Asset Accounts

1. Outcome

After working through this Circular, you will be able to build asset accounts for ocean resources -- the structured records that track what you have, what you gained, what you lost, and what remains. In practice, this means you can:

• Compile physical asset accounts for fish stocks, seabed minerals, and marine ecosystems, recording opening stocks, additions, reductions, and closing stocks in appropriate physical units
• Compile monetary asset accounts that translate physical stocks into economic values, enabling comparison across asset types and calculation of total ocean wealth
• Calculate depletion -- the single most important sustainability signal in these accounts -- by comparing actual extraction against the regenerative capacity of renewable resources
• Track ecosystem extent and condition using the SEEA Ecosystem Accounting framework, capturing whether coral reefs, seagrass meadows, and mangroves are growing or shrinking, improving or degrading
• Integrate asset accounts with national balance sheets, supply-use tables, and indicator frameworks so that asset data feeds directly into budget decisions and policy monitoring

These capabilities connect directly to the budget processes described in TG-1.1 OA and National Budget Processes, where asset accounts provide the evidence base for allocating public resources to ocean conservation and sustainable use. They also underpin the depletion-adjusted income measures that tell governments whether economic growth is coming at the expense of natural capital.

Ocean accounts live in two worlds: the accountant's world (where numbers must balance) and the ecology world (where fish populations follow dynamics that resist neat categorisation). This Circular shows you how to work in both worlds simultaneously -- maintaining the rigorous accounting identities that ensure internal consistency while respecting the ecological complexity of marine systems.

2. Requirements

Essential prerequisites:

• TG-0.1 General Introduction to Ocean Accounts -- for the conceptual framework and domain structure
• TG-0.2 Overview of Relevant Statistical Standards -- for the SNA/SEEA accounting architecture

Helpful background:

• TG-0.7 Quality Assurance Principles -- for data quality and uncertainty management

3. Guidance Material

Asset accounts are where you count what you have and what you have used. They record the stock of marine assets at the beginning and end of an accounting period and every change that happened in between -- growth, extraction, natural losses, discoveries, and measurement revisions^[1]. This structured bookkeeping is what makes it possible to answer the question: are we drawing down our ocean capital faster than it regenerates?

Two complementary accounting frameworks apply to ocean assets. The SEEA Central Framework provides the methodology for accounting for individual environmental assets -- fish stocks, seabed minerals, water resources -- each tracked separately. SEEA Ecosystem Accounting extends this by treating ecosystems as integrated spatial units: a coral reef or seagrass meadow as a whole, not just the fish or carbon within it. For ocean accounting, you will typically need both. The individual asset approach tells you how many tonnes of fish remain; the ecosystem asset approach tells you whether the reef that supports those fish is expanding or contracting.

This section examines the structure and accounting entries for physical and monetary asset accounts, before addressing the specific requirements for individual environmental assets (Section 3.3), ecosystem assets (Section 3.4), and produced assets in the ocean domain (Section 3.5). Section 3.2 provides a compilation procedure detailing the steps from data collection to account entry, while Section 3.6 presents a worked example with synthetic data demonstrating how to populate asset accounts for a hypothetical coastal area. The methodology presented here provides the basis for the thematic circulars on coral reef accounts (TG-6.1), mangrove and wetland accounts (TG-6.2), and seagrass accounts (TG-6.3).

3.1 Physical Asset Accounts

Physical asset accounts are where you count what you have in tangible units -- tonnes of fish, hectares of seagrass, cubic metres of sand. They tell you whether your marine resources are growing or shrinking in actual physical terms, before any monetary valuation comes into play^[2]. This matters because price changes can mask physical depletion: a fish stock can be worth more in dollar terms even as the population collapses, simply because scarcity drives up prices. Physical accounts cut through that illusion.

Basic structure

Every physical asset account follows the same logic^[3]:

1. Opening stock -- what you had at the start of the year
2. Additions to stock -- everything that increased the stock during the year, including natural growth, discoveries, upward reappraisals, and reclassifications
3. Reductions in stock -- everything that decreased the stock, including extractions, natural losses, catastrophic losses, downward reappraisals, and reclassifications
4. Closing stock -- what you have at the end of the year

The closing stock must equal the opening stock plus total additions minus total reductions. If it does not balance, something is missing from your records -- unreported catch, an unobserved storm event, or a measurement error. This accounting identity is your quality control mechanism: it forces you to account for every change.

Figure 3.1.1: Physical asset account identity

Additions to stock

Natural growth is the primary addition for renewable resources like fish stocks. It encompasses recruitment -- new fish joining the stock -- and biomass increase as existing individuals grow^[4]. Natural growth is driven by biological processes that vary with environmental conditions, stock size, and species characteristics. For a fisheries manager, natural growth determines the sustainable harvest ceiling: it represents the largest catch you can take without reducing the stock's long-term productivity^[5]. In practice, this means that if you can measure natural growth accurately, you can set harvest limits that keep the stock stable.

Discoveries apply primarily to non-renewable resources. In ocean accounting, this means identification of new oil and gas deposits, seabed mineral resources such as polymetallic nodules and seafloor massive sulphides, and rare earth elements in marine sediments^[6]. Discoveries can also apply to aquatic resources when previously unknown fish stocks are identified and assessed -- though this is rare in well-surveyed waters.

Upward reappraisals record cases where your estimate of stock size increased, but the stock itself did not physically change^[7]. This distinction matters because it tells you whether something actually happened in the ocean or whether your measurement improved. For fish stocks, a reappraisal occurs when a stock assessment model is updated with new data and produces a revised biomass estimate. The SEEA CF acknowledges that for fish stocks, "it may not be possible to attribute the changes to natural causes or harvesting activity" and in such cases "only a regional or national aggregate resource value will be produced"^[8]. In practice, this means compilers should document whether stock changes reflect real biological dynamics or improved measurement -- even when the boundary between the two is fuzzy.

Reclassifications record transfers between asset categories without any physical change^[9]. A common ocean example: when a wild fish stock becomes subject to aquaculture operations, it transfers from natural aquatic resources to cultivated aquatic resources. The SEEA CF emphasises that "in cases where stocking with cultured seeds is regularly conducted, as commonly observed in freshwater resources, it is important to include the amount of released seeds as a reclassification from cultivated aquatic resources"^[10]. This matters for policy because misclassification can make it look like wild stocks are growing when they are actually being supplemented by hatchery production.

Reductions in stock

Extraction (or harvest, for biological resources) is the removal of natural resources for use in economic activity^[11]. For fish stocks, extraction corresponds to gross catch -- the total live weight of fish caught, including discarded catch but excluding pre-catch losses^[12]. Countries use gross catch rather than landings because discards still remove fish from the ocean, even though they never reach port. The FAO defines the stages of catch as^[13]:

• Gross removal: total live weight of fish caught or killed during fishing operations
• Gross catch: gross removal less pre-catch losses
• Retained catch: gross catch less discarded catch
• Landings: net weight of quantities landed at port
• Nominal catch: live weight equivalent of landings

Normal losses (natural mortality for biological resources) are decreases from natural processes -- death from age, predation, disease, and accidents^[14]. For fish stocks, natural mortality is a key parameter in stock assessment models. It must be estimated alongside fishing mortality to understand population dynamics, because a stock with high natural mortality can tolerate less fishing pressure than one where most individuals die of old age.

Catastrophic losses are "exceptional and significant reductions in the natural resource"^[15] due to discrete events -- storms, disease outbreaks, toxic algal blooms, mass coral bleaching, or oil spills. The distinction from normal losses matters for two reasons. First, if catastrophic losses are increasing in frequency, that is a warning sign that environmental conditions are deteriorating. Second, catastrophic losses are unpredictable and fall outside the scope of normal management, so they need to be tracked separately to avoid distorting assessments of whether routine management is working.

Downward reappraisals and reclassifications mirror their counterparts in additions, recording accounting adjustments from improved information or transfers between categories.

Depletion

Depletion is your most important alarm bell. It tells you whether you are harvesting a renewable resource faster than it can regenerate -- and if so, by how much. For a policy-maker, depletion is the number that answers: "Are we eating our seed corn?"

For renewable resources, depletion occurs when "the extraction of the natural resource is occurring at a level greater than that of regeneration"^[16]. The formula is straightforward:

Depletion = max(0, Extraction - Sustainable yield)

When extraction stays at or below sustainable yield, depletion is zero -- the stock can absorb the harvest. When extraction exceeds sustainable yield, every tonne above that threshold is depletion: a reduction in the stock's future productive capacity^[17]. For non-renewable resources such as mineral deposits, there is no regeneration, so depletion equals extraction. Every barrel of offshore oil pumped is a barrel gone.

Depletion provides a critical link between physical and monetary accounts. In the 2025 SNA, depletion of natural resources is treated as a cost of production analogous to depreciation of produced assets, reducing net measures of income and product^[18]. This means that a country reporting high GDP from fisheries but also high depletion is overstating its true income -- part of what looks like earnings is actually the liquidation of natural capital. The SEEA CF notes that "since the drivers for changes in populations of aquatic resources can only be modelled, it may be difficult to obtain precise and consistent measures of sustainable yield over time"^[19]. For methodological guidance on estimating sustainable yield and depletion for fisheries, see TG-6.7 Fisheries Accounting: Integrating Stock Assessment.

3.2 Compilation Procedure for Asset Accounts

This section walks through the step-by-step procedure for compiling ocean asset accounts -- from assembling raw data to producing balanced accounts that integrate with the broader national accounting system. Following these steps ensures that your accounts are internally consistent and comparable across asset types.

Step 1: Data collection and source identification

Start by identifying the data sources you need for each asset type. Different marine assets draw on very different data ecosystems:

For individual environmental assets (fish stocks, seabed minerals):

• Stock assessment reports from fisheries management agencies, providing estimates of spawning stock biomass, recruitment, natural mortality, and fishing mortality (see TG-6.7 Fisheries Accounting: Integrating Stock Assessment)
• Geological surveys and commercial exploration reports for mineral and energy resources, including estimates of proven reserves and probabilistic resource classifications (see TG-3.10 Offshore Energy Accounts)
• Catch statistics from fisheries agencies and international databases (FAO FishStatJ), recording landings, gross catch, and discards by species and fishing zone

For ecosystem assets (coral reefs, seagrass meadows, mangroves):

• Remote sensing imagery (Sentinel-2, Landsat, MODIS) processed to classify ecosystem extent and detect changes over time (see TG-4.1 Remote Sensing and Geospatial Data)
• Field survey data from ecological monitoring programmes, providing measurements of ecosystem condition variables such as coral cover, seagrass shoot density, mangrove canopy height, and water quality parameters (see TG-4.2 Survey Methods for Ocean Economic Activity)
• National spatial databases delineating protected areas, coastal zone boundaries, and ecosystem type classifications consistent with the IUCN Global Ecosystem Typology

Assess data quality following TG-0.7 Quality Assurance Principles. Pay particular attention to three things: temporal consistency (are opening and closing stocks measured on a comparable basis?), spatial coverage (do the data cover the full accounting area?), and measurement uncertainty (what are the confidence intervals, and are they documented?).

Step 2: Classification and mapping to asset categories

Next, map your source data to the asset classification system used in the accounts. Getting this right matters because it determines how your numbers aggregate and compare with other countries. The SEEA CF classifies environmental assets into major categories including aquatic resources, mineral and energy resources, water resources, timber resources, land, and soil resources^[20]. For ocean accounting, the relevant categories are:

• Natural aquatic resources (wild fish, shellfish, and other uncultivated aquatic organisms)
• Cultivated aquatic resources (farmed fish, shellfish, and seaweed)
• Mineral and energy resources (offshore oil and gas, seabed minerals, marine aggregates)
• Water resources (coastal aquifers subject to saltwater intrusion; seawater abstraction is recorded as an abiotic flow rather than an asset stock)

SEEA EA ecosystem assets are classified using the IUCN Global Ecosystem Typology, with marine ecosystem types including:

• M1 Marine Shelf (seagrass meadows, kelp forests, coral reefs, shellfish beds, subtidal rocky reefs)
• M2 Pelagic Ocean Waters (epipelagic, mesopelagic, bathypelagic zones)
• M3 Deep Sea Floors (abyssal plains, submarine canyons, hydrothermal vents)
• MFT Brackish Tidal Systems (mangroves, saltmarshes, coastal brackish lagoons)
• MT Shorelines (rocky shores, sandy beaches)

If your country uses a national ecosystem type classification that differs from the IUCN GET, document the correspondence between the two. This ensures that your ecosystem extent can be aggregated to standard categories for international comparison.

Step 3: Measurement and quantification

With data sources identified and classifications established, you can quantify opening stocks, additions, reductions, and closing stocks in physical units appropriate to each asset type.

For fish stocks, stock assessment models provide estimates of total biomass (or spawning stock biomass) at the beginning of the accounting period (opening stock). Natural growth is estimated from recruitment models and individual growth rates. Extraction is measured as gross catch from fisheries statistics. Natural mortality is estimated from stock assessment parameters. The accounting identity (opening stock + growth - mortality - catch = closing stock) serves as your reconciliation tool. Where stock assessment provides direct estimates of both opening and closing stock, the identity becomes a consistency check; discrepancies may indicate data errors or unobserved changes requiring reappraisal entries.

For ecosystem extent, remote sensing analysis provides the primary measurement of opening and closing extent, typically expressed in hectares or square kilometres. Change detection algorithms identify areas where ecosystem type has changed (conversions), distinguishing managed changes (restoration, conversion to aquaculture) from natural changes (storm damage, succession). Ground-truthing surveys validate remotely sensed classifications and provide confidence ratings. The SEEA EA recommends recording extent changes in a structured account showing managed expansion, natural expansion, managed reduction, and natural reduction separately^[21].

For ecosystem condition, the measurement process involves selecting condition variables for each ecosystem type following the SEEA EA Ecosystem Condition Typology (ECT), measuring variable values from field surveys or remote sensing, and normalising these values into indicators relative to reference conditions. Condition accounts record the raw variable values rather than derived indicators; the indicator derivation is addressed in TG-2.1 Aggregate Biophysical Indicators of Environmental State.

Step 4: Account entry and balancing

Once physical quantities have been measured, enter them into the asset account structure. The account must balance: closing stock = opening stock + total additions - total reductions. Any imbalance tells you something is wrong -- either a data error or a missing entry (unreported extraction, unobserved natural changes, or a needed reappraisal adjustment).

For marine assets, three issues require particular attention:

• Spatial boundaries: Ensure that opening and closing stocks cover the same geographic area. If jurisdictional boundaries or accounting area definitions changed during the period, record those changes as reclassifications rather than real changes. Otherwise, you will attribute administrative decisions to ecological processes.
• Unit consistency: Keep all entries for a given asset in the same measurement unit. For fish stocks, this typically requires converting landings (wet weight) to live weight equivalent, and converting different life stages to a common biomass measure.
• Temporal alignment: Ensure all entries refer to the same accounting period. Where stock assessments use different time periods or reference dates than the calendar year accounting period, adjustments may be needed.

Step 5: Monetary valuation (for monetary accounts)

For monetary asset accounts, you translate physical stock quantities into monetary values. This is where the two worlds of accounting and ecology create the most tension -- because pricing a fish stock or a mangrove forest requires assumptions about the future that ecology makes uncertain.

For fish stocks and other renewable natural resources, the net present value (NPV) of expected future resource rents provides the standard valuation approach^[22]. The NPV calculation requires:

1. Resource rent estimation: Calculate the annual resource rent as the difference between the value of harvest and all costs (including labour, capital, intermediate inputs, and normal return on produced assets). This tells you the economic surplus that the natural resource itself contributes.
2. Discount rate selection: Determine an appropriate discount rate reflecting the time value of money and asset-specific risk factors (see TG-1.9 Safe Usage of Monetary Valuation for detailed guidance). Higher discount rates reduce asset values -- so this choice matters for policy.
3. Asset life assumption: Estimate the expected duration of the resource rent stream. For sustainably managed renewable resources, this may be assumed to be perpetual.

For ecosystem assets, the monetary value is estimated as the NPV of expected future ecosystem service flows. This requires:

1. Service quantification: Measure the physical supply of ecosystem services from the asset (see TG-3.2 Flows from Environment to Economy)
2. Service valuation: Apply appropriate valuation methods to estimate unit prices for each ecosystem service
3. NPV calculation: Discount the stream of future service values to present value

Changes in monetary value are decomposed into three components:

• Changes due to physical volume changes (additions and reductions in stock)
• Changes due to updated expectations about future rents or service flows (reappraisals)
• Changes due to price movements (revaluations)

This decomposition matters because it prevents price effects from masking physical depletion. A fish stock can be worth more in dollar terms this year than last, even as the population declines -- if prices rose enough. Separating these effects ensures that monetary accounts remain honest about what is happening physically.

Step 6: Integration with balance sheets and other accounts

The final step is integrating asset accounts with national balance sheets and with other ocean accounts. The balance sheet presents the stock of all assets (produced assets, non-produced natural assets, ecosystem assets, financial assets) and liabilities at a single point in time, enabling calculation of net worth^[23]. Changes in net worth during the accounting period are explained by saving, capital transfers, and holding gains/losses recorded in the revaluation account.

For ocean accounting, integration ensures that:

• Depletion entries in asset accounts correspond to depletion costs in production accounts and adjustments to net domestic product
• Extraction entries in asset accounts correspond to natural resource inputs in physical supply-use tables (TG-3.2 Flows from Environment to Economy)
• Ecosystem service flows are consistent with the capacity to supply services implied by ecosystem extent and condition (as illustrated in the worked example below)
• Investment in ocean infrastructure (such as aquaculture facilities and offshore energy) is recorded as gross fixed capital formation in produced assets, enabling joint analysis of natural and produced capital stocks

Cross-stack connections upward to policy circulars include:

• TG-1.1 National Ocean Budgets uses asset values and depletion estimates to inform natural capital budgeting
• TG-2.1 Aggregate Biophysical Indicators of Environmental State derives extent change indicators and condition indices from asset accounts
• TG-2.2 Productivity Indicators uses fish stock biomass and sustainable yield estimates to calculate indicators of resource productivity

Cross-stack connections downward to data circulars include:

• TG-4.1 Remote Sensing and Geospatial Data provides methods for extent mapping that populate ecosystem extent accounts
• TG-4.2 Survey Methods for Ocean Economic Activity provides protocols for condition variable measurement
• TG-4.3 Administrative Data Sources provides the spatial framework for delineating ecosystem assets and aggregating data

These linkages ensure that asset accounts function as an integrated component of the broader ocean accounting system rather than a standalone exercise.

3.3 Monetary Asset Accounts

Monetary asset accounts translate the physical stocks and changes described in Section 3.1 into economic values. This translation enables three things that physical accounts alone cannot do: aggregate across different asset types (you cannot add tonnes of fish to hectares of seagrass, but you can add their monetary values), compare environmental assets with produced and financial assets on common terms, and calculate depletion-adjusted income measures that reveal whether economic growth is genuine or is partly the liquidation of natural capital^[24]. For detailed guidance on valuation methods applicable to ocean assets, see TG-1.9 Safe Usage of Monetary Valuation.

Structure of monetary asset accounts

The structure parallels physical accounts, with one important addition -- a revaluation entry^[25]:

1. Opening stock value -- the monetary value of the asset at the beginning of the accounting period
2. Additions to stock value -- value of growth, discoveries, and upward reappraisals
3. Reductions in stock value -- value of extractions, normal losses, catastrophic losses, and downward reappraisals
4. Revaluations -- changes in value due to price changes, distinct from physical changes
5. Closing stock value -- the monetary value at the end of the accounting period

The revaluation entry captures holding gains and losses -- changes in asset value attributable to changes in prices rather than physical quantities^[26]. This separation is essential: without it, you cannot tell whether an asset became more valuable because the ecosystem improved or because market prices shifted.

Valuation approaches

Environmental assets are typically not traded in markets, which means you cannot simply look up a price. The SEEA CF describes several approaches to work around this^[27]:

Net present value (NPV) is the preferred approach for valuing natural resources. You estimate the stream of expected future resource rents -- the economic surplus accruing to the resource owner after all costs and normal returns are deducted -- and discount this stream to the present^[28]. For natural resources:

Value = Resource rent x Discount factor

where the discount factor adjusts for the time value of money and the expected duration of the income stream. The SEEA CF describes the discount factor (Omega) as linking "future resource rents to the present value of the asset"^[29]. Key considerations for discount rate selection include the real rate of interest, expected asset life, and country-specific factors^[30].

Discount rate selection can significantly affect asset valuations, particularly for long-lived assets such as fish stocks under perpetual management regimes. A lower discount rate makes long-term sustainability more economically visible; a higher rate effectively tells you to worry less about the distant future. The SEEA CF provides extensive guidance on discount rate methodology in Annex A5.2, covering asset life assumptions, risk adjustments, and country-specific factors^[31]. For detailed guidance on discount rate selection and its implications for marine asset valuation, see TG-1.9 Safe Usage of Monetary Valuation.

Market prices may be used where environmental assets or rights to extract them are traded. For aquatic resources, long-term fishing licences and individual transferable quotas (ITQs) may provide market-based valuations^[32]. However, markets for environmental assets are often thin or non-existent, limiting the applicability of this approach. The SEEA CF notes that "in many cases, where the government hands the access rights to fishermen, trading in these access rights is prohibited and there is therefore no directly observable market valuation"^[33].

Restoration cost approaches estimate the cost of restoring an ecosystem asset to a reference condition. This approach is described in SEEA EA as a complement to NPV-based valuation, particularly for ecosystem degradation^[34]. It answers a different question: not "What is this ecosystem worth?" but "What would it cost to replace what we have lost?" See TG-1.9 Safe Usage of Monetary Valuation for detailed comparison of valuation approaches.

Valuation of depletion

The monetary value of depletion is calculated by multiplying physical depletion by the appropriate asset price^[35]. Following the conventions in the SNA and SEEA CF, depletion should be valued at the average of opening and closing prices:

Monetary depletion = Physical depletion x (P_opening + P_closing) / 2

This mid-period valuation "is consistent with the rules in the SNA for the valuation of consumption of fixed capital"^[36] -- which means depletion of a fish stock is treated on the same footing as depreciation of a factory. Both represent the using-up of productive capacity, and both should be deducted from income.

3.4 Individual Environmental Assets

Individual environmental assets are "those environmental assets that may provide resources for use in economic activity"^[37]. Think of them as the specific natural resources you can name and count -- fish stocks, oil deposits, sand reserves. For ocean accounting, the principal categories include aquatic resources (Section 3.4.1), mineral and energy resources (Section 3.4.2), and water resources (Section 3.4.3).

3.4.1 Aquatic resources

Aquatic resources include fish, crustaceans, molluscs, shellfish and other aquatic organisms such as sponges and seaweed, as well as aquatic mammals such as whales^[38]. These organisms are subject to harvest for commercial, subsistence, and recreational purposes. The SEEA CF distinguishes between:

• Natural aquatic resources -- wild fish and other aquatic organisms not under direct cultivation
• Cultivated aquatic resources -- organisms raised in aquaculture facilities, including fish, shellfish, seaweed, and other species

This distinction determines how the assets are classified in the accounts: natural aquatic resources are non-produced assets (nature made them), while cultivated aquatic resources are produced assets (fixed assets for breeding stocks; inventories for stocks held for sale)^[39]. This matters for policy because the management interventions differ fundamentally -- you manage a wild fishery by controlling extraction, but you manage aquaculture by controlling production inputs. Detailed guidance on aquaculture accounting is provided in TG-3.9 Aquaculture Accounts.

Measuring fish stocks. Estimating the absolute size of a fish stock is one of the hardest measurement challenges in ocean accounting. Common approaches include virtual population analysis (VPA), tag-recapture studies, and acoustic or trawl surveys^[40]. When absolute stock estimates are unavailable, catch per unit effort (CPUE) may provide an indicator of relative stock size, on the assumption that population density correlates with catch rates^[41]. The SEEA CF cautions that "a declining trend in the CPUE may be a signal that the rate of harvest is exceeding the renewal rate of the fish stock"^[42]. In practice, this means that even imperfect indicators can signal problems if you track them consistently over time.

Stock assessments should ideally distinguish between the total stock, the spawning stock biomass (the portion capable of reproduction), and the exploitable stock (the portion subject to harvest)^[43]. For accounting purposes, estimates of the total stock and changes therein are required. Remote sensing and other spatial data sources may support stock assessment; see TG-4.1 Remote Sensing and Geospatial Data for guidance on satellite-derived inputs.

Classification of catch types. The SEEA CF recommends using gross catch as the measure of extraction from natural aquatic resources^[44]. Gross catch is defined as the total live weight of fish caught, comprising:

• Retained catch -- fish brought to shore as landings
• Discarded catch -- fish returned to the water, typically dead or dying

Using gross catch rather than landings matters because discards still remove fish from the ocean. In some fisheries, discards constitute a substantial fraction of total catch, so ignoring them would significantly understate the impact on fish stocks. The SEEA CF notes that "the measurement of discarded catch is an important contributory factor to a full understanding of the linkages between economic activity and the impact on aquatic resources"^[45].

Where direct measurement through at-sea observer programmes is unavailable, discard rates may be estimated using species-specific discard ratios from comparable fisheries, gear-type-specific discard rates from regional studies, or logbook-based self-reporting validated against observer data. For guidance on survey methods applicable to discard estimation, see TG-4.2 Survey Methods for Ocean Economic Activity.

Physical asset account for aquatic resources. Table 1 presents the structure of a physical asset account for aquatic resources, distinguishing between cultivated and natural resources.

Table 1: Structure of physical asset account for aquatic resources, with illustrative synthetic values in tonnes (adapted from SEEA CF Table 5.22)^[46]

The table includes "uncompensated seizure" to record illegal fishing by non-residents, following the SEEA CF treatment^[47]. Looking at the numbers: the natural aquatic stock is declining (closing stock 171,000 < opening stock 180,000), driven by extraction and losses exceeding growth. Cultivated resources show modest growth -- a pattern you would expect when aquaculture is expanding while wild fisheries are under pressure.

Comprehensive physical asset account template

Table 1a provides a broader template that compilers can use to record all major categories of marine natural resources within a single accounting structure. It distinguishes natural aquatic resources (fish and other wild aquatic organisms), ecosystem assets (spatially defined marine ecosystem types measured by extent), and mineral and energy resources (seabed deposits). Each asset class follows different dynamics -- fish grow and reproduce, ecosystems expand and contract, minerals only deplete -- and the template captures these differences.

Table 1a: Comprehensive physical asset account template for marine natural resources with illustrative synthetic values (integrating SEEA CF Table 5.4, SEEA EA Table 4.1, and SEEA CF Table 5.22)^[48]

Note: Illustrative values are synthetic. In practice, discoveries of entirely new fish stocks are rare, and stock revisions are typically recorded as reappraisals rather than discoveries.

The "Derived entries" section highlights depletion as a key sustainability indicator. For renewable resources, depletion occurs only when extraction exceeds sustainable yield (24,000 - 22,000 = 2,000 tonnes in this example). For non-renewable resources, all extraction constitutes depletion -- there is no regeneration to offset it. For natural aquatic resources, stock assessment models provide the primary input (see TG-6.7 Fisheries Accounting: Integrating Stock Assessment). For ecosystem assets, remote sensing and spatial survey provide extent mapping (see TG-4.1 Remote Sensing and Geospatial Data). For mineral resources, geological surveys and commercial assessments provide stock estimates.

Sustainable yield and depletion. For fisheries management, sustainable yield represents the maximum catch that can be taken without reducing the stock's long-term productivity. When gross catch exceeds sustainable yield, depletion occurs and should be recorded. Conversely, when catch is below sustainable yield, the excess regeneration contributes to stock growth rather than depletion^[49]. The practical test: compare your catch to your sustainable yield estimate each year. If the gap is widening, management intervention is needed. The SEEA CF recommends that "estimates from biological models be compared with indicators of stock size, such as CPUE, and also that estimation be carried out on an ongoing basis so that the dynamics of the various populations (natural growth, natural losses, etc.) can be better understood"^[50].

Capture fishing by non-residents. Under the SNA and SEEA CF, production is attributed to the country of residence of the harvesting operation, not the location of the resource^[51]. However, for asset accounts, the focus is on changes in the national aquatic resource. The SEEA CF states that "the total catch from the country's aquatic resources--including any resources on the high seas over which ownership rights exist, regardless of the residency of the harvesting operation"^[52] must be recorded as reductions in the national stock. In practice, this means that foreign-flagged vessels fishing in your waters reduce your asset account even though their production appears in another country's GDP.

Illegal fishing. The SEEA CF addresses illegal fishing explicitly: "If residents harvest aquatic resources beyond the scope of their licence, they are harvesting illegally. Nonetheless, following the principles of the SNA, this harvest should still be recorded as production with an income accruing to the fisherman"^[53]. For illegal fishing by non-residents, the physical removals should be recorded as "uncompensated seizures"^[54]. This matters because excluding illegal catch from asset accounts would overstate your closing stock -- the fish are gone regardless of whether they were caught legally.

Monetary valuation of aquatic resources. The SEEA CF describes two main options for valuing natural aquatic resources^[55]:

1. Using the value of long-term fishing licences and quotas where realistic market values are available
2. Applying the NPV of expected resource rents

Where individual transferable quotas (ITQs) are used and traded, the total market value of all quotas may approximate the value of the aquatic resource^[56]. However, quota markets are often imperfect, and quota values may not reflect the full value of the underlying resource. The SEEA CF notes that "because of market imperfections (barriers to entry in the form of specialized fixed assets, knowledge of fishing grounds, etc.), a lack of liquidity in the markets, and uncertainties in the statistical assumptions required for net present value calculations", different approaches "are unlikely to give the same result in practice"^[57]. In practice, this means compilers should document which method they used and why, rather than assuming that any single approach gives the "right" answer.

3.4.2 Mineral and energy resources

Mineral and energy resources in the ocean domain include offshore oil and gas deposits, seabed minerals (including polymetallic nodules, cobalt-rich ferromanganese crusts, and seafloor massive sulphides), sand and gravel, and other extractable minerals^[58]. Unlike fish stocks, these resources do not regenerate on human timescales -- every unit extracted is permanent depletion.

Classification of resources. The SEEA CF classifies mineral resources based on commercial viability, which determines how they enter the accounts^[59]:

• Class A: Commercially recoverable resources -- economically viable to extract under current conditions
• Class B: Potentially commercially recoverable resources -- may become viable with technological advances or price changes
• Class C: Non-commercial and other known deposits -- not currently viable

This classification matters because movements between classes (a deposit becoming commercially viable as technology improves) are recorded as reclassifications, not discoveries. Understanding this distinction prevents double-counting.

Physical asset accounts for minerals. Asset accounts for mineral and energy resources record the opening stock, discoveries, extractions, reappraisals, and closing stock^[60]. Because these are non-renewable resources with no natural growth, depletion equals extraction. Measurement units vary by resource type (barrels for oil, cubic metres for gas, tonnes for minerals).

For deep-sea minerals in areas beyond national jurisdiction (ABNJ), the regulatory framework under the International Seabed Authority (ISA) continues to develop. The BBNJ Agreement, which entered into force on 17 January 2026, establishes new governance arrangements for biodiversity in ABNJ that may have implications for asset accounting boundaries. Compilers should note that mineral resources in ABNJ are designated as the "common heritage of mankind" under UNCLOS and are administered by the ISA. Asset accounts for ABNJ minerals should reflect the access and benefit-sharing provisions of the applicable regulatory framework. For further guidance on deep-sea and ABNJ accounting, see TG-6.6 Deep-Sea and ABNJ.

Monetary valuation. The NPV approach is typically applied, using the expected resource rent -- the surplus after all extraction costs and normal returns to produced assets have been deducted^[61]. Resource rent can be estimated as a residual from operating surplus or from royalty payments where these approximate market rent. For detailed guidance on resource rent estimation, see TG-1.9 Safe Usage of Monetary Valuation.

3.4.3 Water resources

The SEEA CF defines water resources as "freshwater and brackish water in inland water bodies, including groundwater and soil water"^[62]. Seawater is excluded from the asset boundary because "the stocks are too large to be meaningful for analytical purposes"^[63]. In practice, this means the open ocean is not an asset you track -- but the interfaces between freshwater and marine systems are critically important.

For coastal areas, groundwater in coastal aquifers may be subject to saltwater intrusion, and estuarine systems represent transitional zones where freshwater and marine systems interact. Asset accounts for water resources should capture these coastal dynamics where relevant. The SEEA CF notes that "water is in continuous movement through the processes of precipitation, evaporation, run-off, infiltration and flows to the sea"^[64], and this hydrological connectivity must be considered in coastal accounting. A coastal aquifer losing freshwater to saltwater intrusion is an asset account reduction that directly affects the communities and agriculture that depend on that aquifer.

3.5 Ecosystem Assets

SEEA Ecosystem Accounting extends asset accounting to treat ecosystems as integrated spatial units -- not just the fish or minerals within them, but the whole living system^[65]. An ecosystem asset is defined as "contiguous spaces covered by a specific ecosystem type characterized by a distinct set of biotic and abiotic components and their interactions"^[66]. For ocean accounting, ecosystem assets include coral reefs, mangrove forests, seagrass meadows, kelp forests, coastal wetlands, and the various pelagic and deep-sea ecosystem types.

This ecosystem-level view matters because it captures something that individual asset accounts miss: the interdependence of marine systems. A coral reef is not just a source of fish -- it provides coastal protection, supports tourism, sequesters carbon, and maintains biodiversity. Tracking the reef as a whole reveals whether these bundled services are being sustained or degraded. The relationship between ecosystem assets and the individual environmental assets described in Section 3.4 is addressed in TG-0.2 Overview of Relevant Statistical Standards.

3.5.1 Ecosystem extent accounts

Ecosystem extent is the size of an ecosystem asset, measured in units of area (or, for some marine ecosystems, length or volume)^[67]. Extent accounts answer a fundamental question: how much of each ecosystem type do you have, and is it growing or shrinking?

Structure of extent accounts. The structure follows the standard asset account format^[68]:

Table 2: Structure of ecosystem extent account with illustrative synthetic values in hectares (adapted from SEEA EA Table 4.1)^[69]

Ecosystem conversions. Changes in ecosystem extent are termed ecosystem conversions -- situations where "for a given location, there is a change in ecosystem type involving a distinct and persistent change in ecological structure, composition and function"^[70]. Conversions may be human-induced (such as clearing mangroves for aquaculture ponds) or natural (such as succession of wetland types). Within the extent account, the total area of all ecosystem types should equal the total ecosystem accounting area: what one type loses, another type gains.

Marine ecosystem delineation. For marine ecosystems, the SEEA EA recommends that within the continental shelf, "ecosystem assets be delineated based on the areas of the different ecosystem types associated with the seabed, for example, seagrass meadows, subtidal sandy bottoms and coral reefs"^[71]. This seabed-based approach accommodates the three-dimensional nature of marine ecosystems while providing a practical basis for area measurement. The SEEA EA also 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"^[72]. In practice, this means you map from the bottom up -- the seabed type defines the ecosystem boundary, even though the living system extends upward through the water column.

The IUCN Global Ecosystem Typology (GET) provides the reference classification for marine ecosystem types, with the Marine Shelf biome (M1) encompassing key functional groups including seagrass meadows (M1.1), kelp forests (M1.2), photic coral reefs (M1.3), shellfish beds and reefs (M1.4), and subtidal rocky reefs (M1.6)^[73]. For guidance on mapping marine ecosystem extent using remote sensing and spatial data, see TG-4.1 Remote Sensing and Geospatial Data.

The seabed-based delineation approach presents challenges for pelagic ecosystems that are not clearly associated with specific seabed areas. Epipelagic, mesopelagic, and bathypelagic ecosystem types (IUCN GET M2.1--M2.4) require alternative delineation approaches, potentially using water mass characteristics, depth zones, or biogeochemical provinces as boundaries. These challenges are addressed in TG-6.5 Pelagic and Open Ocean Accounts.

3.5.2 Ecosystem condition accounts

Ecosystem condition refers to the quality of an ecosystem -- not just how much of it exists, but how well it is functioning^[74]. A hectare of degraded coral reef is still a hectare in the extent account, but its capacity to deliver services (coastal protection, fish habitat, tourism value) may be a fraction of a healthy reef's. Condition accounts capture this quality dimension.

Structure of condition accounts. The SEEA EA describes a three-stage measurement approach^[75]:

1. Condition variable accounts -- record raw values of condition characteristics (e.g., live coral cover percentage, species richness count, water temperature)
2. Condition indicator accounts -- transform variables into comparable indicators, typically scaled against a reference condition (e.g., coral cover as a percentage of reference condition)
3. Condition indices -- aggregate indicators into composite measures for communication to policy-makers (optional)

Selection of condition variables. Condition variables should be selected based on their relevance to the ecosystem type, data availability, and connection to ecosystem integrity^[76]. For marine ecosystems, the SEEA EA identifies key drivers of condition including bathymetric profile, climate factors (temperature, acidification), substrate type, ocean circulation, salinity, and human pressures^[77]. The SEEA EA groups condition characteristics into six classes^[78]:

• Physical state (including water temperature, salinity)
• Chemical state (including ocean acidification, nutrient levels, pollutant concentrations)
• Compositional state (including species diversity, community composition)
• Structural state (including habitat complexity, biomass)
• Functional state (including productivity, ecosystem processes)
• Landscape/seascape context (including connectivity, fragmentation)

For guidance on indicators derived from ecosystem condition accounts, see TG-2.1 Aggregate Biophysical Indicators of Environmental State.

Relationship between extent, condition, and service capacity

The capacity of an ecosystem to deliver services depends on both its extent and its condition. Think of it as area times quality: a large, healthy reef delivers more services than a small, degraded one, and both dimensions matter. SEEA EA Figure 6.1 illustrates this relationship: ecosystem extent determines the spatial scale of potential service delivery, while ecosystem condition determines the quality and intensity of service flow per unit area^[79]. Together, they determine the overall capacity to deliver ecosystem services, which is then realised through actual service flows to economic units.

Figure 3.5.1: Relationship between ecosystem extent, condition, and service capacity (adapted from SEEA EA Figure 6.1)^[79:1]

This framework has direct implications for ocean accounting. A decline in coral reef extent (recorded in the extent account) reduces the total area available for service delivery. Simultaneously, degradation in reef condition (recorded in the condition account) reduces the service flow per hectare. Both effects are captured when compiling ecosystem service flow accounts as described in TG-3.2 Flows from Environment to Economy.

Illustrative marine condition variables by ECT class

To support the selection of condition variables for marine ecosystem types, Table 3 maps illustrative variables to the six classes of the SEEA EA Ecosystem Condition Typology (ECT)^[80]. The ECT provides a universal structure for organising condition characteristics across all ecosystem types, grouping them into abiotic, biotic, and landscape-level categories.

Table 3: Illustrative marine condition variables structured by ECT class (based on SEEA EA Table 5.1)^[80:1]

The SEEA EA recommends selecting at least one variable for each of the six ECT classes for each ecosystem type, providing a minimum level of comprehensiveness^[81]. Each variable should be compared against a reference condition representing "good" or "natural" state, enabling transformation into condition indicators that are comparable across ecosystem types and accounting periods. Based on evaluation of existing ecosystem condition accounts, a set of approximately 6 to 10 well-selected indicators for a given ecosystem type should provide sufficient information to assess the overall condition of an ecosystem asset^[82]. For biome-specific indicative variables, including those relevant to marine shelf, pelagic, and deep-sea biomes, compilers should consult the indicative variable sets presented in SEEA EA Table 5.7^[83].

Reference conditions. Condition indicators are typically expressed relative to a reference level representing "good" or "natural" condition^[84]. The choice of reference condition -- historical baseline, minimally impacted area, or policy target -- has significant implications for interpretation and should be clearly documented. A reef measured against a pristine 1950s baseline may show 60% decline; the same reef measured against a 2010 baseline may appear stable. The reference you choose shapes the story your accounts tell. The SEEA EA links condition to the concept of ecosystem integrity: "the ecosystem's capacity to maintain its characteristic composition, structure, functioning and self-organization over time within a natural range of variability"^[85].

3.5.3 Monetary ecosystem asset accounts

Monetary ecosystem asset accounts record the value of ecosystem assets in currency units. The SEEA EA presents two main valuation approaches^[86]:

1. NPV of ecosystem services -- summing the discounted future flows of all ecosystem services supplied by the asset
2. Restoration cost -- the cost of restoring an ecosystem to a reference condition

The NPV approach aligns with valuation methods for other natural assets but requires monetary valuation of ecosystem services, which remains methodologically challenging, particularly for non-market services like coastal protection and biodiversity maintenance. The restoration cost approach provides an alternative that does not require service valuation but may not reflect the full economic value of the asset -- a coral reef may cost a certain amount to restore, but its actual value to the economy and society may be far higher. For detailed guidance on valuation of ecosystem services, see TG-1.9 Safe Usage of Monetary Valuation and TG-2.4 Ecosystem Goods and Services.

Structure of monetary ecosystem asset accounts

The SEEA EA monetary ecosystem asset account (Chapter 10) records the monetary values of all ecosystem assets within an ecosystem accounting area at the beginning and end of each accounting period, together with the changes in those values^[87]. Table 4 presents the standard structure.

Table 4: Structure of monetary ecosystem asset account with illustrative synthetic values in thousand currency units (adapted from SEEA EA Table 10.1)^[87:1]

The accounting entries distinguish five broad types of change in the monetary value of ecosystem assets^[88]. Understanding what each one captures helps you interpret your accounts correctly:

• Ecosystem enhancement records increases in monetary value associated with improving condition during the accounting period. This includes the effects of active restoration, reductions in harmful activities that allow recovery, and natural improvements in condition. Enhancement is the monetary good news -- it tells you that investment in ecosystem health is generating economic returns.
• Ecosystem degradation records decreases in monetary value associated with declining condition. Degradation plays an analogous role to depletion for individual natural resources -- it represents a cost that should be deducted in calculating adjusted income measures. If your degradation entries are consistently large, your economy is drawing down ecosystem capital.
• Ecosystem conversions record additions and reductions resulting from changes in ecosystem type at a given location. When mangroves are cleared for aquaculture, a decrease in value is recorded for mangroves and an increase for the new aquaculture pond ecosystem. There is no expectation that these will be offsetting in monetary terms -- mangroves typically provide far more total ecosystem services than the aquaculture ponds that replace them.
• Other changes in volume capture two categories: catastrophic losses, which are decreases in value due to large-scale, discrete events that cause significant loss in ecosystem condition (a mass bleaching event, an oil spill); and reappraisals, which record changes in value due to updated information about expected future demand for ecosystem services or changed expectations about future condition (SEEA EA para 10.38)^[89].
• Revaluation records changes in monetary value due solely to movements in the unit prices of ecosystem services, distinct from any physical changes in the asset. Following the SEEA CF convention, changes in value resulting from changes in the quantity or quality of future flows of ecosystem services are not revaluations and should be recorded under the appropriate entry above (SEEA EA para 10.41)^[90].

The distinction between reappraisals and revaluation is essential for interpreting changes in ecosystem asset values. Reappraisals concern changes in expectations -- for example, revised demographic projections that alter the expected future demand for ecosystem services, or rezoning decisions that change the expected pattern of ecosystem use. Revaluations, by contrast, concern changes in unit prices only and are conceptually equivalent to the holding gains and losses recorded for other asset types in the SNA.

For marine contexts, monetary values of ecosystem assets can be estimated using the NPV of expected future ecosystem service flows. For market services such as fish provisioning and blue carbon sequestration, resource rent methods may be applied. For non-market services such as coastal protection, water purification, and recreational amenity, replacement cost or avoided damage cost methods from the preference hierarchy in TG-1.9 Safe Usage of Monetary Valuation should be used.

It should be noted that the UN Statistical Commission, in adopting the SEEA EA in 2021, identified "outstanding methodological concerns related to chapters 8 to 11 on valuation" (SEEA EA Preface para 8). Accordingly, the valuation chapters have the status of "internationally recognised statistical principles and recommendations" rather than a full international statistical standard. Compilers should be aware that monetary ecosystem asset valuation methods continue to evolve, and should document the methods and assumptions used in their valuations transparently.

Degradation. In SEEA EA, ecosystem degradation represents the decline in condition multiplied by the associated loss of future ecosystem service flows, valued in monetary terms^[91]. It is the ecosystem equivalent of depletion: the cost of using an ecosystem harder than it can sustain. The SEEA EA states that the approach "involves measuring the value of degradation in terms of loss in future value of ecosystem services due to a decline in ecosystem condition"^[92]. For a policy-maker, degradation is the number to watch -- it tells you what ecosystem decline is costing your economy.

3.6 Produced Assets in the Ocean Domain

While this Circular focuses primarily on environmental (non-produced) assets, ocean accounts must also include produced assets -- the infrastructure, vessels, and equipment that humans have built for ocean-based activities. Produced assets are "assets that have come into existence as outputs of production processes"^[93]. Including them alongside environmental assets provides a complete picture of total ocean wealth and reveals important interdependencies.

Types of produced assets in the ocean domain

Key categories include:

Aquaculture fixed assets -- cages, pens, nets, and other infrastructure used for raising cultivated aquatic resources, as well as breeding stocks held as fixed assets^[94]. These are produced assets resulting from deliberate investment decisions. For detailed guidance, see TG-3.9 Aquaculture Accounts.

Offshore energy infrastructure -- platforms, drilling equipment, pipelines, subsea facilities for oil and gas extraction, as well as wind turbines, wave energy converters, and tidal energy devices for marine renewable energy. For detailed guidance, see TG-3.10 Offshore Energy Accounts.

Port infrastructure -- wharves, jetties, breakwaters, dredged channels, container terminals, and other facilities for maritime transport and trade.

Coastal protection structures -- sea walls, groynes, revetments, and other engineered structures designed to protect coastal areas from erosion and flooding. These can be considered alongside the natural coastal protection services provided by ecosystems such as mangroves and coral reefs^[95] -- a comparison that often reveals that natural defences are more cost-effective than engineered ones.

Vessels -- fishing vessels, cargo ships, cruise ships, offshore service vessels, and other maritime transport equipment.

Accounting treatment

Produced assets are accounted for following standard SNA methodology, with opening stocks, gross fixed capital formation (investment), consumption of fixed capital (depreciation), and closing stocks recorded in physical and monetary terms^[96].

For ocean accounting, the important insight is how produced assets interact with environmental assets:

• Aquaculture investment creates produced assets (infrastructure) that depend on environmental assets (water quality, marine ecosystems providing supporting services). If the environmental asset degrades, the produced asset becomes less productive.
• Offshore oil and gas investment creates produced assets that enable extraction of environmental assets (mineral resources). The produced asset depreciates through use, while the environmental asset depletes through extraction.
• Coastal infrastructure may protect inland assets but may also affect coastal ecosystem extent and condition -- a sea wall can prevent erosion of property while simultaneously destroying the beach ecosystem in front of it.

These connections should be reflected in integrated accounts that present produced and environmental assets together, enabling analysis of total ocean wealth and its composition. Combined presentations of environmental and economic accounts are addressed in TG-3.8 Combined Presentations.

3.7 Worked Example: Coastal Ecosystem Asset Accounts

This section demonstrates how to compile physical and monetary asset accounts for a hypothetical coastal area. The example uses synthetic data for three marine ecosystem types (mangroves, seagrass meadows, and continental shelf waters) and one individual environmental asset (fish stocks), following the compilation procedure from Section 3.2 and the account structures from Sections 3.4 and 3.5. Working through this example will show you how the pieces fit together -- and how physical declines can be masked by monetary gains.

Scenario description

The accounting area is a coastal zone extending 12 nautical miles offshore and encompassing 500 km² of mangrove forest, 200 km² of seagrass meadows, and 15,000 km² of continental shelf marine waters. The area supports commercial fisheries targeting demersal fish species that depend on mangroves and seagrass as nursery habitat. The accounting period is calendar year 2025.

Step 1: Ecosystem extent accounts

Extent data were compiled from Sentinel-2 satellite imagery processed using supervised classification algorithms, validated by field surveys at 150 ground-truth sites (see TG-4.1 Remote Sensing and Geospatial Data). Change detection analysis identified conversions between ecosystem types and losses to non-natural land cover.

Physical extent account (area in km²)

Table 5: Physical ecosystem extent account for coastal zone, 2025

What does this tell you? The coastal zone lost 3.2 km² of mangroves and 0.8 km² of seagrass during 2025. Managed reductions (conversion to aquaculture ponds) exceeded managed expansion (restoration projects) -- meaning that deliberate human decisions drove more loss than restoration could offset. Natural reductions from storm damage and coastal erosion added to the deficit. Continental shelf waters showed no change in extent because the accounting area boundary remained fixed.

Step 2: Ecosystem condition accounts

Condition data were compiled from 45 monitoring stations distributed across the three ecosystem types, measuring variables in each of the six ECT classes. Measurements were taken quarterly and averaged for the year.

Condition variable account for mangroves (selected variables)

Table 6: Ecosystem condition variables for mangroves, 2025

What does this tell you? Every condition variable is below its reference value, and most are moving in the wrong direction. Canopy density dropped from 72% to 70% (reference: 85%). Connectivity -- how well mangrove patches are linked to each other -- fell from 0.68 to 0.65 (reference: 0.80), likely because the conversion of mangrove patches to aquaculture created gaps in the landscape. Leaf litter production, a proxy for ecosystem productivity, declined from 480 to 465 g/m²/yr. Together, these numbers tell a story of an ecosystem under pressure from both direct conversion and degradation of remaining areas.

Step 3: Fish stock asset account

Fish stock data were compiled from stock assessment models for the dominant demersal species complex, using data from commercial catch statistics, fishery-independent trawl surveys, and length-frequency analysis. The stock assessment estimated total biomass, natural mortality, recruitment, and fishing mortality using age-structured models.

Physical asset account for fish stocks (biomass in tonnes)

Table 7: Physical asset account for demersal fish stock, 2025

What does this tell you? The fish stock declined by 4,000 tonnes (9.5%) during 2025. Gross catch of 7,200 tonnes exceeded the sustainable yield estimate of 6,500 tonnes by 700 tonnes -- that 700 tonnes is depletion, the amount by which the fishery is drawing down its capital base. The stock also experienced a catastrophic loss of 500 tonnes from a hypoxia event in coastal waters during summer, potentially linked to the nutrient loading and habitat fragmentation visible in the mangrove condition data. The combination of overfishing and environmental stress drove the decline.

Step 4: Monetary valuation

Monetary values were estimated for ecosystem assets using the NPV of expected future ecosystem service flows, and for fish stocks using the NPV of expected future resource rents.

Mangrove ecosystem services valuation (values expressed per km² for this illustrative example):

• Coastal protection service: Replacement cost method based on equivalent seawall construction, estimated at USD 12,000 per km² per year
• Carbon sequestration service: Social cost of carbon (USD 50 per tonne CO₂) applied to sequestration rate, USD 400 per km² per year
• Fish nursery service: Productivity change method estimating contribution to commercial fisheries, USD 800 per km² per year
• Total annual ecosystem service value: USD 13,200 per km² per year
• Discount rate: 4% real
• Asset value (perpetual annuity): USD 13,200 / 0.04 = USD 330,000 per km² (equivalent to USD 3,300 per hectare)

Seagrass ecosystem services valuation (values expressed per km²):

• Carbon sequestration: USD 300/km²/yr
• Water filtration: Avoided treatment cost USD 150/km²/yr
• Fish nursery: USD 600/km²/yr
• Total annual service value: USD 1,050/km²/yr
• Asset value (4% discount): USD 1,050 / 0.04 = USD 26,250 per km² (equivalent to USD 262.5 per hectare)

Fish stock valuation:

• Resource rent per tonne caught: USD 450 (gross value of landings USD 1,200/tonne, less all costs)
• Sustainable yield: 6,500 tonnes/yr
• Annual sustainable resource rent: 6,500 x USD 450 = USD 2,925,000
• Asset value (4% discount, perpetual): USD 2,925,000 / 0.04 = USD 73,125,000

Monetary ecosystem asset account (values in USD thousand)

Table 8: Monetary ecosystem asset account, 2025

Fish stock monetary asset account (values in USD thousand)

Table 9: Monetary fish stock asset account, 2025

What does this tell you -- and what might it hide? Despite physical decline in ecosystem extent and fish stock biomass, monetary values increased slightly due to revaluation (holding gains from price increases). For mangroves, the 5% increase in unit service values (driven by updated coastal protection valuations following storm damage in a neighbouring region) generated USD 8.25 million in holding gains, offsetting degradation and conversion losses. For fish stocks, a 10% increase in market prices generated USD 7.3 million in holding gains, offsetting the value of depletion.

This result illustrates a critical lesson: price increases can mask physical depletion in monetary accounts. If you looked only at closing values, you might conclude that ocean wealth is stable or growing. The physical accounts tell a different story -- declining fish stocks, shrinking mangroves, degrading condition. Adjusted income measures should deduct depletion at constant prices to avoid the misleading signal that rising resource prices can compensate for unsustainable extraction. The SEEA CF emphasizes that "depletion should be valued at the average of opening and closing prices" to provide a neutral mid-period valuation^[36:1], and changes in asset value due to price effects should be reported separately as revaluations rather than real income.

Step 5: Integration and policy implications

The worked example demonstrates several key linkages in the ocean accounting system:

Upward linkages to policy (TG-1.x and TG-2.x):

• TG-1.1 National Ocean Budgets: The USD 1,755 thousand in ecosystem degradation plus USD 315 thousand in fish stock depletion (700 tonnes above sustainable yield at USD 450/tonne resource rent; total USD 2,070 thousand at constant prices) represents the natural capital cost of economic activity during 2025. This cost should be deducted from gross income measures to calculate environmentally adjusted net domestic product.
• TG-2.1 Aggregate Biophysical Indicators of Environmental State: The 0.64% decline in ecosystem extent (4.0 km² loss from 15,700 km² total) and 9.5% decline in fish stock biomass provide headline indicators of environmental state for policy monitoring.
• TG-2.4 Ecosystem Goods and Services: The estimated ecosystem service flows (coastal protection, carbon sequestration, fish nursery) quantified in the valuation step provide inputs to ecosystem service accounts.

Downward linkages to data (TG-4.x):

• TG-4.1 Remote Sensing and Geospatial Data: Sentinel-2 imagery provided the primary data source for extent change detection, demonstrating the operational feasibility of satellite-based ecosystem monitoring.
• TG-4.2 Survey Methods for Ocean Economic Activity: Field monitoring at 45 stations provided condition variable measurements, highlighting the continuing need for in-situ data to complement remote sensing.
• TG-4.3 Administrative Data Sources: Integration of extent, condition, and stock assessment data within a common spatial framework enabled consistent attribution of ecosystem services to ecosystem assets and analysis of nursery habitat contributions to fisheries.

Cross-account consistency:

• Fish stock depletion of 700 tonnes corresponds to extraction recorded in physical supply-use tables (see TG-3.2 Flows from Environment to Economy)
• Ecosystem extent changes are reconciled with ecosystem conversion matrices
• Monetary degradation and depletion entries correspond to physical condition decline and stock reduction multiplied by appropriate unit values

This worked example illustrates how asset accounts provide a structured framework for organizing diverse data sources, maintaining accounting identities that ensure internal consistency, and deriving policy-relevant measures of sustainability. The procedure can be scaled to national accounting areas and extended to additional asset types following the same logical structure.

Implementation Considerations

For minimum institutional capacity, data infrastructure, and human skills requirements for compiling these accounts, see TG-0.8 Implementation Readiness Assessment. For guidance on adapting these methods to sub-national scales, see TG-3.11 Sub-National Ocean Accounts.

4. Acknowledgements

Authors: [To be confirmed]

Reviewers: [To be confirmed]