Pelagic and Open Ocean Accounting

1. Outcome

This Circular provides guidance on compiling accounts for pelagic and open ocean ecosystems--the vast water column environments that extend from coastal waters to the high seas and from the sunlit surface to the dark abyssal depths. As an Emerging circular, it acknowledges that methodologies for pelagic ocean accounting remain less developed than for coastal and benthic ecosystems, with significant uncertainties in spatial delineation, stock measurement, and the attribution of assets and services across jurisdictional boundaries^[1]. The pelagic realm presents unique accounting challenges: ecosystems are defined by water masses rather than fixed substrates, primary production occurs diffusely throughout enormous volumes, key species are highly migratory and cross multiple jurisdictions, and much of the domain lies beyond national jurisdiction in areas governed by the common heritage of mankind principle^[2].

Despite these challenges, pelagic ecosystems underpin critical decision use cases. National governments depend on pelagic fisheries accounts to inform quota allocation decisions for tuna, billfish, and other high-value species managed through Regional Fisheries Management Organizations (RFMOs), with quota values often representing millions of dollars in annual fishing rights. Countries implementing ocean carbon sink quantification programs require pelagic ecosystem accounts to measure the biological carbon pump--the process by which surface primary production exports carbon to the deep ocean--to support nationally determined contributions (NDCs) under the Paris Agreement^[3]. Coastal States navigating the implementation of the BBNJ Agreement (entered into force January 2026) require accounts that extend beyond national jurisdiction to inform area-based management tools, environmental impact assessments, and benefit-sharing arrangements for marine genetic resources in the high seas^[4].

This Circular also supports upward connections to the indicator and climate policy circulars. Pelagic primary productivity measurements feed into TG-2.1 Biophysical Indicators as a key functional indicator of ecosystem condition. The biological carbon pump quantification connects directly to TG-2.8 Climate Change Indicators, where ocean carbon sequestration is recorded as a regulating service contributing to climate mitigation. Ocean acidification effects on calcifying plankton link condition accounts to climate impact assessment. These connections ensure that pelagic accounts are not compiled in isolation but function as inputs to broader policy frameworks.

The foundational concepts for Ocean Accounts--including the relationship between environmental and economic accounting frameworks and the spatial scope within which pelagic ecosystems are recorded--are established in TG-0.1 General Introduction to Ocean Accounts. The physical and monetary asset accounting methodology that underpins the treatment of aquatic resources and ecosystem assets in pelagic waters is described in TG-3.1 Asset Accounts. As an Emerging circular, this document should be reviewed in two to three years as methodologies mature, with particular attention to standardised spatial delineation for pelagic ecosystems, attribution protocols for transboundary migratory species, and valuation of carbon sequestration services.

2. Requirements

This Circular requires familiarity with:

• TG-0.1 General Introduction to Ocean Accounts -- provides the conceptual framework and key components of Ocean Accounts, including the spatial scope definitions that determine how pelagic ecosystems within and beyond national jurisdiction are treated in the accounting framework.

• TG-3.1 Asset Accounts -- for the methodology of physical and monetary asset accounts, including the treatment of aquatic resources, ecosystem assets, and the relationship between individual environmental assets (such as fish stocks) and the ecosystem assets in which they reside.

Readers may also benefit from consulting:

• TG-6.7 Fisheries Stock Assessment -- for detailed guidance on fish stock measurement methods relevant to pelagic species, including acoustic surveys, tagging, and close-kin mark-recapture techniques.
• TG-6.6 Deep Sea and Seabed Accounting -- for complementary guidance on benthic ecosystems underlying pelagic waters, including the distinction between water column and seabed accounting units.
• TG-2.8 Climate Change Indicators -- for guidance on recording the carbon sequestration and climate regulation services that pelagic ecosystems provide.

3. Guidance Material

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^[5]. 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. These properties create distinct vertical zones that host fundamentally different ecological communities and present different opportunities and challenges for environmental-economic accounting.

This section addresses the spatial delineation of pelagic ecosystems (Section 3.1), extent and condition measurement (Section 3.2), the particular challenges of accounting for highly migratory species (Section 3.3), the treatment of accounting boundaries for pelagic activities (Section 3.4), ecosystem services from pelagic systems (Section 3.5), the data sources and methods available to support pelagic accounting (Section 3.6), and a compilation procedure with worked example (Section 3.7). Throughout, the guidance acknowledges methodological uncertainty and identifies areas where further development is needed.

For contextual understanding of how pelagic ecosystem assets relate to individual environmental assets such as fish stocks, see the discussion of asset relationships in TG-3.1 Asset Accounts Section 3.4, which notes the challenge that "pelagic ecosystems that are not clearly associated with specific seabed areas" present for the seabed-based delineation approach recommended by SEEA EA.

3.1 Spatial Delineation

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

Vertical zonation

The IUCN Global Ecosystem Typology (GET) classifies pelagic ocean waters as biome M2, with functional groups defined by depth and light availability^[6]:

• 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 organisms^[7]
• 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 migration^[8]
• 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 webs^[9]
• M2.4 Abyssopelagic ocean waters (3,000-6,000m) -- extreme depths with very low biomass and specialised fauna adapted to high pressure and nutrient scarcity^[10]
• M2.5 Sea ice -- the seasonally frozen surface of polar oceans supporting specialised ice-associated communities^[11]

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^[12]. The mesopelagic zone is increasingly recognised as a significant carbon reservoir and potential future fishery resource, though exploitation remains limited. The deeper zones (bathypelagic and abyssopelagic) have minimal direct economic use but provide essential supporting services through nutrient cycling and carbon sequestration.

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 continental shelf waters, SEEA EA recommends delineating ecosystem assets based on seabed-associated ecosystem types. However, this seabed-based approach is less suitable for the open ocean where pelagic ecosystems extend over abyssal plains without clear seabed associations. Alternative approaches may include:

1. Water-mass-based delineation -- defining ecosystem units by oceanographic characteristics such as temperature, salinity, and circulation patterns
2. Biogeographic provinces -- using established marine biogeographic classifications such as Longhurst provinces^[14]
3. Depth-zone layers -- treating vertical zones as separate accounting units overlaying the horizontal extent

Countries operationalising pelagic ecosystem delineation within their EEZs may find it practical to begin with a simplified approach that combines Longhurst biogeographic provinces (for horizontal delineation) with depth-zone layers based on the GET M2 functional groups (for vertical stratification). For initial implementation, focusing on the epipelagic zone alone--where the majority of commercially relevant species and ecosystem services are concentrated--may be most tractable. As methodologies mature and data improve, countries can progressively extend their accounts to include mesopelagic and deeper zones.

Biogeographic regions

Horizontal variation in pelagic ecosystems reflects large-scale oceanographic processes. Productive regions include:

• Eastern boundary upwelling systems -- the Canary, Benguela, California, and Humboldt currents where nutrient-rich deep water surfaces, supporting exceptional fisheries productivity^[15]
• Tropical and subtropical gyres -- large circulation systems with relatively low productivity but high biodiversity
• Polar and subpolar regions -- areas of strong seasonality with intense productive periods
• Frontal zones and convergences -- boundaries between water masses where productivity and biodiversity concentrate

The GET recognises coastal upwelling zones as a distinct functional group (M1.9) within the Marine Shelf biome, noting that "the most productive upwelling zones are coastal, notably in four major eastern-boundary current systems"^[16]. Weaker upwelling processes in the open ocean, such as along the intertropical convergence zone, are included within M2.1 Epipelagic ocean waters.

Countries with significant upwelling-influenced waters (e.g., Peru, Chile, Namibia, Morocco, USA west coast) should consider whether to distinguish upwelling ecosystems as separate accounting units given their disproportionate contribution to fisheries production.

Jurisdictional boundaries

The United Nations Convention on the Law of the Sea (UNCLOS) establishes the legal framework for ocean jurisdiction, creating distinct zones with different rights and responsibilities^[17]:

• 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 for the purpose of exploring and exploiting, conserving and managing the natural resources, whether living or non-living, of the waters superjacent to the seabed"^[18]
• High seas -- areas beyond national jurisdiction, open to all States with freedom of navigation, fishing (subject to conservation obligations), and scientific research^[19]

For accounting purposes, these jurisdictional distinctions are fundamental. The SEEA CF asset boundary includes aquatic resources within a country's EEZ throughout their life cycles, but resources on the high seas pose attribution challenges^[20]. The 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"^[21].

The Agreement under UNCLOS on the Conservation and Sustainable Use of Marine Biological Diversity of Areas Beyond National Jurisdiction (BBNJ Agreement), which entered into force in January 2026, establishes new provisions relevant to pelagic accounting in high seas areas^[22]. Key implications include: area-based management tools that may create defined spatial units for ecosystem accounting beyond national jurisdiction; provisions for environmental impact assessments that generate data on pelagic ecosystem condition; and a framework for benefit-sharing from marine genetic resources that introduces new economic flows requiring recording. Future revisions of this Circular should incorporate detailed guidance on how BBNJ instruments affect the compilation of extent, condition, and services accounts for pelagic ecosystems in areas beyond national jurisdiction.

For guidance on how these jurisdictional boundaries interact with ecosystem extent and condition accounts, see TG-0.1 General Introduction to Ocean Accounts on the spatial scope of ocean accounts.

3.2 Extent and Condition

Measuring the extent and condition of pelagic ecosystems requires different approaches than for fixed coastal ecosystems such as coral reefs or mangroves. The fundamental concepts of ecosystem extent and condition accounts described in TG-3.1 Asset Accounts apply, but implementation differs significantly.

Extent measurement

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

The GET provides indicative global distributions for each functional group. For example, M2.1 Epipelagic ocean waters encompass "the surface layer of the entire open ocean beyond the near-shore zone"^[23], while M2.2 Mesopelagic waters extend "from a depth of ~200 m or where <1% of light penetrates, down to 1,000 m"^[24].

For national accounts within the EEZ, extent may be calculated as the area of the EEZ multiplied by relevant depth zones. Changes in extent over time reflect changes in the boundaries of water masses due to climate-driven shifts in oceanographic conditions rather than the conversion processes typical of coastal ecosystems.

Table 1 presents an illustrative structure for a pelagic ecosystem extent account. For initial implementation, countries may find it practical to focus on the epipelagic zone alone, where extent changes are more readily detected through satellite-observable indicators such as sea surface temperature and chlorophyll-a fronts. Changes in pelagic extent tend to reflect continuous oceanographic variation--such as shifts in water mass boundaries, deepening or shoaling of the thermocline, and changes in the depth of the photic zone--rather than the discrete conversions characteristic of benthic ecosystems. Compilers should document whether changes are recorded as gradual trends or attributed to specific oceanographic events.

Table 1: Illustrative structure for pelagic ecosystem extent account within EEZ

Condition variables

The SEEA EA condition framework applies to pelagic ecosystems with appropriate variable selection. Key condition characteristics include^[25]:

Abiotic characteristics:

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

Biotic characteristics:

• Chlorophyll-a concentration as a proxy for phytoplankton biomass^[26]
• Primary productivity rates (net primary production, NPP)
• Zooplankton biomass and community composition
• Fish biomass and species composition
• Marine mammal and seabird abundance

Functional characteristics:

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

The epipelagic zone's primary production "largely by diatoms, accounts for around half of all global carbon fixation"^[27], making productivity metrics particularly important. The mesopelagic zone contains fish biomass estimated at "two orders of magnitude larger than global fisheries landings"^[28], though precise quantification remains uncertain.

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

Reference conditions

Establishing reference conditions for pelagic ecosystems is complicated by high natural variability driven by climate oscillations (El Nino-Southern Oscillation, Pacific Decadal Oscillation, etc.) and the lack of historical baseline data for most parameters. Possible approaches include:

• Pre-industrial baselines (e.g., for temperature, acidification) where reconstructions exist
• Minimally impacted reference sites (difficult for the open ocean given pervasive climate change and fishing pressure)
• Model-based reference states derived from ecosystem models
• Policy-based targets such as those established under regional fisheries agreements

Given these challenges, condition accounts for pelagic ecosystems may need to focus on tracking trends and rates of change rather than departures from fixed reference conditions.

3.3 Migratory Species

Highly migratory species present particular challenges for asset accounting because their distribution spans multiple EEZs and the high seas, making national attribution problematic. 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

Article 64 of UNCLOS addresses highly migratory species: "The coastal State and other States whose nationals fish in the region for the highly migratory species listed in Annex I shall cooperate directly or through appropriate international organizations with a view to ensuring conservation and promoting the objective of optimum utilization of such species throughout the region, both within and beyond the exclusive economic zone"^[29].

UNCLOS Annex I lists highly migratory species including^[30]:

• 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
• Cetaceans -- all whale and dolphin families

The 1995 UN Fish Stocks Agreement further elaborates obligations for straddling and highly migratory stocks, requiring cooperation through Regional Fisheries Management Organizations (RFMOs)^[31].

Accounting approaches

The SEEA CF provides guidance on attributing migratory stocks: "Migrating and straddling fish stocks are considered to belong to a country during the period when those stocks inhabit its EEZ"^[32]. This residence-based approach creates practical difficulties when stocks move continuously across boundaries.

Alternative approaches include:

1. Proportional allocation -- attributing stock shares based on time spent in each EEZ or catch proportions
2. RFMO-based allocation -- using catch quotas allocated by regional management bodies as the basis for national asset shares
3. Flag State attribution -- attributing catches to the flag State of the harvesting vessel (as done for production in the SNA)
4. Ecosystem-based accounts -- maintaining accounts at the scale of the stock's range rather than national boundaries

For highly migratory species managed under RFMOs such as the International Commission for the Conservation of Atlantic Tunas (ICCAT), the Commission for the Conservation of Southern Bluefin Tuna (CCSBT), or the Western and Central Pacific Fisheries Commission (WCPFC), the allocation of quotas provides a practical basis for attributing asset shares. As the SEEA CF notes, "that portion of agreed access rights to those aquatic resources can be considered to belong to the country"^[33].

A key implementation challenge is that RFMO quotas are typically expressed as catch limits (flows) rather than stock shares (assets). Converting from quota allocations to asset shares requires additional steps: first, the total stock biomass must be estimated from the RFMO's stock assessment; second, each country's share of the total allowable catch is calculated from its quota allocation; third, this proportional share is applied to the total stock estimate to derive an attributed asset value. This conversion introduces uncertainties arising from stock assessment precision, the assumption that catch shares proxy for asset shares, and the temporal mismatch between assessment periods and accounting periods. Collaboration with RFMO science bodies can improve the robustness of these conversions.

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

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

Cetaceans and marine mammals

UNCLOS Article 65 addresses marine mammals separately: "States shall cooperate with a view to the conservation of marine mammals and in the case of cetaceans shall in particular work through the appropriate international organizations for their conservation, management and study"^[34]. Unlike fish stocks, cetaceans are not typically harvested commercially (with limited exceptions), so their treatment in accounts focuses on their contribution to ecosystem condition, ecosystem services (tourism, existence value), and biodiversity indicators.

For countries where whale watching or marine mammal tourism is significant, see TG-2.4 Ecosystem Goods and Services for guidance on cultural ecosystem services.

3.4 Accounting Boundaries for Pelagic Activities

Compiling accounts for pelagic ecosystems requires clear rules for attributing activities and assets across jurisdictional and ecological boundaries. Table 3 summarises the principal boundary challenges and their treatment within the accounting framework, 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. This approach is consistent with the SEEA CF residence principle for aquatic resources (see Section 3.3 above).

Stocks shared with neighbouring countries require joint stock assessments conducted through bilateral or multilateral arrangements, often coordinated by the relevant RFMO. Each country's asset share reflects its agreed access rights, and compilers should document the basis for attribution.

High seas activity by resident economic units (vessels flagged to the compiling country) is included in national production accounts in accordance with the SNA residence principle: the output of fishing vessels operating on the high seas is attributed to the economy of the flag State. Flag state reporting through catch documentation schemes provides the data basis.

High seas activity by non-resident units (foreign-flagged vessels) is excluded from national production but may appear in other accounts where foreign vessels operate under access agreements within the EEZ. The distinction between resident and non-resident activity depends on flag state identification, which is increasingly supported by automatic identification system (AIS) data.

Water column versus seabed ecosystems are treated as separate extent accounts to avoid double-counting of the three-dimensional marine environment. Pelagic ecosystem extent accounts record water column volumes (as described in Section 3.2), while benthic ecosystem extent accounts record seabed areas. The relationship between these overlapping accounts is addressed in TG-6.6 Deep Sea and Seabed Accounting.

3.5 Ecosystem Services

Pelagic ecosystems generate significant 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.

Provisioning services

Fisheries production is the dominant provisioning service from pelagic ecosystems. The SEEA AFF distinguishes among fish categories including "Pelagic fish, including Tunas, bonitos, billfishes" and "Other pelagic fish"^[35]. Global tuna and tuna-like species catches exceed 7 million tonnes annually, with high commercial value. Stock condition directly affects sustainable yield, linking condition accounts to provisioning service flows.

For pelagic fisheries accounting, the relevant guidance includes TG-3.2 Flows from the Environment to the Economy and TG-6.7 Fisheries Stock Assessment.

Genetic resources from pelagic organisms, including marine microorganisms, represent an emerging category of provisioning service with applications in pharmaceuticals, biotechnology, and industry. The BBNJ Agreement establishes a framework for the equitable sharing of benefits from marine genetic resources in areas beyond national jurisdiction^[36].

Regulating services

Climate regulation through carbon sequestration is perhaps the most globally significant ecosystem service from pelagic systems. Primary production in the epipelagic zone fixes atmospheric CO2 into organic matter. A portion of this carbon is exported to depth through sinking particles, vertical migration of organisms, and physical mixing--collectively termed the "biological pump"^[37]. Carbon that reaches the deep ocean is effectively sequestered from the atmosphere for centuries to millennia.

The mesopelagic zone plays a crucial role: "Consumers of this material, including detritivorous copepods, deplete oxygen levels in the mesopelagic zone" while "many species undertake diel vertical migration into the epipelagic zone during the night to feed... and increase the flow of carbon between ocean layers"^[38]. This active biological transport enhances carbon sequestration beyond passive sinking alone.

Quantifying carbon sequestration services requires estimates of:

• Net primary production rates
• Export production (fraction of NPP exported below the mixed layer)
• Sequestration efficiency (fraction reaching long-term storage depths)

The SEEA EA notes that "net primary productivity is considered a condition indicator for terrestrial ecosystems and is categorized in the functional class of the SEEA EA Ecosystem Condition Typology"^[39]; the same applies to marine ecosystems.

Attribution of carbon sequestration services to national accounts raises particular challenges. The service is generated by global ocean circulation not confined to any single EEZ; the benefit of climate regulation is global rather than national; and carbon exported within one EEZ may be sequestered in another jurisdiction's deep waters. Countries may address these challenges by recording carbon sequestration within the EEZ as a nationally generated service while acknowledging that the beneficiaries are global. Alternatively, carbon sequestration in the open ocean may be reported as a supplementary global commons account separate from national ecosystem service accounts. The choice of approach should be documented in metadata and applied consistently across accounting periods. 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 receiving upwelled water.

Cultural services

Research and education -- the open ocean supports marine scientific research, oceanographic monitoring, and educational programmes.

Recreation and tourism -- pelagic tourism includes whale watching, recreational fishing (particularly for billfish and tuna), and ocean cruising. For guidance on measuring cultural ecosystem services, see TG-2.4 Ecosystem Goods and Services.

Existence and bequest values -- many people value the existence of healthy ocean ecosystems, charismatic marine megafauna (whales, sharks, sea turtles), and marine biodiversity independently of any direct use.

Monetary valuation of these services faces particular challenges in the pelagic context. Carbon sequestration services may be valued using carbon prices, but attribution to specific national accounts is problematic for a global commons. Tourism services can be measured through expenditure data where they occur within national jurisdiction. Existence values require stated preference methods (contingent valuation, choice experiments) that are not part of the core SEEA framework. For valuation approaches and their limitations, see TG-1.9 Valuation.

3.6 Data and Methods

Pelagic ecosystems present data challenges due to their remoteness, vast scale, and dynamic nature. This section surveys available data sources and methods.

Satellite oceanography

Remote sensing provides essential data for pelagic ecosystem monitoring at scales not achievable by in situ methods^[40]:

• Ocean colour -- sensors such as MODIS, VIIRS, and Sentinel-3 OLCI measure chlorophyll-a concentration as a proxy for phytoplankton biomass and primary productivity
• Sea surface temperature (SST) -- thermal infrared and microwave sensors provide global SST data
• Sea surface height -- altimetry detects ocean circulation patterns, mesoscale eddies, and fronts
• Sea ice extent -- passive microwave sensors monitor polar sea ice

Satellite data support both condition monitoring (trends in chlorophyll, temperature anomalies) and biogeographic delineation. For guidance on using remote sensing data in ocean accounts, see TG-4.1 Remote Sensing Data.

Limitations include: penetration limited to the near-surface (chlorophyll algorithms represent ~1 optical depth); cloud cover affecting visible-band sensors; and the need for atmospheric correction. Validation against in situ measurements is essential.

In situ oceanography

Oceanographic surveys and autonomous platforms provide subsurface observations:

• Research vessels -- conduct transects with CTD (conductivity-temperature-depth) profiles, water sampling for nutrients and plankton, and net sampling
• Argo floats -- the global array of >4,000 autonomous profiling floats provides temperature and salinity profiles to 2,000m
• Biogeochemical Argo -- emerging network of floats with oxygen, nitrate, pH, chlorophyll, and particle sensors
• Moorings and buoys -- fixed platforms for time-series observations

The Global Ocean Observing System (GOOS) coordinates international oceanographic observation with Essential Ocean Variables (EOVs) relevant to ecosystem accounting.

Fish stock assessment

Assessing pelagic fish stocks employs methods distinct from demersal fisheries due to the species' mobility and aggregating behaviour:

• Acoustic surveys -- scientific echosounders estimate biomass of schooling pelagic fish
• Tagging and tracking -- electronic tags (archival, satellite) reveal migration patterns and population structure
• Virtual population analysis -- age-structured models using catch-at-age data and indices of abundance
• Close-kin mark-recapture -- genetic methods for estimating absolute abundance, particularly for high-value species like southern bluefin tuna

Stock assessments for transboundary and highly migratory species are typically conducted by RFMOs using data contributed by member States. These assessments provide the scientific basis for catch limits and allocation.

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

Given the difficulty of direct observation at scale, pelagic ecosystem accounting may rely substantially on models:

• Biogeochemical models -- simulate primary production, carbon cycling, and nutrient dynamics
• Ecosystem models (e.g., Ecopath with Ecosim, Atlantis) -- represent trophic interactions and biomass flows
• Species distribution models -- predict habitat suitability and distribution shifts
• Stock assessment models -- integrate fisheries data into population estimates

Models provide spatial and temporal coverage not achievable by observation alone but introduce structural and parametric uncertainties. Best practice involves ensemble approaches, uncertainty quantification, and validation against available observations.

Uncertainty characterisation

The substantial reliance on models for pelagic accounting introduces uncertainties that compilers should document and communicate to account users. Key sources of uncertainty include: structural uncertainty in model formulations (e.g., alternative representations of trophic interactions); parametric uncertainty in model calibration; observational uncertainty in the sparse in situ data used for validation; and spatial extrapolation from point observations to large pelagic domains. Compilers should report confidence intervals or uncertainty ranges where feasible, use ensemble modelling approaches to capture structural uncertainty, and clearly distinguish between observation-based and model-derived entries in published accounts. For systematic guidance on quality assurance and uncertainty communication, see TG-0.7 Quality Assurance.

3.7 Compilation Procedure and Worked Example

This section provides a step-by-step compilation procedure for pelagic ecosystem accounts, followed by a worked example demonstrating the procedure with synthetic data.

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:

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

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

Step 2: Compile extent accounts

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

Data requirements:

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

Calculation:

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

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

Step 3: Compile condition accounts

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

Data requirements:

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

Calculation:

• Normalise each condition variable against reference conditions
• Apply scaling formula: Indicator = (V - VL) / (VH - VL), where V = observed value, VH = reference high, VL = degraded low
• Document reference condition selections

Output: Condition variable account with raw measurements and normalised indicators for each ecosystem type.

Step 4: Compile fish stock asset accounts

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

Data requirements:

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

Calculation:

• Opening stock from most recent stock assessment
• 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.

Step 5: Estimate ecosystem service flows

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

Data requirements:

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

Calculation:

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

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

Step 6: Value ecosystem assets (optional)

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

Data requirements:

• Annual ecosystem service values from Step 5
• Discount rate (4% real is commonly applied)
• Assumptions about service flow duration (perpetual for renewable services)

Calculation:

• Asset value = Annual service value / discount rate (for perpetual annuity)
• Apply to carbon sequestration and fisheries services separately
• Document uncertainties and sensitivity to discount rate

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

Worked Example

This worked example demonstrates the compilation of pelagic and open ocean ecosystem accounts for a hypothetical Pacific small island developing state (SIDS). The example follows the extent-condition-services-valuation sequence presented in Sections 3.1-3.5 and illustrates the key accounting entries and calculations. 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 exclusive economic zone (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 the EEZ's southern boundary.

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

Note: Total EEZ volume is unchanged. The epipelagic zone contracted slightly as warming shifted the thermocline and warm pool boundaries. These changes are recorded as reclassifications between depth zones rather than net additions or reductions, consistent with the guidance in Section 3.2.

Step 2: Condition account

Condition indicators are derived from satellite oceanography, Argo float data, and RFMO stock assessments. Reference levels are based on 1990-2010 climatological means:

Note: For SST anomaly, lower values indicate better condition. The indicator formula is inverted: Indicator = (VL - V) / (VL - VH).

Composite condition index (equal weights): (0.65 + 0.72 + 0.55 + 0.60) / 4 = 0.63

Step 3: Ecosystem services (annual flows)

Note: The tuna fisheries value reflects only the national quota allocation managed through WCPFC, representing the country's attributed share of the transboundary stock. Licence fees paid by distant-water fishing nations for EEZ access (approximately USD 30,000,000/yr for this hypothetical SIDS) are recorded as economic flows in the national accounts but are not ecosystem service values. Carbon sequestration is valued at a conservative USD 20/t CO2, reflecting the methodological uncertainty in attributing open-ocean carbon sequestration to national accounts. Compilers may use higher social cost of carbon values with appropriate documentation.

Step 4: Asset valuation

Applying a 4% social discount rate over a 25-year projection horizon:

Asset value = 104,000,000 x present value annuity factor (4%, 25 years) Asset value = 104,000,000 x 15.62 = 1,624,500,000 USD

This large asset value reflects the immense scale of the EEZ and the globally significant ecosystem services generated by pelagic systems. However, the estimate carries substantial uncertainty, particularly regarding carbon sequestration attribution. A sensitivity analysis using only fisheries services (USD 54,000,000/yr) yields an asset value of approximately USD 843,500,000, illustrating the sensitivity of the total to carbon valuation assumptions.

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. The example values are illustrative and should not be used as benchmarks for specific national contexts.

4. Acknowledgements

Authors: Jordan Gacutan (GOAP Secretariat), Mitchell Lyons (GOAP Secretariat)

Reviewers: [To be confirmed]