Pelagic and Open Ocean Accounting

Key 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 Agreement, 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 jurisdiction.

Pelagic 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.

This Circular requires familiarity with:

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

• 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.

• 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.

Readers may also benefit from:

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

Readiness and sequencing. Whether a country is positioned to compile this account at all — mandate, data architecture, capacity and the order in which to build — is assessed under TG-0.8 Implementation Readiness Assessment, and is not restated here.

The 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 trenches. 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.

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 upwelling zones fall under the Marine Shelf biome (M1) and are outside TG-6.5. No circular in this edition covers the M1 shelf ecosystems other than M1.1 (TG-6.3), M1.2 and M1.6 (TG-6.4) and M1.3 (TG-6.1); until one is issued, compile M1.9 using the generic ecosystem asset method in TG-3.1 Asset Accounts with condition variables selected under TG-4.8 and TG-4.9, and document the departure. 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.

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

Pelagic 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. Table 3.1.1 summarises the M2 functional groups relevant to this Circular.

For 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 relevance. 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”. 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.

Countries 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.

Productive 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 biome, whilst weaker open-ocean upwelling (e.g., intertropical convergence zone) falls within M2.1.

The UNCLOS maritime zones, the SEEA CF asset-boundary treatment of areas beyond national jurisdiction, the extended continental shelf claim states, and the BBNJ Agreement provisions are set out once in TG-6.6 Deep Sea and ABNJ Accounting Section 3.1, which owns them for the publication, , , , . That treatment applies to the pelagic water column exactly as it does to the seabed.

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

3.2 Extent and Condition

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

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

For 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.

Table 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.

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

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

• A regional habitat map that predates the benthic/pelagic split supports the seabed account of TG-6.6 Deep Sea and Seabed Accounting but not the pelagic extent account of this Section. Compile pelagic extent from oceanographic fields and biogeographic provinces per Section 3.1 instead of forcing a benthic classification into the water column.
• Where both a benthic and a pelagic classification are available from the same regional product, they remain separate extent accounts over the same footprint, per the double-counting rule in Section 3.4. Record the vintage of the classification used in account metadata, as the class definitions have changed.

The epipelagic zone’s primary production “largely by diatoms, accounts for around half of all global carbon fixation”. 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”, though precise quantification remains uncertain.

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

Establishing 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.

Highly 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 3.4.1. This section provides supplementary guidance specific to highly migratory pelagic species.

Article 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 EEZ. UNCLOS Annex I lists highly migratory species. Table 3.3.1 summarises the principal groups.

For 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”. 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

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

1. Use the most recent assessment as the opening stock for the accounting period.
2. Update 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. Flag the resulting entry as “model-projected” rather than “assessment-based” in account metadata.
4. When 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.

Cetaceans and marine mammals

UNCLOS Article 65 addresses marine mammals separately, requiring States to cooperate for conservation through appropriate international organisations. 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

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

Table 3: Pelagic accounting boundary treatments

Migratory 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.

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

High 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.

High 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.

Water 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.

Pelagic 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.

• Record the service against the set of pelagic ecosystem types plausibly supplying the stock, labelled as a group, rather than apportioning it to one type on an unstated assumption.
• Where an apportionment is required, derive it from VMS or observer effort distribution rather than from landings, and disclose the effort series and the period used.
• Do not read a service flow recorded against a group of ecosystem types as evidence of the condition of any one of them.

Access 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.

Licence 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).

Genetic 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 jurisdiction.

Mesopelagic 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:

• Where 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).
• Countries 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.
• When commercial exploitation begins and exploitation control is established, the resource moves into the core asset account.

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

Quantifying 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.

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

Pelagic 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

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

Remote sensing provides essential data for pelagic ecosystem monitoring at scales not achievable by in situ methods. Principal satellite data streams include (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.

Pelagic 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.

Key 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

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

Data requirements:

• Condition 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.
• Document reference condition selections and the treatment of any negative indicators.

Output: 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

Data 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.

Calculation:

• Opening 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)
• Natural growth = recruitment + somatic growth (from stock assessment model)
• Extraction = gross catch (landings + discards) attributed to national fleet
• Natural mortality from stock assessment parameter M applied to population size
• Closing stock = opening stock + growth — extraction — natural mortality

Output: 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

• Asset value = Annual service value x PV annuity factor (r, n), where n is the projection horizon and r is the real discount rate
• A 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
• Apply to carbon sequestration and fisheries services separately
• Document uncertainties and sensitivity to discount rate and horizon

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

This 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.

Setting: 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.

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

The 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.

Step 2: Condition account

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):

Composite 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.

Step 3: Ecosystem services (annual flows)

The carbon sequestration physical rate of 4.5 t CO2/km²/yr applied below is derived from biological-pump parameters as follows:

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

Carbon 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.

Note: 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.

Step 4: Asset valuation

Applying 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 USD 2,635,875,000

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

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

The 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.

This 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.