Pelagic and Open Ocean Accounting

TG-6.5 Section 6: Thematic Methodologies Emerging Draft

Circular ID	TG-6.5
Version	7.0
Badge	Emerging
Status	Draft
Last Updated	May 2026

1. Outcome

1This Circular provides guidance on compiling accounts for pelagic and open ocean ecosystems, the water column environments that extend seaward to the high seas and downward to the abyssal depths. As an Emerging circular, it acknowledges that methodologies for pelagic ocean accounting remain less developed than for coastal and benthic ecosystems, with substantial uncertainties in spatial delineation, stock measurement, and the attribution of assets and services across jurisdictional boundaries¹. Pelagic ecosystems present distinctive accounting challenges. They are defined by water masses rather than fixed substrates, primary production occurs diffusely throughout enormous volumes, and key species are highly migratory across multiple jurisdictions. Much of the domain also lies beyond national jurisdiction, in high seas areas governed by the freedom of the seas principle, whilst the seabed beneath is subject to the common heritage of mankind regime².

2Key decision use cases include quota allocation for tuna and billfish managed through Regional Fisheries Management Organisations (RFMOs), ocean carbon sink quantification to measure the biological carbon pump for nationally determined contributions (NDCs) under the Paris Agreement³, and BBNJ Agreement implementation, where area-based management tools, environmental impact assessments, and benefit-sharing arrangements for marine genetic resources require accounts extending beyond national jurisdiction⁴.

3Pelagic primary productivity measurements feed into TG-2.1 Biophysical Indicators and biological carbon pump quantification connects to TG-2.8 Climate Change Indicators. The foundational concepts for Ocean Accounts are established in TG-0.1 General Introduction to Ocean Accounts. Physical and monetary asset accounting methodology is in TG-3.1 Asset Accounts. As an Emerging circular, this document should be reviewed in two to three years as methodologies mature.

2. Requirements

1This Circular requires familiarity with:

• 2

  TG-0.1 General Introduction to Ocean Accounts — conceptual framework, key components, and spatial scope definitions for pelagic ecosystems within and beyond national jurisdiction.

• 3

  TG-3.1 Asset Accounts — methodology for physical and monetary asset accounts, including aquatic resources, ecosystem assets, and the treatment of assets not clearly associated with specific seabed areas.

• 4

  TG-6.7 Fisheries Stock Assessment — stock assessment concepts (biomass, natural mortality M, fishing mortality F, recruitment, VPA, close-kin mark-recapture) required before attempting Steps 4 and 5 of the compilation procedure.

5Readers may also benefit from:

• 6TG-6.6 Deep Sea and Seabed Accounting — complementary guidance on benthic ecosystems underlying pelagic waters.
• 7TG-2.8 Climate Change Indicators — carbon sequestration and climate regulation services from pelagic ecosystems.

3. Guidance Material

1The pelagic ocean is the largest biome on Earth, comprising the open-ocean water column across all latitudes and spanning depths from the surface to nearly 11 kilometres in the deepest trenches⁵. Unlike coastal and benthic ecosystems where spatial units can be delineated by substrate or shoreline, pelagic ecosystems are structured by the physical and chemical properties of water masses: temperature, salinity, light penetration, oxygen concentration, and nutrient availability.

3.1 Spatial Delineation

Scope boundary: TG-6.5 covers only open-ocean M2 biome units (M2.1 Epipelagic, M2.2 Mesopelagic, M2.3 Bathypelagic, M2.4 Abyssopelagic, M2.5 Sea ice). M1.9 coastal upwelling systems fall under the Marine Shelf biome (M1) and are accounted for under the shelf/coastal biome circular, not TG-6.5. Open-ocean upwelling features (e.g., intertropical convergence) are included within M2.1. See TG-6.6 Deep Sea and Seabed Accounting for the shelf/benthic boundary treatment.

1Spatial delineation must address three considerations: vertical zonation of the water column, horizontal biogeographic regions, and jurisdictional boundaries under international law.

Vertical zonation

1Pelagic ocean waters are classified within the IUCN Global Ecosystem Typology as biome M2. For the GET realm/biome/EFG hierarchy and the national crosswalk obligation, see TG-4.1 Remote Sensing and Geospatial Data Section 3.2.4.⁶ Table 3.1.1 summarises the M2 functional groups relevant to this Circular.

Functional group	Description
M2.1 Epipelagic ocean waters (0-200m)	The sunlit surface layer where photosynthesis occurs, supporting primary production by phytoplankton and the highest biodiversity and biomass of pelagic organisms7.
M2.2 Mesopelagic ocean waters (200-1,000m)	The “twilight zone” receiving insufficient light for photosynthesis, dominated by detritivores and predators, and characterised by high biomass of small fishes and extensive diel vertical migration8.
M2.3 Bathypelagic ocean waters (1,000-3,000m)	Dark waters dependent on organic fallout from above, with low biomass, long-lived organisms, and truncated food webs9.
M2.4 Abyssopelagic ocean waters (3,000-6,000m)	Extreme depths with low biomass and specialised fauna adapted to high pressure and nutrient scarcity10.
M2.5 Sea ice	The seasonally frozen surface of polar oceans supporting specialised ice-associated communities11.

2For practical accounting purposes, the epipelagic zone is of primary importance as it contains most commercially exploited fish stocks, supports virtually all primary production, and generates the ecosystem services of greatest current economic relevance¹². The mesopelagic zone is now recognised as a large carbon reservoir and a potential future fishery resource. The SEEA EA notes that “marine ecosystems are not concentrated near one surface (i.e. the air-land/water interface) but extend throughout the water column and include the underlying sediment and seabed”¹³. For the open ocean, where the seabed-based delineation approach recommended by SEEA EA is less suitable, alternative approaches are summarised in Table 3.1.2.

Approach	Description
Water-mass-based delineation	Defining ecosystem units by oceanographic characteristics such as temperature, salinity, and circulation patterns.
Biogeographic provinces	Using established marine biogeographic classifications such as Longhurst provinces14.
Depth-zone layers	Treating vertical zones as separate accounting units overlaying the horizontal extent.

3Countries operationalising pelagic ecosystem delineation within their EEZs may find it practical to begin with Longhurst biogeographic provinces (for horizontal delineation) combined with GET M2 depth-zone layers (for vertical stratification), and to focus on the epipelagic zone alone for initial implementation.

Biogeographic regions

1Productive pelagic regions include eastern boundary upwelling systems (Canary, Benguela, California, and Humboldt currents), tropical and subtropical gyres, polar and subpolar regions, and frontal zones and convergences. The GET recognises coastal upwelling zones as functional group M1.9 within the Marine Shelf biome¹⁵, whilst weaker open-ocean upwelling (e.g., intertropical convergence zone) falls within M2.1.

Jurisdictional boundaries

1The United Nations Convention on the Law of the Sea (UNCLOS) establishes distinct zones with different rights and responsibilities¹⁶. Table 3.1.4 summarises the four zones relevant to pelagic accounting.

Zone	Description
Internal waters	Waters landward of the baseline, under full sovereignty.
Territorial sea	Extending 12 nautical miles from the baseline, under sovereignty subject to innocent passage.
Exclusive Economic Zone (EEZ)	Extending up to 200 nautical miles, where the coastal State has sovereign rights over natural resources of the superjacent waters17.
High seas	Areas beyond national jurisdiction, open to all States with freedom of navigation, fishing (subject to conservation obligations), and scientific research18.

2The SEEA CF states that “when exploitation control over migrating and straddling fish stocks, and fish stocks that complete their life cycle in international waters (high seas), has been established and the access rights of a country to them are defined in international agreements, that portion of agreed access rights to those aquatic resources can be considered to belong to the country”¹⁹. The BBNJ Agreement (entered into force 17 January 2026) introduces area-based management tools and environmental impact assessments that generate spatial units and data for pelagic condition accounts in areas beyond national jurisdiction. It also establishes a benefit-sharing framework for marine genetic resources, which requires recording as a new economic flow²⁰. Future revisions of this Circular should incorporate detailed guidance on these BBNJ instruments.

3For how jurisdictional boundaries interact with extent and condition accounts, see TG-0.1 General Introduction to Ocean Accounts.

3.2 Extent and Condition

Extent measurement

1Unlike terrestrial or benthic ecosystems measured in hectares, pelagic ecosystem extent may be expressed in multiple dimensions:

• 2Surface area — the horizontal extent of water masses or biogeographic provinces (km2)
• 3Volume — the three-dimensional extent of depth zones (km3)
• 4Mixed metrics — surface area combined with characteristic depth range

5For national accounts within the EEZ, extent may be calculated as the area of the EEZ multiplied by relevant depth zones. For initial implementation, the epipelagic zone is the most tractable focus, as extent changes there are detectable through satellite-observable indicators such as sea surface temperature and chlorophyll-a fronts. Changes in pelagic extent reflect continuous oceanographic variation (water mass boundary shifts, thermocline movement, photic zone depth changes) rather than the discrete conversions characteristic of benthic ecosystems.

6Table 1 presents an illustrative structure for a pelagic ecosystem extent account (Pacific SIDS, 500,000 km2 EEZ, two depth zones). Derivation: epipelagic 500,000 km2 x 0.2 km = 100,000 km3, and mesopelagic 500,000 km2 x 0.8 km = 400,000 km3.

Accounting entry	M2.1 Epipelagic	M2.2 Mesopelagic	Total
Opening extent (km3)	100,000	400,000	500,000
Changes
— Water mass shifts	-500	+500	0
— Depth zone changes	+200	-200	0
Net change	-300	+300	0
Closing extent (km3)	99,700	400,300	500,000

7Table 1: Illustrative structure for pelagic ecosystem extent account within EEZ (Pacific SIDS, 500,000 km2 EEZ)

8Crosswalk to SEEA EA standard extent change categories. The change categories used here reflect the continuous rather than discrete nature of pelagic extent changes.

Pelagic-specific category	SEEA EA standard category	Notes
Water mass shift (climate-driven warm pool expansion/contraction)	Natural reduction / natural expansion	Where a net volume change of a water mass type occurs
Depth zone reclassification (thermocline movement with no net volume change)	Reclassification (not an extent change)	Per SEEA EA para 6.14, statistical reclassifications are recorded separately from extent losses and gains
Sea ice seasonal/multi-year change (M2.5)	Natural reduction / natural expansion	Where multi-year ice extent shifts

9Thermocline-driven movements that shift volume between depth zones without changing total water column volume should be recorded as reclassifications, not as paired losses and gains, to preserve consistency with SEEA EA para 6.12-6.15.

Condition variables

1The SEEA EA condition framework applies to pelagic ecosystems with appropriate variable selection. Key condition characteristics include²¹:

2Abiotic characteristics:

• 3Sea surface temperature and thermal stratification
• 4Ocean acidification (pH, aragonite saturation)
• 5Dissolved oxygen concentration (with attention to oxygen minimum zones)
• 6Nutrient concentrations (nitrate, phosphate, silicate, iron)
• 7Salinity and density structure

8Biotic characteristics:

• 9Chlorophyll-a concentration as a proxy for phytoplankton biomass²²
• 10Primary productivity rates (net primary production, NPP)
• 11Zooplankton biomass and community composition
• 12Fish biomass and species composition
• 13Marine mammal and seabird abundance

14Functional characteristics:

• 15Primary productivity (carbon fixation rates)
• 16Export production (flux of organic matter to depth)
• 17Community structure and trophic organisation

18The epipelagic zone’s primary production “largely by diatoms, accounts for around half of all global carbon fixation”²³. Productivity metrics are therefore of particular importance in pelagic condition accounts. The mesopelagic zone contains fish biomass estimated at possibly “two orders of magnitude larger than global fisheries landings”²⁴, though precise quantification remains uncertain.

19For how condition variables translate into indicators, see TG-2.1 Biophysical Indicators.

Reference conditions

1Establishing reference conditions for pelagic ecosystems is complicated by high natural variability from climate oscillations (El Nino-Southern Oscillation, Pacific Decadal Oscillation) and the lack of historical baseline data. Possible approaches include pre-industrial baselines (for temperature, acidification), model-based reference states from ecosystem models, and policy-based targets established under regional fisheries agreements. Given these challenges, condition accounts may need to focus on tracking trends and rates of change rather than departures from fixed reference conditions.

3.3 Migratory Species

1Highly migratory species present particular challenges for asset accounting, as their distribution spans multiple EEZs and the high seas. The treatment of aquatic resources in asset accounts is addressed in TG-3.1 Asset Accounts Section 3.3.1. This section provides supplementary guidance specific to highly migratory pelagic species.

UNCLOS provisions

1Article 64 of UNCLOS requires coastal States and other fishing States to cooperate through appropriate international organisations to ensure conservation and optimal utilisation of highly migratory species throughout the region, both within and beyond the EEZ²⁵. UNCLOS Annex I lists highly migratory species. Table 3.3.1 summarises the principal groups²⁶.

Group	Species included
Tunas	Albacore, bluefin (Atlantic, Pacific, southern), bigeye, skipjack, yellowfin, blackfin, little tuna, frigate mackerel.
Billfish	Marlins, sailfishes, swordfish.
Other pelagic fish	Sauries, pomfrets, dolphinfish (mahi-mahi).
Oceanic sharks	Various families including Carcharhinidae, Alopiidae, Isurida (now Lamnidae).
Cetaceans	All whale and dolphin families.

2The 1995 UN Fish Stocks Agreement further elaborates obligations for straddling and highly migratory stocks, requiring cooperation through RFMOs²⁷.

Accounting approaches

1The SEEA CF provides that “migrating and straddling fish stocks are considered to belong to a country during the period when those stocks inhabit its EEZ”²⁸. Alternative attribution approaches are summarised in Table 3.3.2.

Approach	Description
Proportional allocation	Attributing stock shares based on time spent in each EEZ or catch proportions.
RFMO-based allocation	Using catch quotas allocated by regional management bodies as the basis for national asset shares.
Flag State attribution	Attributing catches to the flag State of the harvesting vessel (as done for production in the SNA).
Ecosystem-based accounts	Maintaining accounts at the scale of the stock’s range rather than national boundaries.

2For highly migratory species managed under RFMOs, quota allocations provide a practical basis for attributing asset shares, since “that portion of agreed access rights to those aquatic resources can be considered to belong to the country”²⁹. RFMO quotas are expressed as catch limits (flows) rather than stock shares (assets). Converting to asset shares requires (1) total stock biomass from the RFMO stock assessment, (2) each country’s proportional quota share, and (3) the application of that share to the total stock estimate to derive attributed asset value. This conversion introduces uncertainties from stock assessment precision, the catch-share-as-asset-share assumption, and temporal mismatch.

Handling temporal mismatch between stock assessments and accounting periods

1RFMO stock assessments are conducted every 3 to 5 years. In years without a new assessment, compilers should:

1. 2Use the most recent assessment as the opening stock for the accounting period.
2. 3Update to a closing stock using the stock account flow identity: closing stock = opening stock + natural growth — catch — natural mortality, with growth, M, and F parameters carried forward from the last assessment.
3. 4Flag the resulting entry as “model-projected” rather than “assessment-based” in account metadata.
4. 5When a new stock assessment becomes available, retrospectively revise the affected accounting periods and record the change as a statistical revision (reappraisal) consistent with SEEA CF para 5.393.

6For revision protocols, see TG-0.7 Quality Assurance.

7Table 2 presents an approach to structuring highly migratory species asset accounts using RFMO-based allocation.

Accounting entry	National EEZ stock	High seas allocation (quota-based)	Total attributed
Opening stock (tonnes)	8,200	4,100	12,300
Additions
— Natural growth	1,200	600	1,800
— Immigration/movement in	500	0	500
— Reappraisals	0	0	0
Reductions
— Catch — landings (by residents)	850	520	1,370
— Catch — discards (by residents)	50	30	80
— Catch (by non-residents, licensed)	200	0	200
— Emigration/movement out	400	0	400
— Natural mortality	800	400	1,200
— Reappraisals	0	0	0
Closing stock (tonnes)	7,600	3,750	11,350

8Table 2: Illustrative structure for highly migratory species asset account with synthetic values

9Methodological note. The “Total attributed” column sums the National EEZ stock (physical biomass) and the High seas allocation (quota-based financial asset share). These two quantities are not dimensionally homogeneous: the EEZ column records physical fish location, whilst the high seas column records a treaty-defined access right. Compilers should treat the “Total attributed” column as a memo aggregate only. Immigration and emigration entries appear only in the National EEZ stock column: movement out of the EEZ does not transfer biomass into the quota-based high seas allocation, as that allocation is defined by treaty share rather than physical residence. Changes to the quota share (renegotiated allocations) are recorded as reappraisals, not as flows. The approach chosen should be documented in metadata.

Cetaceans and marine mammals

1UNCLOS Article 65 addresses marine mammals separately, requiring States to cooperate for conservation through appropriate international organisations³⁰. Unlike fish stocks, cetaceans are not typically harvested commercially, so their treatment in accounts focuses on ecosystem condition, ecosystem services (tourism, existence value), and biodiversity indicators. For whale watching and marine mammal tourism, see TG-2.4 Ecosystem Goods and Services.

3.4 Accounting Boundaries for Pelagic Activities

1Table 3 summarises the principal boundary challenges and their treatment, drawing on UNCLOS jurisdictional provisions, SEEA CF asset boundary rules, and SNA residence principles.

Accounting Challenge	Treatment Approach	Data Implications
Migratory species crossing EEZ	Allocate by RFMO quota share (recommended); or by residence time / catch location where quotas unavailable (see Section 3.3)	VMS data, tagging studies, RFMO quota records
Stocks shared with neighbours	Joint stock assessment	Regional cooperation
High seas activity by residents	Include in national accounts	Flag state reporting
High seas activity by non-residents	Exclude from national production	Distinguish by flag
Water column vs seabed ecosystems	Separate extent accounts	3D spatial data

2Table 3: Pelagic accounting boundary treatments

3Migratory species crossing EEZ boundaries are allocated to the EEZ in which they reside at the accounting date or, where continuous tracking is unavailable, in proportion to catch location data from vessel monitoring systems (VMS) and scientific tagging programmes.

4Stocks shared with neighbouring countries require joint stock assessments through bilateral or multilateral arrangements. Each country’s asset share reflects its agreed access rights.

5High seas activity by resident economic units is included in national production accounts per the SNA residence principle: output of fishing vessels on the high seas is attributed to the flag State’s economy.

6High seas activity by non-resident units (foreign-flagged vessels) is excluded from national production but may appear where foreign vessels operate under access agreements within the EEZ. AIS data supports flag state identification.

7Water column versus seabed ecosystems are treated as separate extent accounts to avoid double-counting. The relationship between these overlapping accounts is addressed in TG-6.6 Deep Sea and Seabed Accounting.

3.5 Ecosystem Services

1Pelagic ecosystems generate a range of ecosystem services, though many are difficult to measure and value. This section should be read in conjunction with TG-2.4 Ecosystem Goods and Services for the general framework. For valuation approaches, see TG-1.9 Valuation.

Provisioning services

1Fisheries production is the dominant provisioning service. The SEEA AFF distinguishes “Pelagic fish, including Tunas, bonitos, billfishes” and “Other pelagic fish”³¹. For pelagic fisheries accounting, see TG-3.2 Flows from the Environment to the Economy and TG-6.7 Fisheries Stock Assessment.

2Access licence fees vs ecosystem service supply. A common compilation issue for SIDS whose primary ocean revenue is access fee income is the distinction between the ecosystem service flow value and the economic transaction value represented by licence fees.

Item	Conceptual nature	Recording location
Ecosystem service supply (sustainable yield x resource rent per tonne)	The value of the provisioning service generated by the stock	SEEA EA ecosystem service supply table; physical and monetary ecosystem service accounts
Access licence fee receipts from foreign fleets	A payment for the right to exploit the provisioning service; an economic transaction	SNA current account (current transfers / service exports under the balance of payments); recording basis described in TG-3.2 Flows from the Environment to the Economy

3Licence fees should not be added to the ecosystem service supply value, as that addition would double-count the underlying provisioning service. The resource rent component embedded in the licence fee is conceptually part of the ecosystem service value, but the fee itself is recorded as a current transaction (see SEEA EA para 8.40-8.42 and SEEA CF para 5.395).

4Genetic resources from pelagic organisms represent an emerging provisioning service category. The BBNJ Agreement establishes a benefit-sharing framework for marine genetic resources in areas beyond national jurisdiction³².

5Mesopelagic resources as a prospective asset. Mesopelagic fish biomass may be two orders of magnitude larger than global fisheries landings (estimate carries high uncertainty from acoustic backscatter conversion). As commercial exploitation has not yet emerged at scale, the recommended treatment is:

• 6Where no commercial exploitation control has been established, the monetary value within the SEEA CF asset boundary is zero (per SEEA CF para 5.6 asset recognition criterion requiring exploitation control).
• 7Countries may include mesopelagic stock in a physical-only experimental account recording biomass estimates with explicit uncertainty ranges, clearly distinguished from core SEEA EA/CF tables.
• 8When commercial exploitation begins and exploitation control is established, the resource moves into the core asset account.

9See TG-3.1 Asset Accounts on the asset boundary criterion and contingent resources.

Regulating services

1Climate regulation through carbon sequestration. Primary production in the epipelagic zone fixes atmospheric CO2 into organic matter. A portion is exported to depth through sinking particles, vertical migration, and physical mixing. These processes, collectively the “biological pump”³³, sequester carbon from the atmosphere for centuries to millennia. Diel vertical migration in the mesopelagic zone enhances this transport³⁴.

2Quantifying carbon sequestration services requires estimates of net primary production rates, export production (fraction of NPP exported below the mixed layer), and sequestration efficiency (fraction reaching long-term storage depths). Attribution to national accounts raises challenges, as the service is generated by global ocean circulation that is not confined to any single EEZ, whilst the benefits are global rather than national. For initial compilations, recording carbon sequestration within the EEZ as a nationally generated service is the recommended default, with metadata noting that benefits are global and the recording does not imply exclusive national claim. For guidance on integrating carbon sequestration into accounts, see TG-2.8 Climate Change Indicators.

3Nutrient cycling through upwelling and vertical mixing redistributes nutrients that support productivity throughout the ocean and in coastal zones.

Cultural services

1Pelagic cultural services include marine scientific research and oceanographic monitoring, whale watching and recreational fishing, and ocean cruising. Existence and bequest values attach to healthy ocean ecosystems and charismatic marine megafauna (whales, sharks, sea turtles). For measuring cultural ecosystem services, see TG-2.4 Ecosystem Goods and Services.

3.6 Data and Methods

1Pelagic ecosystems present data challenges due to their remoteness, vast scale, and dynamic nature.

Satellite oceanography

1Remote sensing provides essential data for pelagic ecosystem monitoring at scales not achievable by in situ methods. Principal satellite data streams include³⁵ (1) ocean colour (MODIS, VIIRS, Sentinel-3 OLCI) for chlorophyll-a and primary productivity, (2) sea surface temperature from thermal infrared and microwave sensors, (3) sea surface height from altimetry for circulation, eddies, and fronts, and (4) sea ice extent from passive microwave sensors. Limitations include penetration limited to the near-surface, cloud cover affecting visible-band sensors, and the need for atmospheric correction. For detailed guidance on remote sensing in ocean accounts, see TG-4.1 Remote Sensing Data.

In situ oceanography

1Principal in situ platforms are (1) research vessels (CTD profiles, water sampling, net sampling), (2) Argo floats (global array of >4,000 profiling floats to 2,000m), (3) Biogeochemical Argo (oxygen, nitrate, pH, chlorophyll, particle sensors), and (4) moorings and buoys for time-series observations. The Global Ocean Observing System (GOOS) coordinates international observation with Essential Ocean Variables (EOVs) relevant to ecosystem accounting.

Fish stock assessment

1Pelagic fish stock assessment employs methods distinct from demersal fisheries due to species mobility and aggregating behaviour: acoustic surveys (echosounders estimating biomass of schooling fish), electronic tagging and tracking (archival, satellite tags for migration patterns), virtual population analysis (age-structured models from catch-at-age data), and close-kin mark-recapture (genetic methods for absolute abundance). For detailed guidance on stock assessment methods, see TG-6.7 Fisheries Stock Assessment. For survey methodologies, see TG-4.2 Survey Methods.

Model-based approaches

1Pelagic ecosystem accounting may rely substantially on models: biogeochemical models (primary production, carbon cycling, nutrient dynamics), ecosystem models such as Ecopath with Ecosim and Atlantis (trophic interactions and biomass flows), species distribution models (habitat suitability and distribution shifts), and stock assessment models. Best practice involves ensemble approaches, uncertainty quantification, and validation against available observations.

Uncertainty characterisation

1Key sources of uncertainty include structural uncertainty in model formulations, parametric uncertainty in model calibration, observational uncertainty in sparse in situ validation data, and spatial extrapolation from point observations to large pelagic domains. Compilers should report confidence intervals or uncertainty ranges where feasible, use ensemble modelling to capture structural uncertainty, and clearly distinguish between observation-based and model-derived entries in published accounts. See TG-0.7 Quality Assurance for systematic guidance on uncertainty communication.

3.7 Compilation Procedure and Worked Example

Step 1: Define spatial accounting units

1Action: Delineate the pelagic accounting area within the EEZ, distinguishing depth zones and, where relevant, biogeographic provinces.

2Data requirements:

• 3Bathymetric data to determine water column depth
• 4Oceanographic data (temperature, salinity) to identify water mass boundaries
• 5EEZ boundary data from national maritime authorities

6Output: A spatial framework defining accounting units (e.g., “Epipelagic zone within EEZ”, “Mesopelagic zone within EEZ”).

Step 2: Compile extent accounts

1Action: Measure the opening and closing extent of each pelagic ecosystem type, recording changes during the accounting period.

2Data requirements:

• 3Volume calculations from bathymetry and depth zone definitions
• 4Time-series satellite data for sea surface temperature and chlorophyll-a to detect water mass shifts
• 5Oceanographic model outputs for thermocline depth changes

6Calculation:

• 7Epipelagic extent (km3) = EEZ surface area (km2) x mean depth of photic zone (km)
• 8Record changes as water mass boundary shifts or depth zone reclassifications

9Output: Physical extent account showing opening extent, changes (water mass shifts, depth zone changes), and closing extent by ecosystem type.

Step 3: Compile condition accounts

1Action: Measure condition variables for each ecosystem type at representative monitoring stations.

2Data requirements:

• 3Satellite-derived chlorophyll-a and SST (for epipelagic zone)
• 4Argo float profiles for subsurface temperature, salinity, dissolved oxygen
• 5Research vessel surveys for nutrient concentrations and plankton biomass
• 6Stock assessment outputs for fish biomass

7Calculation:

• 8Condition variables are normalised using the standard formula defined in TG-2.1 Biophysical Indicators for Ocean Accounts Section 3.4.1. For pelagic negative indicators (e.g., SST anomaly, acidification stress), transform the variable to its negative form (e.g., use -1 x SST anomaly) so that the standard formula applies directly. See the SST worked cell in the worked example below.
• 9Document reference condition selections and the treatment of any negative indicators.

10Output: Condition variable account with raw measurements and normalised indicators for each ecosystem type. Composite indices are optional. Where compiled, the weighting scheme must be documented (see Step 2 of the worked example).

Step 4: Compile fish stock asset accounts

1Action: Record opening stock, natural growth, catch, mortality, and closing stock for commercially important pelagic species.

2Data requirements:

• 3RFMO stock assessment reports providing biomass estimates, recruitment, natural mortality, fishing mortality
• 4National catch statistics by species, distinguishing landings and discards
• 5VMS data for catch location (to attribute shared stocks)

6Data note on discards. Discard estimates for highly migratory pelagic species should be drawn from RFMO observer programme data where available. For the Western and Central Pacific, the WCPFC Regional Observer Programme is the recommended primary source: it places trained independent observers on fishing vessels with mandated coverage of 100% for purse-seine vessels (since 2010) and 5% for longline vessels (since 2012). Actual coverage rates vary by fleet and year and must be documented in account metadata. Where RFMO observer data are unavailable or coverage is insufficient, compilers may use FAO global discard estimates as a fallback.³⁶

7Calculation:

• 8Opening stock from most recent stock assessment (flag “model-projected” if the assessment predates the accounting year, and see Section 3.3 for the temporal-mismatch protocol)
• 9Natural growth = recruitment + somatic growth (from stock assessment model)
• 10Extraction = gross catch (landings + discards) attributed to national fleet
• 11Natural mortality from stock assessment parameter M applied to population size
• 12Closing stock = opening stock + growth — extraction — natural mortality

13Output: Physical fish stock asset account with entries in tonnes, distinguishing EEZ and high seas components for migratory species, and reporting landings and discards on separate rows.

Step 5: Estimate ecosystem service flows

1Action: Quantify the annual supply of ecosystem services from pelagic ecosystems.

2Data requirements:

• 3Primary productivity estimates from satellite ocean colour and biogeochemical models
• 4Export production rates from sediment trap data or models
• 5Fish catch data and resource rent estimates for fisheries provisioning services

6Calculation:

• 7Carbon sequestration (t CO2/yr) = NPP (t C/yr) x export ratio x sequestration efficiency x 44/12
• 8Fisheries provisioning service (physical) = sustainable yield (from stock assessment)
• 9Fisheries provisioning service (monetary) = sustainable yield x resource rent per tonne

10Output: Ecosystem service supply table (physical and monetary) by service type and ecosystem type.

Step 6: Value ecosystem assets (optional)

1Action: Estimate the monetary value of pelagic ecosystem assets as the NPV of expected future service flows.

2Data requirements:

• 3Annual ecosystem service values from Step 5
• 4Discount rate (4% real is commonly applied)
• 5Assumptions about service flow duration and projection horizon

6Calculation:

• 7Asset value = Annual service value x PV annuity factor (r, n), where n is the projection horizon and r is the real discount rate
• 8A finite 25-year horizon is recommended as the default, as RFMO stock assessment projections and SEEA EA practice for ecosystem services with substantial methodological uncertainty typically use a horizon comparable to the stock assessment projection period rather than perpetuity. Compilers adopting a perpetual annuity (Asset value = Annual service value / r) should document the assumption explicitly and apply it consistently
• 9Apply to carbon sequestration and fisheries services separately
• 10Document uncertainties and sensitivity to discount rate and horizon

11Output: Monetary ecosystem asset account showing opening value, changes (enhancement, degradation), and closing value.

Worked Example

1This worked example demonstrates the compilation of pelagic and open ocean ecosystem accounts for a hypothetical Pacific small island developing state (SIDS), following the extent-condition-services-valuation sequence in Sections 3.1-3.5. Given the Emerging status of this Circular, compilers should treat the methods and values below as indicative rather than prescriptive.

2Setting: A Pacific SIDS with an EEZ of 500,000 km2, predominantly comprising epipelagic waters (M2.1) over a deep ocean basin. The nation’s economy depends heavily on tuna fisheries managed through the Western and Central Pacific Fisheries Commission (WCPFC). The EEZ intersects the warm pool region of the western Pacific, with seasonal upwelling along its southern boundary.

3Step 1: Extent account (year t to t+1)

Accounting entry	M2.1 Epipelagic (km3)	M2.2 Mesopelagic (km3)	Total (km3)
Opening extent	100,000	400,000	500,000
— Water mass boundary shifts (warming-driven)	-500	+500	0
— Thermocline depth change (reclassification)	+200	-200	0
Net change	-300	+300	0
Closing extent	99,700	400,300	500,000

4The thermocline depth change is a reclassification (no net volume change, per SEEA EA para 6.14). The water mass shift is a natural reduction in epipelagic with a corresponding natural expansion in mesopelagic.

5Step 2: Condition account

6Condition indicators are derived from satellite oceanography, Argo float data, and RFMO stock assessments. Reference levels are based on 1990-2010 climatological means. The four indicators are presented separately rather than aggregated, consistent with TG-2.1 Biophysical Indicators on indicator selection and weighting.

Condition variable	Observed value	VH (reference good)	VL (degraded)	Indicator score
Primary productivity (mg C/m2/day)	280	350	150	0.65
Dissolved oxygen at 200m (ml/L)	3.8	4.5	2.0	0.72
Tuna stock biomass (% of unfished)	42%	60%	20%	0.55
SST anomaly, transformed (-1 x C above 1990-2010 mean)	-0.8	0.0	-2.0	0.60

7Worked cell — SST anomaly normalisation:

Raw SST anomaly = +0.8 C above the 1990-2010 mean. As lower SST anomalies indicate better condition, the variable is transformed to V = -0.8. With VH = 0.0 (reference good, no anomaly) and VL = -2.0 (degraded, +2 C anomaly transformed), applying the standard normalisation formula (see TG-2.1 §3.4.1):

Indicator = (V - VL) / (VH - VL) = (-0.8 - (-2.0)) / (0.0 - (-2.0)) = 1.2 / 2.0 = 0.60

8Composite index (optional): An equal-weighted arithmetic mean of the four indicators yields (0.65 + 0.72 + 0.55 + 0.60) / 4 = 0.63. Compilers should consider alternative weightings (e.g., management-relevance weights placing greater weight on the tuna stock variable for a fisheries-dependent SIDS) and report sensitivity to weighting choices. See SEEA EA para 5.72-5.76 and TG-2.1 Biophysical Indicators.

9Step 3: Ecosystem services (annual flows)

10The carbon sequestration physical rate of 5.0 t CO2/km2/yr applied below is derived from biological-pump parameters as follows:

Parameter	Value	Source
Net primary production (NPP)	280 mg C/m2/day x 365 days = 102.2 g C/m2/yr = 102.2 t C/km2/yr	Satellite ocean colour mean for the EEZ (matches Step 2 observed productivity)
Export ratio (e-ratio, fraction of NPP exported below mixed layer)	0.12	Henson et al. (2011), open-ocean global mean for the biological carbon pump
Sequestration efficiency (fraction of exported flux reaching long-term storage depths > 1,000 m)	0.10	Siegel et al. (2014), Global Biogeochemical Cycles, on mesopelagic transfer efficiency in the open ocean
Derived rate	4.5 t CO2/km2/yr	102.2 x 0.12 x 0.10 x 3.667 = 4.50 t CO2/km2/yr (×44/12 to convert C to CO₂-equivalent)

11Total carbon sequestration over the EEZ: 500,000 km2 x 4.5 t CO2/km2/yr = 2,250,000 t CO2/yr.

Service	Physical quantity	Monetary value (USD)
Fisheries (tuna — national quota allocation)	45,000 tonnes	54,000,000 (resource rent)
Carbon sequestration (biological pump within EEZ)	2,250,000 t CO2/yr (at 4.5 t CO2/km2/yr)	Range: 45,000,000 (low) — 114,750,000 (central) — 303,750,000 (high)
Climate regulation (ocean heat uptake)	(not separately valued)	—
Genetic resources	(qualitative — emerging)	—
Total valued services (central)		168,750,000

12Carbon price range applied: (1) USD 20/t CO2 (low, marginal abatement cost reference), (2) USD 51/t CO2 (central, US IWG 2021 Social Cost of Carbon, 3% discount rate, 2020 USD), and (3) USD 135/t CO2 (high, IPCC AR6 WG3 lower bound of the 1.5 C-compatible carbon price range for 2030). Compilers should present sensitivity analysis across the full range and check for updates to the US IWG SCC at the time of compilation. See TG-1.9 Valuation for carbon price selection.

13Note: The tuna fisheries value reflects only the national quota allocation managed through WCPFC. Licence fees paid by distant-water fishing nations (approximately USD 30,000,000/yr for this hypothetical SIDS) are recorded as economic flows in the national accounts (SNA current account) and are not added to the ecosystem service supply value — the conceptual basis is described in the Section 3.5 mapping table.

14Step 4: Asset valuation

15Applying a 4% real social discount rate over a 25-year projection horizon (basis: a horizon broadly comparable to the RFMO stock assessment projection period, and compilers electing a perpetual annuity should substitute 1/r = 25.0 and document the change):

Asset value (central carbon price) = 168,750,000 x PV annuity factor(4%, 25) = 168,750,000 x 15.62 = approximately 2,635,875,000 USD

16Sensitivity to carbon price (annual service value x 15.62 PV annuity factor):

• 17Low (USD 20/t CO2): annual services 99,000,000 -> asset value approximately 1,546,380,000 USD
• 18Central (USD 51/t CO2): annual services 168,750,000 -> asset value approximately 2,635,875,000 USD
• 19High (USD 135/t CO2): annual services 357,750,000 -> asset value approximately 5,588,055,000 USD
• 20Fisheries only (excluding carbon): annual services 54,000,000 -> asset value approximately 843,480,000 USD

21The wide range illustrates the dominant role of the carbon price in pelagic ecosystem asset valuation. Compilers adopting a perpetual annuity at 4% would multiply annual service values by 25.0 instead of 15.62. The choice should be documented and applied consistently.

22This worked example illustrates the full accounting sequence for pelagic ecosystems. Actual compilations will require nationally specific oceanographic data, RFMO stock assessments, and careful consideration of how transboundary services are attributed. The treatment of carbon sequestration in pelagic accounts remains an active area of methodological development.

Figure 6.5.1 Vertical cross-section of four IUCN GET M2 pelagic depth zones as accounting units, crossed by the EEZ/ABNJ water-column boundary. M2.5 sea ice is a hatched surface band, not a depth zone. Depths: 200 m, 1 000 m, 3 000 m. Seabed/Area out of scope (TG-6.6). Source: TG-6.5 (Pelagic Zone Ecosystem Accounts), Sections 2--3; IUCN Global Ecosystem Typology v2.1, M2 biome (Keith et al. 2022); SEEA CF 2012, Chapter 5 (ecosystem asset classification); BBNJ Agreement (adopted 19 June 2023; entered into force 17 January 2026), Art. 1 (area definitions). Adapted from: IUCN GET v2.1 M2 biome schematic, with GOAP extensions: SEEA CF asset-account framing, EEZ/ABNJ jurisdictional divide, EOV annotation, biological carbon pump overlay, and depth-luminance fill encoding.

4. Acknowledgements

1Authors: [To be confirmed]

2Reviewers: [To be confirmed]

Footnotes

1. 1

   This Circular carries the “Emerging” badge, indicating that accounting methodologies are less developed and uncertainties remain. Approaches should be considered provisional pending further international consensus. ↩

2. 2

   UNCLOS, Article 136. “The Area and its resources are the common heritage of mankind.” ↩

3. 3

   Under the Paris Agreement, countries may include ocean-based climate mitigation measures in their nationally determined contributions (NDCs). Pelagic carbon sequestration quantification supports NDC formulation and tracking. ↩

4. 4

   Agreement under the United Nations Convention on the Law of the Sea on the conservation and sustainable use of marine biological diversity of areas beyond national jurisdiction (BBNJ Agreement), adopted 19 June 2023. Entry-into-force date and implementation status are stated in Section 3.1. ↩

5. 5

   IUCN GET, M2 Pelagic ocean waters biome. “The Pelagic ocean biome is the largest on earth, comprising the open-ocean water column across all latitudes.” ↩

6. 6

   IUCN GET, M2 Pelagic ocean waters biome classification. ↩

7. 7

   IUCN GET, M2.1 Epipelagic ocean waters. ↩

8. 8

   IUCN GET, M2.2 Mesopelagic ocean waters. “The diverse organisms within this layer consume and reprocess allochthonous organic material sinking from the upper, photosynthetic layer.” ↩

9. 9

   IUCN GET, M2.3 Bathypelagic ocean waters. “Total biomass declines exponentially from an average of 1.45 mg.C.m-3 at 1,000 m deep to 0.16 mg.C.m-3 at 3,000 m.” ↩

10. 10

    IUCN GET, M2.4 Abyssopelagic ocean waters. “Due to extreme conditions and limited resources, biodiversity is very low.” ↩

11. 11

    IUCN GET, M2.5 Sea ice. “The seasonally frozen surface of polar oceans… is one of the most dynamic ecosystems on earth.” ↩

12. 12

    IUCN GET, M2.1. “Autochthonous productivity in the epipelagic layer, largely by diatoms, accounts for around half of all global carbon fixation.” ↩

13. 13

    SEEA EA, para 3.11. ↩

14. 14

    Longhurst, A.R. (2007). Ecological Geography of the Sea. Academic Press. Longhurst provinces are widely used biogeographic units for pelagic ecosystems. ↩

15. 15

    IUCN GET, M1.9. “The most productive upwelling zones are coastal, notably in four major eastern-boundary current systems (the Canary, Benguela, California and Humboldt).” ↩

16. 16

    UNCLOS, Parts II, V, VII. ↩

17. 17

    UNCLOS, Article 56(1)(a). ↩

18. 18

    UNCLOS, Article 87. “The high seas are open to all States, whether coastal or land-locked.” ↩

19. 19

    SEEA CF, para 5.399. ↩

20. 20

    Agreement under the United Nations Convention on the Law of the Sea on the conservation and sustainable use of marine biological diversity of areas beyond national jurisdiction (BBNJ Agreement), adopted 19 June 2023; see Section 3.1 for entry-into-force date and accounting implications. ↩

21. 21

    SEEA EA, para 5.32 and Table 5.1, on ecosystem condition characteristic classes. ↩

22. 22

    FDES 2013, Topic 1.3.3 on marine water quality indicators including chlorophyll-a. ↩

23. 23

    IUCN GET, M2.1 Epipelagic ocean waters, Ecological Traits. ↩

24. 24

    IUCN GET, M2.2. Mesopelagic fish biomass estimates remain uncertain. ↩

25. 25

    UNCLOS, Article 64(1). ↩

26. 26

    UNCLOS, Annex I Highly Migratory Species. ↩

27. 27

    UN Fish Stocks Agreement (1995). Agreement for the Implementation of the Provisions of the United Nations Convention on the Law of the Sea relating to the Conservation and Management of Straddling Fish Stocks and Highly Migratory Fish Stocks. Treaty Series, vol. 2167, No. 37924. ↩

28. 28

    SEEA CF, para 5.398. ↩

29. 29

    SEEA CF, para 5.399. ↩

30. 30

    UNCLOS, Article 65. ↩

31. 31

    SEEA AFF, Table 3.10 aquatic products classification. ↩

32. 32

    BBNJ Agreement (2023), Part II (Marine genetic resources) establishes benefit-sharing provisions. ↩

33. 33

    The biological pump refers to the suite of biologically mediated processes that transport carbon from the surface to the deep ocean, including sinking particles, vertical migration, and physical mixing. ↩

34. 34

    IUCN GET, M2.2 Mesopelagic ocean waters. ↩

35. 35

    See TG-4.1 Remote Sensing Data for detailed guidance on satellite data applications in ocean accounting. ↩

36. 36

    Sources: WCPFC Regional Observer Programme, Western and Central Pacific Fisheries Commission (https://www.wcpfc.int/regional-observer-programme); Perez Roda, M.A. et al. (2019), A third assessment of global marine fisheries discards, FAO Fisheries and Aquaculture Technical Paper No. 633, FAO, Rome (ISBN 978-92-5-131869-9); Kelleher, K. (2005), Discards in the world’s marine fisheries: An update, FAO Fisheries Technical Paper No. 470, FAO, Rome. FAO global discards 2010-2014 are estimated at 9.1 million tonnes (95% CI: 6.7-16.1 million tonnes), with pelagic longline fisheries averaging a 6.7% discard rate. ↩