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Closing critical gaps in global fisheries oversight requires standardized identification
Key takeaways from article
- Permanent IMO numbers help combat illegal fishing by allowing authorities to track vessels despite changes in name, flag or ownership.
- Standardized vessel identification strengthens fisheries transparency by linking registries, satellite tracking and enforcement systems across jurisdictions.
- Wider adoption of IMO numbers can improve vessel tracking, seafood traceability and global fisheries governance while closing key oversight gaps.
Overview
Every year, vessels engaged in illegal, unreported and unregulated (IUU) fishing evade detection by changing their name, flag or registration. Without a reliable way to identify a vessel across jurisdictions and over time, these actors operate with near impunity — undermining fisheries governance, threatening food security and distorting the data on which fisheries authorities depend.
Established in 1987 to enhance maritime safety, reduce fraud and prevent maritime pollution, the International Maritime Organization (IMO) ship identification number scheme provides eligible vessels with permanent, unique identifiers called IMO numbers. While originally limited to passenger and cargo ships, the scheme has since expanded to include the fishing sector, becoming a critical tool for transparency — the act of making specific ocean and vessel data, as well as the policies and decisions that surround them, available to those who need them. By establishing a permanent identifier that acts as a vessel’s “DNA”, IMO numbers are fostering greater accountability by enabling authorities to track vessels across changes in name, flag, ownership and registration.
They also play a crucial role in the seafood supply chain by enhancing traceability and providing a means to verify information. This consistency allows for the seamless tracking of vessels across different regulatory environments over extended periods of time — a requirement for effective monitoring and surveillance of fishing activity.
At Global Fishing Watch, the IMO number is the most valued way to identify vessels. They not only improve the accuracy of the online platform for visualization and analysis of human activity at sea, but they also help transform raw data into insights that bolster governance decisions.
What are IMO numbers?
IMO numbers are a standardized method for identifying ships worldwide. Each vessel is assigned a unique permanent seven-digit number preceded by the letters “IMO,” which remains unchanged throughout a vessel’s lifetime regardless of ownership transfers, name changes or changes in flag. Issued and managed by S&P Global Market Intelligence (S&P Global), IMO numbers serve as unique vessel identifiers (UVIs), providing vessels with an identity that supports data sharing and enforcement. These numbers are provided free of charge to encourage universal adoption and ensure monitoring and control across the maritime sector.
Figure 1. Evolution of the IMO Number
The IMO number scheme now applies to all fishing vessels 100 gross tonnage (GT) and above operating anywhere in the world, as well as vessels less than 100 GT and over 12 meters in length operating outside their flag State waters. For vessels below those thresholds operating only within national waters, governments are encouraged to uphold robust national UVI systems.
Maintaining a comprehensive database which holds essential details about each vessel — including construction information, registration, ownership and status — S&P Global assigns IMO numbers to all ships of applicable size, with the exception of the following.
- Ships without mechanical means of propulsion, floating docks and other similar structures
- Pleasure yachts
- Ships engaged in special services such as lightships or search and rescue vessels
- Hopper barges
- Hydrofoils and air cushion vehicles
- Warships and troop transports
- Wooden ships
The IMO number is one type of UVI, but it stands apart because it was designed to be both unique and permanent. Unlike a vessel’s call sign or maritime mobile service identity (MMSI) number, which may change over time, an IMO number stays with the vessel until scrapping. That permanence makes IMO numbers especially valuable for transparency and traceability in fisheries, where vessel operators may otherwise obscure a vessel’s identity through administrative changes. Acting as essential components for linking vessel identities, these numbers allow satellite tracking data to be linked to a verifiable identity and create a continuous record of a vessel’s history, ownership and behavior across different jurisdictions.
Table 1.Vessel Identification Schemes
| Identification scheme | Scope | Lifetime |
|---|---|---|
| IMO number | Cargo and passenger ships as well as eligible fishing vessels | Permanent and unique: It remains with a vessel for its lifetime. |
| Hull identification number | Recreational vessels and small fishing vessels under 24 meters in length | Not permanent: It may be changed following major reconstructions, to correct formatting errors and/or, to meet modernization standards, or in rare cases of ownership transfer. |
| Tuna unique vessel identifier | Tuna fishing vessels | Not permanent: It may be reassigned if a vessel is significantly reconstructed or rebuilt altogether. |
| National registration number | Vessels registered locally or nationally | Not permanent: It may be updated following changes in ownership, jurisdictional registration or legal registration status. |
| Maritime mobile service identity | Vessels using radio or communication systems | Not permanent: It may be reassigned following reflagging or changes in radio licensing and telecommunications administration. |
| International radio call sign | All commercial and government ships, vessels equipped with high frequency or satellite gear and private craft intending to transit international waters | Not permanent: It may be changed following reflagging, changes in radio licensing authority or other registration-related administrative updates. |
International and regional requirements for IMO numbers
The Global Record
A vessel must have an IMO number in the UN Food and Agriculture Organization’s (FAO) Global Record of Fishing Vessels, Refrigerated Transport Vessels and Supply Vessels. The Global Record uses this number as the primary way to link vessel data across multiple information systems. In 2014, the FAO Committee on Fisheries adopted the IMO number as the UVI for the first phase of implementation of the Global Record. An FAO-commissioned technical analysis on unique vessel identifiers paved the way for this decision, concluding that the IMO number was the most appropriate UVI for the initial phase, which targeted vessels 100 GT and above. The study also noted that the system could be effectively extended to vessels under 100 GT due to its practical data requirements, viable management structure and cost considerations.
The IMO number scheme was found to offer the highest system integrity, lowest duplication risk and greatest compatibility with existing maritime vessel systems, all while ensuring a rapid launch at the lowest probable cost. A survey of 22 countries revealed that most national registers could already meet the data requirements for an IMO-based UVI scheme. Furthermore, because many nations already comply with the vessel information requirements of tuna regional fisheries management organizations (RFMOs), the study concluded that using IMO numbers would not require extensive additional effort.
Although the IMO number is widely recognized as the preferred UVI, debate continues over whether other UVIs could satisfy this requirement in certain contexts such as the FAO Global Record. A March 2025 Global Fishing Watch analysis of the data feeding into the Global Record found no duplications between IMO numbers and other UVIs across 13,840 identifiers. This indicates that both identifiers can coexist within the registry without conflict.
However, as Global Fishing Watch works to improve vessel transparency and data sharing, it has noted that some countries have not uploaded their registries to the Global Record because their vessels lack IMO numbers. While the application process is free and relatively simple, vessel owners often face administrative costs to gather the required documentation. Additionally, national registries may need to adjust their own information standards to qualify for IMO number allocation.
Regional fisheries management organizations and agreements
RFMOs increasingly encourage or require vessels operating in their convention areas to have IMO numbers. These identifiers improve compliance monitoring and allow RFMOs to track vessels seamlessly across jurisdictions. Several RFMOs already mandate IMO numbers for authorized fishing vessels above specific size thresholds.
Table 2. Requirements for IMO numbers across RFMOs
| RFMO | Regulation | Size requirements for IMO number | Rule |
|---|---|---|---|
| International Commission for the Conservation of Atlantic Tunas (ICCAT) | Amended Recommendation 13-13 on the ICCAT record of vessels | ≥ 20 meters | Conditional Large-scale fishing vessels must obtain an IMO or Lloyd’s Register number. |
| South Pacific Regional Fisheries Management Organisation | CMM 05-2022 – Conservation and Management Measure on the Commission Record of Vessels Authorized to Fish | ≥ 12 meters | Mandatory |
| General Fisheries Commission for the Mediterranean | GFCM/48/2025/8 – Recommendation on the Application of IMO Numbers | ≥ 20 meters | Mandatory |
| Indian Ocean Tuna Commission (IOTC) | Resolution 19/04 on the Record of Vessels Authorized to Operate in the IOTC Area | ≥ 12 meters | Mandatory |
| Inter-American Tropical Tuna Commission | Amended Resolution C-18-06 on the Regional Vessel Register | ≥ 12 meters | Mandatory, excluding recreational fishing vessels |
| Western and Central Pacific Fisheries Commission | CMM 2018-06 - Conservation and Management Measure on the Record of Fishing Vessels and Authorization to Fish. | ≥ 12 meters | Mandatory |
While national UVIs work well for individual governments tracking vessels within their own waters, they complicate data exchange during international cooperation or when submitting records to regional organizations. For vessels operating outside national jurisdictions, a standardized identification scheme is essential. All parties must be able to understand the identifier instantly, without needing to decode it or rely on supplementary information from individual flag States.
Wider adoption of IMO numbers across RFMOs provides this standardization. Expanding their use will strengthen monitoring, control and surveillance, improve enforcement of conservation measures, and support consistent assessments of fishing effort and vessel compliance. Such a unified system can help drive greater transparency and accountability in global fisheries governance.
How Global Fishing Watch works with IMO numbers
Global Fishing Watch is dedicated to advancing ocean governance through open data and analysis that reveal patterns of human activity at sea. At the core of this work is the ability to identify vessels consistently across multiple datasets. IMO numbers serve as the vital bridge between official registry data and behavioral insights from satellite tracking technologies such as automatic identification systems (AIS) and the vessel monitoring systems (VMS). Consequently, IMO numbers are integrated throughout Global Fishing Watch’s platform and analytical tools, supporting work to:
Strengthen vessel monitoring and identification
By linking IMO numbers to AIS data and registries, Global Fishing Watch can reliably distinguish individual vessels over time. This cross-referencing improves identity matching and the accuracy of long-term analysis of vessel activity and operations.
Understand vessel history and activity
Governments, RFMOs, civil society and other stakeholders can use IMO numbers to determine a vessel’s ownership, registration and compliance history. This level of transparency is essential for verifying legal status, identifying risks and supporting monitoring, enforcement or legal proceedings.
Distinguish between legal and potentially illegal activities
When paired with tracking and authorization data, IMO numbers support the analysis of vessel behavior and fishing activity. They help distinguish between authorized and potentially unauthorized vessels that appear across multiple registries or those exhibiting patterns that warrant further investigation. Monitoring these patterns in near-real-time enables timely interventions and more informed decision making.
Streamline investigations and accountability
When suspicious activity is detected, IMO numbers allow authorities to cross-reference vessels across registries, licensing systems, port controls and enforcement records. This reduces the chances of vessels operating under false pretenses, enhances international cooperation and improves the ability to assign responsibility.
Reveal hidden identities
Vessels engaged in IUU fishing often change names, flags or registrations to evade detection. Because the IMO number remains permanent, it can be used to reveal links between seemingly unrelated vessels and expose vessels attempting to conceal their histories. Using IMO numbers, Global Fishing Watch can detect vessels that attempt to mask their identity or operate in multiple jurisdictions illicitly. This helps build comprehensive profiles of suspect vessels and coordinate multiagency actions.
Improve data sharing and interoperability
IMO numbers make it easier to integrate vessel information across national, regional and international registries and systems. They reduce confusion caused by duplicate vessel names or changing administrative details. Global Fishing Watch is developing solutions to strengthen vessel data systems, enhance national registries and support more accurate, standardized vessel data submissions from States to RFMOS and the FAO Global Record.
Challenges and opportunities in the use of IMO numbers for vessel identification
Challenges
-
Limitations in data coverage and adoption
Not all eligible fishing vessels have IMO numbers, and only some national systems consistently use them. These gaps reduce transparency, limit data alignment across systems and create blind spots in vessel monitoring and enforcement. -
Resource and capacity constraints
Many countries lack the regulatory frameworks or administrative capacity to manage IMO-based registries effectively. In these cases, technical hurdles and registry management must be addressed before universal adoption is possible. -
Vessel size and applicability issues
Small-scale and artisanal vessels often fall outside current IMO technical or regulatory requirements for inclusion in the numbering scheme. Furthermore, some authorities view IMO numbers as redundant, preferring to rely on domestic license numbers as a form of identification and tracking.
Opportunities
-
Enhanced data integration and transparency
Wider use of IMO numbers can improve data sharing between national authorities, RFMOs and international platforms. This interoperability reduces duplication, enables real-time tracking and enforcement, and fosters a more transparent, cooperative and efficient international governance system. -
Stronger support for governments
Integrating IMO numbers into national registries improves domestic tracking abilities and risk analysis while aligning countries with FAO and RFMO requirements. Global Fishing Watch can help countries by facilitating connections with S&P Global and supporting the organization of vessel data for official review. -
Strengthened RFMO Engagement
Because RFMOs rely on accurate Member State submissions, IMO numbers allow for the reliable matching of public registries with AIS tracking data and RFMO records. This improves the detection of unauthorized operations, supports compliance analysis and simplifies the cross-referencing of vessels on IUU lists and activity reports.
Key recommendations for governments
To advance transparency, vessel tracking and international fisheries governance, Global Fishing Watch calls on governments to take the following actions:
-
Mandate IMO numbers for eligible vessels:
Require IMO numbers for all fishing vessels 100 GT and over, as well as any vessel over 12 meters in length that operates outside their national jurisdiction -
Audit and update national registries:
Identify eligible vessels that lack IMO numbers and fast-track their registration. Governments should either instruct vessel owners to obtain them or submit bulk registration requests to S&P Global on behalf of the fleet. -
Leverage technical assistance:
Seek support from Global Fishing Watch to prepare vessel data in advance of the application process and facilitate communications with S&P Global to ensure that all IMO number requests meet the necessary requirements. -
Implement permanent national UVIs where IMO numbers do not apply:
For vessels not eligible for IMO numbers — particularly small-scale and domestically operating fleets — governments should assign a unique, hull-linked national identifier to all vessels operating exclusively within domestic waters. This UVI must be permanent, persisting even if the vessel undergoes modifications to ownership, flag, authorization or physical characteristics. -
Integrate identifiers across all tracking and documentation:
Include the relevant unique identifier — whether an IMO or national UVI — into all positional messages, such as AIS and/or VMS, and mandate its inclusion on all authorization, registration, catch and landing documents.
Conclusion
Reliable vessel identification is the foundation for transparent and accountable fisheries governance. As a globally recognized, permanent and unique identifier, the IMO number ensures vessels can be tracked consistently across jurisdictions, databases and time.
For Global Fishing Watch, IMO numbers are a critical element in linking vessel identity data with activity at sea, strengthening risk analysis and supporting efforts to combat illegal fishing. Expanding their adoption is not a simple technical improvement; it is a governance imperative that will strengthen transparency, enhance accountability and data interoperability, and equip governments to manage their fishing fleets more effectively, closing critical gaps in global fisheries oversight.
Global Fishing Watch stands ready to support governments in integrating IMO numbers into their national vessel registries, driving a new standard of transparency across the world’s ocean.