged by timeframe and
applied the previous equation. All analyses were done in R-Studio version
4.2.086 using the packages tidyverse87, janitor88, sf89, sp90, rnaturalearth91
and viridis92.

DATA AVAILABILITY

The datasets generated during the current study are available in the repository
ccz_tuna available at https://github.com/jepa/ccz_tuna. Original tuna
distribution data analysed in the current study can be found in the repository
Tuna_Redistribution and can be accessed via this link https://osf.io/qa8w4/.

CODE AVAILABILITY

The underlying code for this study is available in the repository ccz_tuna and
can be accessed via this link https://github.com/jepa/ccz_tuna.

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ACKNOWLEDGEMENTS

We wish to thank Johann Bell, Graham Pilling and Simon Nico for support with
data processing. DJM, NN and DJA received funding from UC Santa Barbara’s
Benioff Ocean Science Laboratory. J.P.A. received funding from NSERC and SSHRC
Partnership Grant. The funders played no role in study design, data collection,
analysis and interpretation of data, or the writing of this manuscript.

AUTHOR INFORMATION

AUTHORS AND AFFILIATIONS

1. Marine Science Institute, University of California, Santa Barbara, Santa
Barbara, CA, USA

Diva J. Amon, Neil Nathan & Douglas McCauley

2. SpeSeas, D’Abadie, Trinidad and Tobago

Diva J. Amon

3. Institute for the Oceans and Fisheries, The University of British Columbia,
Vancouver, Canada

Juliano Palacios-Abrantes

4. University of Hawaii at Manoa, Honolulu, HI, USA

Jeffrey C. Drazen

5. Independent Consultant, London, UK

Hannah Lily

6. South Atlantic Environmental Research Institute, Stanley, Falkland Islands

Jesse M. A. van der Grient

Authors
1. Diva J. Amon
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