At-sea transshipment of fishery catches: a systematic literature review

At-sea transshipment is the process of transferring cargo, fuel, crew, and supplies between vessels at sea. Fishery catch transshipment specifically refers to the transfer of fish catches from fishing vessels to refrigerated cargo carriers (reefers) operating beyond national jurisdictions. By enabling fishing vessels to remain at sea for up to 13.3 months, compared with approximately 3 months for non-transshipping vessels, the practice enhances operational efficiency for distant-water fleets while simultaneously functioning as the primary logistical channel for illegal, unreported, and unregulated (IUU) fishing, catch laundering, labor abuses, and the depletion of global marine stocks; IUU losses linked to these activities are estimated at USD 10-23.5 billion annually. A systematic literature review was conducted on 35 peer-reviewed articles published between 2011 and 2025. A multi-database search of Scopus, Web of Science, and Springer Nature returned 2,826 initial records, which were filtered, deduplicated, and screened in full text to yield the final sample. Articles were coded across five thematic categories: monitoring and technology, policy and governance, geopolitical and international relations, economic and social aspects, and environmental sustainability. Research interest surged after 2018, with peak output in 2024. Quantitative methodologies dominated (65.7%), followed by mixed-methods and qualitative approaches (17.1% each). Machine learning and satellite-based tracking applied to AIS data are the most common detection tools, but only 50%–70% of events are captured because of AIS disabling, signal manipulation, and dark-vessel activity. Activities concentrate in the Northwest Pacific, West Africa, and the Indian Ocean, frequently involving vessels registered under flags of convenience. Comparative analysis of 17 regional fisheries management organizations showed that only three mandate full observer coverage. A persistent misalignment between surveillance capabilities and regulatory frameworks remains, sustained by inconsistent reporting and weak enforcement. Three priority gaps emerge: the spatial relationship between transshipment hotspots and ecologically sensitive zones is unexamined; no study has linked vessel registries with port inspection databases into an integrated compliance system; and interdisciplinary research tracing the full chain from at-sea transfer through catch laundering to market entry and labor conditions is absent. Mandatory electronic monitoring, standardized data sharing, and increased transparency in vessel ownership are needed to transform transshipment from a regulatory blind spot into a leverage point for sustainable ocean governance.

Importance of fish for global food security

Marine foods contribute 15%−15.3% of the global animal protein supply, and provide more than 20% of the animal protein intake for 3.2 billion people. They contain essential nutrients indispensable to human health (Boyd et al., 2022; FAO, 2024). However, production is increasingly threatened by factors such as overfishing, climate change, and pollution (Cabral et al., 2018; Sharma et al., 2025; Wagner et al., 2021). Global production reached 223.2 million tons in 2022, which was split almost equally between aquaculture and capture fisheries (FAO, 2024). Asia is the largest producer and consumer, accounting for 71% of the world's consumption (FAO, 2024). Consumption rose from 9.1 kg/cap in 1961 to 20.7 kg/cap in 2022, but is still low among low-income groups (FAO, 2024). In 2022, the fisheries sector employed 61.8 million people and generated USD 452 billion (FAO, 2024). To meet the projected global population of 9.7 billion people by 2050, an increase of 22% will be required, which will only be achievable through an increase in aquaculture because capture fisheries globally will continue to face sustainability challenges (Boyd et al., 2022; FAO, 2024).

Marine fishery stock status and unsustainable levels

State of marine resources

The (Sharma et al. 2025) assessment of 2,570 marine stocks, representing 74.2% of global landings, found that 77.2% of production by volume came from biologically sustainable stocks, although only 64.5% were sustainable by count. Overfished stocks comprised 35.5% in 2021, an increase from 10% in the 1970s, reflecting a persistent decline in global sustainability and divergence from the Sustainable Development Goal (SDG) Target 14.4, which aimed to end overfishing by 2020 (Chaisse et al., 2024; Sharma et al., 2025; Pham et al., 2023). Recovery remains limited, with some studies indicating that 48% of stocks are still overfished and 40% face ongoing overexploitation (Britten et al., 2021; Elleby et al., 2025). Several regions face critical overfishing rates, including the Mediterranean and Black Sea (64.9%), Southeast Pacific (53.6%), and Eastern Central Atlantic (52.6%) (Sharma et al., 2025). High fishing pressure is also evident in the Western Central Pacific (47.1%) and Northwest Pacific (37%), particularly in Chinese waters, where only 20% of stocks are healthy (Sharma et al., 2025; Zhai et al., 2020). Moderate but impactful overfishing affects the Western Indian Ocean, Western Central Atlantic, and Eastern Central Pacific (30%−38%), targeting shrimp, lobsters, and reef fishes (Sharma et al., 2025). Highly migratory and deep-sea species are under severe pressure, with 43.5% of shark and 13% of tuna stocks overfished, and only 29% of deep-sea stocks classified as sustainable, reflecting weak governance in the high seas (Sharma et al., 2025).

Overfishing: definition, causes, and impacts

Overfishing refers to the action of harvesting fish at a rate that reduces biomass below levels capable of producing the maximum sustainable yield (MSY), leading to stock depletion, food web disruption, and habitat degradation (FAO, 2024; Issifu et al., 2022; Jackson et al., 2001; Pham et al., 2023; Sumaila et al., 2020; Vidal-León, 2024). A stock is considered “overfished” when its biomass falls below safe biological limits, often defined as less than 80% of the biomass that can produce the MSY (Elleby et al., 2025; FAO, 2024; Vidal-León, 2024). The act of overfishing and the state of being overfished are not synonymous; a healthy stock may be overfished, while a depleted one may be fished at sustainable levels to allow recovery (Vidal-León, 2024).

Overfishing manifests in four main forms (Cánovas-Molina et al., 2021; Gough et al., 2020; Link and Watson, 2019; Rahangdale et al., 2025; Tidd et al., 2025): Growth overfishing: harvesting fish before they reach the optimal size, resulting in the removal of juveniles; Recruitment overfishing: depleting adult populations to the point that they cannot sustain reproduction; Economic overfishing: fishing mortality exceeding maximum economic yield thresholds, reducing profitability; Ecosystem overfishing: excessive fishing effort preventing marine ecosystems from sustaining fished communities, fundamentally disrupting food webs through the systematic depletion of key species.

The impacts of overfishing are biological, ecological, and economic. When extraction rates exceed the natural reproductive capacity of fish populations, mortality surpasses the MSY, leading to stock depletion (Cánovas-Molina et al., 2021; Sathianandan et al., 2021; Sumaila and Tai, 2020). The removal of juvenile and immature individuals further compromises reproductive output, driving growth and recruitment overfishing (Gebremedhin et al., 2021; González-Mon et al., 2021; Gough et al., 2020; Kalhoro et al., 2025; Rahangdale et al., 2025). Species with low reproductive rates and long lifespans are particularly vulnerable (Dulvy et al., 2021; Huynh et al., 2025; Rahangdale et al., 2025). Continued fishing pressure degrades stock structure, reducing age, size, and genetic diversity, and diminishes population resilience (Issifu et al., 2022). The depletion of larger predators disrupts food web dynamics and weakens ecosystem stability (Issifu et al., 2022; Pauly et al., 2005; Pham et al., 2023; Sumaila and Tai, 2020). Overfished systems are less capable of adapting to additional stressors such as climate change and ocean warming (Issifu et al., 2022; Sumaila and Tai, 2020). Although some depleted stocks have shown signs of recovery, many remain below sustainable levels, highlighting the long-term consequences of overexploitation (Hilborn et al., 2020). Key drivers of overfishing include high market demand, poverty-driven fishing pressure, and international competition (FAO, 2024; Pham et al., 2023; Prellezo et al., 2024). Contributing factors include fleet overcapacity, harmful subsidies, destructive gear types, and governance failures (Chaisse et al., 2024; Hilborn et al., 2020; Tidd et al., 2025; Vidal-León, 2024). Weak enforcement, politically driven quotas, and limited stock assessment, especially in developing countries, compound overexploitation risks (Baloch et al., 2025; FAO, 2024; Froese et al., 2021; Gebremedhin et al., 2021; Hilborn et al., 2020; Prellezo et al., 2024; Tidd et al., 2025). Illegal, unreported, and unregulated (IUU) fishing further exacerbates unsustainable practices by bypassing controls and weakening accountability within global seafood supply chains (Chaisse et al., 2024; FAO, 2024; Issifu et al., 2022; Oceana, 2019; Pham et al., 2023; Sumaila and Tai, 2020).

Illegal, unreported, and unregulated fishing and transshipment

Illegal, unreported, and unregulated (IUU) fishing substantially contributes to overfishing by depleting stocks and distorting markets (Agnew et al., 2009; Oceana, 2019). Global IUU-related losses between 2000 and 2003 were estimated at 11–26 million tons annually, worth USD 10–23.5 billion (Agnew et al., 2009). From 2005 to 2014, unreported catches reached 7.7–14 million tons per year, causing USD 25.5–49.5 billion in losses, particularly affecting Asia and Africa (Sumaila et al., 2020). Transshipment, the transfer of fish, crew, or materials between vessels at any location (FAO, 2023), is a key logistical practice supporting distant-water fishing operations (Nascimento et al., 2024). It occurs in ports or at sea, each presenting regulatory risks. Port transshipment is more visible but inconsistently enforced, allowing misreported catches to enter supply chains (Chen et al., 2017, 2022; Pauly and Zeller, 2016). At-sea transshipment, typically involving refrigerated cargo vessels (reefers/carriers), often occurs beyond national jurisdictions, enabling longer trips, reduced costs, and catch preservation (Boerder et al., 2018; Miller et al., 2018), but it creates governance vulnerabilities including inspection avoidance, data manipulation, and regulatory loopholes (Bengtsson et al., 2024; Ewell et al., 2017; Petrossian et al., 2022). Transshipping vessels may remain at sea for up to 13.3 months vs. approximately 3 months for others, severely limiting oversight (EJF, 2023). Evidence increasingly links at-sea transshipment to IUU fishing and labor abuses, especially among vessels flying flags of convenience (FOCs) that obscure ownership (Bengtsson et al., 2024; Daniels et al., 2016; Pauly et al., 2022). Indonesian case studies show higher involvement in shark finning, wildlife targeting, and labor exploitation compared to non-transshipping vessels (EJF, 2023; Selig et al., 2022; Tickler et al., 2018).

Global transshipment is substantial but poorly reported. The first global AIS-based assessment by Global Fishing Watch and SkyTruth identified 5,783 likely rendezvous between fishing vessels and refrigerated cargo carriers from 2012 to July 2017 (Kroodsma et al., 2017), and a parallel peer-reviewed analysis documented over 10,500 at-sea events over the same period, mainly involving longliners (21%) and trawlers (53%) (Boerder et al., 2018). FAO reported 22,641 events in 2017 totaling 12.24 million tons, while Regional fisheries management organizations (RFMO) data recorded only 688,876 tons across 4,551 events, indicating major discrepancies and underreporting (Cabanelas et al., 2020; EJF, 2023). Authorized transshipping fleets are diverse, led by longliners, squid jiggers, trawlers, and purse seiners, with transshipped biomass consisting predominantly of tuna and pelagic species, followed by demersals and cephalopods (Cabanelas et al., 2020). Monitoring increasingly relies on Automatic Identification System (AIS) and Vessel Monitoring System (VMS), yet only 50%−70% of events are detected due to AIS disabling or manipulation (Deng et al., 2022; Rodríguez et al., 2024). While port inspections, VMS, and monitoring centers are widely used, tools like vessel traffic services and satellite sensing remain limited (Cabanelas et al., 2020). Transshipment is often inferred from AIS-detected encounters and loitering (a vessel remaining stationary or moving slowly in one area for an extended period). Encounters involve vessels within 500 m for ≥2 h at low speeds far from shore (Boerder et al., 2018; Miller et al., 2018), though confirmation requires additional evidence (Greenpeace, 2020). Such events cluster in regions with weak governance and high IUU risk, including the Northwest Pacific and West Africa (Rodríguez et al., 2021). Loitering, while sometimes legitimate, is frequently linked to rendezvous between fishing vessels and reefers, especially when AIS signals are manipulated (Boerder et al., 2018; Miller et al., 2018; Welch et al., 2022). The transshipment network involves diverse vessel types. Longliners dominate high-value tuna fisheries and are associated with IUU and labor risks (ISSF, 2025; McCoy, 2012; Miller et al., 2021), while trawlers account for over half of transshipment events (Cabanelas et al., 2020; SIF (Stop Illegal Fishing) et al., 2021). Purse seiners land most global tuna but are often restricted from at-sea transshipment, though violations occur (ISSF, 2025). Squid jiggers and small-scale fleets also contribute, including informal exchanges such as Ghana's illegal transshipment and Thailand's blue swimming crab fishery (Cabanelas et al., 2020; SIF (Stop Illegal Fishing) et al., 2021). Reefers are central, transporting catch and supplies. A global fleet of 1,228 reefers was identified, with 38% controlled by three corporations (Bengtsson et al., 2024). In certain regions, container and coastal vessels also support logistics. FOC-registered reefers further reduce transparency by exploiting weak regulatory jurisdictions (Bengtsson et al., 2024; Borland and Bailey, 2019).

The governance of transshipment is complex, involving multiple institutional and commercial actors. Fishing vessel owners and operators manage at-sea operations, while reefer owners, often obscured by FOCs and corporate opacity, handle catch logistics (Bengtsson et al., 2024; Holmes et al., 2020). Port authorities provide regulatory checkpoints, but inspection capacity is limited, even in major hubs such as Las Palmas and Busan (Rodríguez et al., 2024). Flag states frequently lack the resources or will to enforce compliance, and vessel reflagging further complicates oversight (Borland and Bailey, 2019). AIS and VMS monitoring is undermined by deliberate tampering (Deng et al., 2022), and supply-chain actors, including processors and distributors, remain challenged by the absence of end-to-end traceability (González-Mon et al., 2021). Regulation spans RFMOs, national authorities, and certification schemes such as Marine Stewardship Council (MSC), but gaps persist. MSC certification, for instance, does not independently verify transshipment events (Borland and Bailey, 2019; Selig et al., 2022).

Recent research highlights the need to better understand transshipment networks and behavior. This study therefore examines current research on fishery transshipment and its links to overfishing, IUU fishing, and sustainability. It reviews detection methods, network structures, and governance implications, addressing knowledge gaps in high-seas operations. Section 2 describes the methodology, Section 3 presents the results, including both a descriptive overview and thematic synthesis, and Section 4 offers concluding remarks and identifies priority research gaps.

A systematic literature review was conducted to examine the key themes related to fish transshipment, particularly its links to IUU fishing and overfishing. The approach followed established practices in transportation and maritime research (Hermann et al., 2022; Seuring and Müller, 2008), emphasizing transparency, reproducibility, and bias reduction through well-defined inclusion and exclusion criteria. The process included formulating a research question, defining search terms, conducting database searches, and applying selection criteria (Ermel et al., 2021). The review also aimed to assess the quality of individual studies, identify research gaps, and map conceptual developments and advancements in the field. The study selection process followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (Page et al., 2021), as illustrated in Figure 1.

Scope of the literature review

The review was guided by the research question: What are the prevailing themes and discussions in the literature concerning fishery transshipment, and how do they elucidate its role in overfishing, IUU fishing, and sustainability? This question informed the selection of the key terms and concepts used in the literature search (Table 1). A combination of keywords commonly associated with fish transshipment and IUU fishing was employed to capture relevant studies. Inclusion criteria were established to ensure consistency and quality (Ermel et al., 2021). Only peer-reviewed articles published in English between January 1, 2000, and April 30, 2025, were considered. The search was conducted in April 2025 to incorporate the most recent research available.

Database search

To identify relevant literature on transshipment activities within the fisheries sector, a systematic keyword-based search strategy was implemented across three major academic databases: Scopus, Web of Science (WOS), and Springer Nature. The objective was to capture a broad yet focused collection of scholarly works by targeting key terms commonly associated with maritime, fisheries, and enforcement literature (Table 1). In the Scopus database, four separate search queries were executed, each focusing on different combinations of transshipment and fisheries-related terminology: (1) TITLE-ABS-KEY (“transshipment” OR “trans-shipment”) AND TITLE-ABS-KEY (“fish” OR “fishery” OR “fishing”), which returned 78 articles; (2) TITLE-ABS-KEY (illegal AND fish * AND detect), yielding 92 articles; (3) TITLE-ABS-KEY-AUTH (fish * AND (transship * OR trans-ship *)), resulting in 90 articles; (4) TITLE-ABS-KEY (fish *) AND (transship *), which retrieved 392 articles.

These results were exported and compiled into CSV files. To refine the dataset, a filtering procedure was applied using Python. Two keyword groups were defined: fisheries-related terms (e.g., “fish,” “fishery,” “fishing,” “vessel,” “ship”) and transshipment or enforcement-related terms (e.g., “transship,” “transship,” “IUU,” “detect,” “illegal”). Articles were retained if their title or abstract contained at least one keyword from each group. This filtering process reduced the dataset to 351 relevant articles for further review.

A parallel search was conducted using the Web of Science database to ensure broader coverage. Four targeted topic searches (TS) were employed: (1) ((transship * OR trans-ship *) AND (fish OR fishery OR fishing)), yielding 85 articles; (2) (illegal AND fish * AND detect), yielding 393 articles; (3) (fish transshipment), retrieving 65 articles; (4) (fish * AND transship *), resulting in 17 articles.

After removing duplicate entries based on article titles, a total of 558 unique records remained. The same filtering criteria used in the Scopus dataset were applied, requiring the presence of at least one keyword from each defined group in either the title or abstract. This process resulted in a final set of 445 articles from the Web of Science deemed suitable for further analysis.

To further enhance the comprehensiveness of the review, the Springer Nature database was also queried. Four keyword-based searches were conducted: (1) ((transshipment OR trans-shipment) AND (fish OR fishery OR fishing)), yielding 343 articles; (2) (illegal AND fish * AND detect), returning 1,000 articles (although the search originally produced 2,611 results, the database's export limit allowed only the first 1,000 records to be downloaded); (3) (fish transshipment), retrieving 136 articles; (4) (fish * AND transship *), yielding 135 articles.

The combined dataset consisted of 1,614 articles before filtering. Consistent with the approach used for Scopus and Web of Science, the dataset was filtered based on the presence of at least one fisheries-related and one transshipment- or enforcement-related keyword in the title or abstract. This process yielded 86 relevant articles from Springer Nature. All datasets were then standardized by aligning field names (e.g., title, abstract, year, authors, document type), and missing fields were added as needed. Finally, the Scopus, Web of Science, and Springer Nature datasets were merged, and duplicate records were removed based on article titles, resulting in a final consolidated dataset comprising 590 unique articles for systematic analysis.

Selection of articles for thematic analysis

All bibliographic information for each article (author, year, title, abstract, journal/book name, number of citations, and doi number) was exported into an Excel file for analysis. This first stage of the analysis focused on a preliminary comparison of article titles to remove any duplicate articles from the dataset. This process resulted in 590 unique articles from the three databases. Subsequently, a detailed review of this sample was conducted to reduce the dataset to a focused number of articles providing in-depth information about the critical issues tackled by the research question. Each article's title and abstract were read, and only those explicitly focused on fishery transshipment at sea were retained. This screening reduced the sample to 71 articles, which were then assessed at full-text level. Articles that discussed transshipment only incidentally as part of broader logistics topics or non-fishery topics were excluded, yielding a final sample of 35 articles that were read and analyzed thematically. Supplementary File 1 (Appendix A) presents a list of the 35 selected articles, including their study design, methodology, data sources, thematic focus, and key findings.

Thematic analysis and synthesis

A thematic analysis was conducted on the final selection of 35 articles. For this analysis, four key aspects of each article were summarized (Table 2):

(1) The article's general bibliographic information (including year of publication and journal, but also the focus region and methods);

(2) The article's disciplinary approach to at-sea transshipment and its associated risk of IUU fishing;

(3) The specific transshipment issue(s) addressed in the article;

(4) How the article addresses fishery transshipment in the context of IUU fishing.

Parts (1) and (2) address the descriptive aspects of the research question: “What are the key discussions and themes in regard to at-sea transshipment?” Parts (3) and (4) are directly connected to the reflective approach adopted in the second part of the research question: “How do these discussions inform the role of transshipment in IUU activities?”

The synthesis phase of a systematic literature review involves more than a simple summary of the identified literature; it requires the integration of insights from multiple sources to identify gaps or advances in the field through conceptual development (Ermel et al., 2021). This study adopted a configurative synthesis approach using an integrative technique that is commonly used in qualitative research, the purpose of which is to refine theories. This technique is appropriate when the overall objective is to identify gaps in the extant literature and propose a research agenda (Ermel et al., 2021). An integrative review, therefore, reduces the primary research into codes that are subsequently organized into categories. To synthesize the literature, each article was coded according to the elements listed in Table 2.

These relationships are synthesized thematically in Section 3.2. The following results section is organized in two parts. Section 3.1 provides a descriptive overview of the 35 reviewed publications, examining trends by year, journal, subject area, methodology, author affiliation, and geographical focus. Section 3.2 presents the thematic synthesis, organized into seven subsections: data sources and monitoring systems (Section 3.2.1), detection, surveillance, and analytical methods (Section 3.2.2), geographical patterns (Section 3.2.3), key actors (Section 3.2.4), governance, policy, and compliance monitoring (Section 3.2.5), risk factors (Section 3.2.6), and social, ecological, and economic impacts (Section 3.2.7). Section 3.1 synthesizes parts (1) and (2) of the coding procedure, while Section 3.2 integrates the thematic insights from parts (3) and (4).

General overview of publications focused on at-sea transshipment

This section presents a descriptive analysis using frequency distribution techniques to provide trends and insights into previous research on at-sea fishery transshipment. It determines the frequency distribution of publications by year, popular journals, subject area, methodology, authors' location, and geographical focus. The analysis revealed that the earliest literature on at-sea fishery transshipment in this dataset appeared in 2011, with a substantial surge in research interest over the past 8 years (2018–2025). The most popular scientific publishing outlets in this area are Marine Policy, Frontiers in Marine Science, Sensors, and Science Advances. The articles were classified into five subject areas: Monitoring and Technology (21 articles), Policy and Governance (16), Economic and Social Aspects (seven), Geopolitical and International Relations (seven), and Environmental Sustainability (four). Quantitative methodologies, particularly machine learning and statistical analysis, were the most commonly adopted research methodologies in the reviewed articles. The USA, Australia, Indonesia, and China were the major contributors, with nearly 67% of articles (37 author affiliations) contributed by authors located in these four countries. Additionally, reflecting the global nature of transshipment and its association with IUU fishing, global studies (12 articles, 30.77%) dominated the geographical focus, followed by Indonesia (five articles, 12.82%).

Figure 2 presents the distribution of the 35 selected articles on at-sea fishery transshipment by the year of publication. The earliest article in the dataset was published in 2011, with only four publications appearing before 2018, each in a different year. From 2018 onward, the number of publications varied but followed a clear upward trend. Notably, 89% of the articles (31 out of 35) were published in the last 8 years (2018–2025), with peaks in 2018 (seven articles) and 2024 (eight articles).

This analysis incorporated two primary source types: journal articles (32 sources, 91.43% of the sample) and journal review articles (three sources, 8.57%). Table 3 details the journals with the highest publication counts, revealing a broad spectrum of outlets (17 journals) contributing to the study of at-sea fishery transshipment. Leading the field were Marine Policy (11 publications), Frontiers in Marine Science (five publications), and both Sensors and Science Advances (three publications each), followed by Wireless Personal Communications (two publications).

Figure 3 presents the distribution of articles by subject area and year of publication for the 35 studies. Some articles span multiple categories due to their interdisciplinary nature. For example, (Petrossian et al. 2024) addresses both port regulations and flags of convenience, appearing in both Policy and Governance and Geopolitical and International Relations. Of the 35 articles, 21 focus on Monitoring and Technology, 16 on Policy and Governance, 7 each on Economic and Social Aspects and Geopolitical and International Relations, and 4 on Environmental Sustainability.

Quantitative methodologies dominate the reviewed literature, accounting for 65.7% of articles (23 of 35), while qualitative and mixed-methods studies each represent 17.1% (six articles each) (Figure 4). Within the quantitative strand, machine learning and AI techniques constitute the largest subgroup (39.1%), followed by statistical modeling (30.4%), remote sensing and data fusion (17.4%), and network or spatial analysis (13.0%). This methodological distribution reflects the field's trajectory from rule-based spatial filtering toward increasingly data-driven detection, with supervised and unsupervised classifiers, self-supervised deep learning, and physics-based trajectory models collectively addressing limitations of fixed-threshold approaches (Bernabé et al., 2024; Chuaysi and Kiattisin, 2020; Deng et al., 2022; Jones et al., 2023; Nascimento et al., 2024; Saxena et al., 2024; Sharma et al., 2024; Welch et al., 2022). Statistical modeling has been applied to catch estimation, port risk assessment, vessel behavior ranking, and spatial triangulation of AIS data against fishery records (Aanes et al., 2011; Ford et al., 2018b; James et al., 2018; Petrossian et al., 2015, 2024; Saito et al., 2022; Seto et al., 2022), while network analysis has revealed structural concentration within transshipment systems through centrality metrics, community detection, and ownership mapping (Bengtsson et al., 2024; Bichler et al., 2024; Petrossian et al., 2022). Remote sensing studies integrating satellite imagery with AIS data remain few but address a critical gap in non-cooperative vessel detection (Kurekin et al., 2019; Miller et al., 2018; Utama et al., 2021; Ying et al., 2023). Qualitative approaches, including policy analysis, legal review, expert elicitation, and stakeholder case studies, provide essential governance and community-level context that quantitative monitoring alone cannot capture (Ewell et al., 2017, 2020; Mensah et al., 2025; Owusu et al., 2025; Riskas et al., 2018; Telesetsky, 2014), and mixed-methods designs have been employed to bridge these perspectives by combining stakeholder engagement with indicator development and spatial analysis (Ford et al., 2018a; Khan et al., 2024; Khoerunnisa et al., 2021; Satria et al., 2018; Selig et al., 2022; Zhu et al., 2021). A notable gap is the absence of comparative benchmarking across methods: no study has tested multiple detection approaches on the same dataset, limiting evidence-based method selection for operational deployment.

The distribution of author affiliations across the 35 analyzed articles revealed a strong concentration of scholarly activity among a few key countries. The United States had the dominant share, contributing to 17 articles, which represented nearly half of all publications in the dataset. This was followed by Australia with seven articles, and both Canada and China, each contributing to four articles. A smaller yet notable presence was observed from Ghana and Indonesia (three articles each), and Japan, Norway, Sweden, and the UK (two articles each). The remaining contributions were distributed across various countries, including India, Ireland, Tuvalu, New Caledonia, Brazil, Spain, Thailand, France, and the Netherlands, each represented in a single publication. These patterns suggest that research on IUU fishing and transshipment is primarily concentrated in a few global research hubs, particularly in North America, Oceania, and parts of East and Southeast Asia, while also attracting contributions from smaller or regionally impacted states (Figure 5).

In terms of the geographical focus of the selected publications, as summarized in Table 4, 12 articles addressed the topic from a global perspective. Several studies concentrated on specific national exclusive economic zones (EEZs), including Indonesia (five articles), China (three articles), and Ghana (three articles). Eight articles explored other EEZs, including Norway, the USA, Cuba, France, Russia, Brazil, Tuvalu, and Thailand. Four articles conducted regional analyses, covering parts of the Indian Ocean, including the Western, Southern, and Eastern Indian Ocean, as well as the Indian Ocean and Southeast Asia region (IOSEA). The Pacific Ocean region was represented by one article focused on the Western and Central Pacific Ocean, with another article examining the Northern Pacific's Bering Sea. Three studies concentrated on RFMOs, such as the Western and Central Pacific Fisheries Commission (WCPFC) and the North Pacific Fisheries Commission (NPFC). Finally, two articles investigated protected or disputed maritime areas, specifically the Galapagos Marine Reserve and the Greyzone.

Thematic analysis

Research into at-sea transshipment draws on a heterogeneous evidence base whose limitations and complementarities have shaped the field's analytical trajectory. The following subsections synthesize findings across the 35 reviewed articles, organized around seven interconnected themes: data sources and monitoring systems; detection, surveillance, and analytical methods; geographical patterns; key actors; governance, policy, and compliance monitoring; risk factors; and social, ecological, and economic impacts. Rather than describing individual studies sequentially, the analysis foregrounds cross-cutting patterns, convergences, divergences, and gaps that emerge from the collective body of evidence.

Data sources and monitoring systems

Transshipment research relies overwhelmingly on cooperative vessel tracking, yet the field increasingly recognizes that no single data source can capture the full scope of at-sea transfer activity. AIS data underpin the majority of studies, serving as the primary means of identifying vessel encounters, loitering behavior, signal anomalies, and network structures (Table 5). As an open broadcast system carried by all vessel types above a certain tonnage threshold, AIS provides publicly accessible positional data, including vessel identity and name, through platforms such as Global Fishing Watch (GFW), enabling large-scale transparency research that has processed billions of position messages to map global transshipment patterns (Boerder et al., 2018; Miller et al., 2018). However, AIS reception in oceanic areas depends on satellite constellation coverage, producing alternating periods of high-frequency positioning and signal gaps whose spatial and temporal variability must be accounted for analytically (Welch et al., 2022). More critically, AIS is fundamentally compromised by the very behavior it seeks to detect: signals can be intentionally disabled or spoofed, and disabling clusters precisely in transshipment hotspots and near EEZ boundaries (Bernabé et al., 2024; Welch et al., 2022). This creates a paradox in which the monitoring tool's coverage gaps are largest where illicit activity is most concentrated.

VMS offers a complementary fisheries-specific alternative that operates under national or regional mandates exclusively for fishing vessels, providing positional data at lower but more regular transmission intervals than satellite-dependent AIS, and unlike AIS, VMS signals cannot be legally disabled nor received by commercial competitors (Welch et al., 2022). However, access restrictions imposed for commercial confidentiality have severely limited VMS use in independent research (Aanes et al., 2011; Chuaysi and Kiattisin, 2020; James et al., 2018; Utama et al., 2021). Even where VMS data are available, transmission interruptions are pervasive, affecting 973 of 1,082 longline vessels in one Indian Ocean study alone (Utama et al., 2021), raising questions about systemic reliability. The analytical advantages of integrating AIS and VMS are widely acknowledged: AIS provides high-resolution behavioral detail in the public domain while VMS fills temporal gaps within managed fisheries (Chuaysi and Kiattisin, 2020; Utama et al., 2021), yet only a small fraction of studies have combined both sources.

Satellite imagery, including Synthetic Aperture Radar (SAR) and optical sensors, provides the only non-cooperative detection capability available for transshipment monitoring, enabling independent visual identification of vessel positions without reliance on self-reported tracking signals. Where deployed, SAR has revealed that up to 75% of radar-detected vessels lack AIS equivalents, exposing a fundamental blind spot in cooperative tracking (Kurekin et al., 2019). This complementary function is critical: by cross-referencing satellite-detected vessel positions against AIS records, analysts can identify non-cooperative vessels that may be involved in IUU fishing, including those operating without AIS, transmitting with disabled transponders, or deliberately misreporting positions (Jones et al., 2023; Kurekin et al., 2019). Resolution constraints have limited SAR's utility for verifying close-proximity transfer events (Saito et al., 2022), while data fusion frameworks integrating optical satellite imagery with AIS have been proposed for behavioral classification but still depend on cooperative signals for trajectory analysis (Jones et al., 2023). Revisit frequency, weather dependence, and cost continue to confine satellite-based approaches to regional applications, and no study has demonstrated independent dark vessel (those operating without transmitting AIS signals) detection at scale.

Operational records, including logbooks, transshipment declarations, and observer reports, provide ground-truth data against which remotely sensed outputs can be validated, but their availability and reliability vary markedly across jurisdictions and reporting formats (James et al., 2018; Saito et al., 2022; Zhu et al., 2021). The transition from paper to electronic reporting has improved timeliness but exposed persistent data quality gaps, with significant discrepancies between the two formats undermining confidence in self-reported records (Zhu et al., 2021). Observer programs, though critical for independent verification, remain constrained by low coverage (only three of 17 RFMOs mandate full observer presence), transparency deficits, and documented safety risks (Ewell et al., 2020). Remote Electronic Monitoring (REM), a system combining onboard cameras, gear sensors, video storage, and satellite positioning, has been proposed to address these coverage limitations by providing visual records of on-deck activity including transshipment operations (Ewell et al., 2020; Saito et al., 2022). Video recording of transshipment events could serve as valuable evidence to validate information in transshipment reports, particularly when combined with AIS data analysis (Saito et al., 2022), though REM faces unresolved challenges in camera angle limitations, data storage capacity, review workflows, and cross-jurisdictional standardization (Ewell et al., 2020). Enforcement records, including coastguard inspections, IUU vessel databases, and maritime law enforcement case histories, have also served as ground-truth validation sources, but such data remain scarce, restricted by privacy and confidentiality constraints, and geographically fragmented (Aanes et al., 2011; Petrossian et al., 2015; Ying et al., 2023).

Geospatial contextual layers have been incorporated in some studies to reduce false positives and improve classification accuracy, though integration remains limited. Port and coastline proximity filtering helps exclude benign rendezvous in congested zones by defining buffer areas around ports and shorelines (Deng et al., 2022), while static boundary layers [EEZs, Marine Protected Areas (MPAs), undersea infrastructure] enable spatial differentiation between legitimate activity and suspicious behavior within protected or restricted zones (Jones et al., 2023; Nascimento et al., 2024). One study collected weather, tidal, and air quality data alongside static boundary layers to construct a multi-layered contextual database, though the contribution of these dynamic variables to classification accuracy was not systematically evaluated (Jones et al., 2023). Fishing effort density derived from AIS data has been used to optimize transshipment monitoring station placement through cost-coverage clustering (Saxena et al., 2024). However, no study has fully integrated dynamic oceanographic variables (such as sea surface temperature or chlorophyll concentration) as functional inputs to transshipment detection models, and data incompleteness and the difficulty of isolating contextual effects from primary behavioral indicators remain unresolved limitations.

Qualitative evidence from expert elicitation, stakeholder interviews, and focus groups provides contextual depth that quantitative monitoring alone cannot capture, particularly regarding governance dynamics, community impacts, and compliance behavior (Mensah et al., 2025; Owusu et al., 2025; Riskas et al., 2018; Satria et al., 2018). These studies collectively reveal that transshipment regulation often produces unintended governance outcomes, including the repositioning of illegal practices under state-facilitated structures rather than their elimination, and that qualitative indicators remain essential where quantitative monitoring is absent or unreliable. Secondary source analysis of government data, enforcement records, and media reports has further documented links between IUU transshipment and organized crime, including forced labor and trafficking (Khan et al., 2024). However, qualitative evidence remains constrained by restricted access, reactive bias, and partial geographic coverage.

A persistent structural weakness across the literature is the fragmentation of data sources. No study has achieved comprehensive integration of cooperative tracking (AIS/VMS), non-cooperative sensing (SAR), operational records, and qualitative evidence within a single analytical framework. Most studies rely on one or two data types, and the resulting partial coverage creates systematic blind spots that undermine both detection accuracy and governance credibility. A detailed summary of data types and corresponding studies is presented in Table 5.

Detection, surveillance, and analytical methods

The analytical toolkit for identifying transshipment has evolved through three broad methodological generations, each addressing limitations of its predecessor while introducing new constraints. A clear convergence has emerged: detection accuracy improves substantially with each generation, but no single method has achieved comprehensive coverage, and the absence of standardized benchmarking prevents rigorous cross-method comparison.

Rule-based spatial filtering constitutes the foundational detection approach, defining transshipment encounters through proximity thresholds (typically less than 500 m between vessels), minimum duration (at least 2 h), low speed (below 2 knots), and minimum distance from shore or anchorage (Boerder et al., 2018; Miller et al., 2018). Single-vessel loitering analysis, which infers transshipment from sustained low-speed offshore presence rather than observed vessel pairs, yields roughly four times more detections than two-vessel encounters (46,570 loitering events vs. 10,233 encounters globally between 2012 and 2017), suggesting that much transshipment activity involves carrier vessels waiting rather than synchronized rendezvous (Miller et al., 2018). While effective for large-scale global screening, fixed-threshold methods generate high false positive rates in dense traffic areas and miss events that fall just outside parameter boundaries (Deng et al., 2022). These limitations have prompted context-specific calibration, with sensitivity analysis in the NPFC convention area reducing encounter and loitering duration thresholds from the standard values to 1 h (Saito et al., 2022). A further constraint is the inability of uniform thresholds to accommodate the diversity of transshipment operational modes, which range from direct ship-to-ship berthing at distances under 100 m to boat-based transfers at distances exceeding 500 m (Ying et al., 2023).

Machine learning (ML) and adaptive classification methods address these limitations by learning decision boundaries from data rather than imposing fixed parameters. Supervised classification consistently achieves accuracies exceeding 90%, with the highest-performing models reaching 97% overall accuracy across four vessel behavior categories (Nascimento et al., 2024), class-specific accuracies of 90%−98% for vessel type identification (Chuaysi and Kiattisin, 2020), and 99.5%−99.8% accuracy for intentional AIS shutdown detection (Bernabé et al., 2024). AIS signal anomaly detection has been addressed across multiple methodological paradigms, from self-supervised deep learning and ensemble regression to physics-based trajectory modeling, collectively identifying tens of thousands of suspected intentional disabling events concentrated near EEZ boundaries, transshipment zones, and piracy-prone areas (Bernabé et al., 2024; Sharma et al., 2024; Welch et al., 2022). A notable methodological divergence separates approaches requiring labeled training data from those that do not: self-supervised and unsupervised frameworks avoid the labeling bottleneck, critical given the scarcity of verified transshipment ground truth, but have been validated only in limited geographic contexts (Bernabé et al., 2024; Deng et al., 2022). Hybrid approaches that combine rule-based interpretability with data-driven adaptability show particular promise for reducing false positives that arise when non-fishing vessels exhibit fishing-like movement patterns with high probability (Deng et al., 2022; Nascimento et al., 2024). Data fusion frameworks that integrate AIS with satellite imagery and expert rules further extend detection capability to non-cooperative vessels (Jones et al., 2023; Kurekin et al., 2019). Despite these advances, no study has benchmarked multiple ML approaches on the same dataset, making direct performance comparison impossible, a critical gap for operational deployment that requires evidence-based method selection.

Network analysis represents a fundamentally different analytical paradigm, shifting from event-level detection to structural characterization of transshipment systems. Social network analysis (SNA), community detection algorithms, and centrality metrics have collectively revealed that transshipment networks are highly concentrated: only 130 central carriers (22% of the fleet) account for 72% of all recorded events globally, with 12 distinct communities detected through modularity analysis (Petrossian et al., 2022). In the Western Central Pacific, community detection identified five factions, with three subgroups accounting for almost half of all transshipments, and highly central FOC-flagged carriers forming shadow networks with operationally deficient fishing vessels (Bichler et al., 2024). Network approaches have also extended beyond fishing and carrier vessels to include bunker/fuel tankers, which play a crucial logistical role in sustaining at-sea operations and have been identified as central actors within transshipment networks through SNA in the southern Indian Ocean (Ford et al., 2018a). Centrality metrics and ownership mapping have further traced corporate networks and flag-of-convenience patterns that obscure accountability (Bengtsson et al., 2024). The convergent finding across network studies is that structural concentration creates both amplified risk and a governance opportunity: regulatory engagement with a small number of central actors could yield disproportionate improvements in oversight.

A persistent verification gap undermines all detection tiers. Even the comparatively robust WCPFC framework left 68% of AIS-identified potential transshipments unsubstantiated after triangulation with diverse data sources, due to restricted access to VMS and observer records (Seto et al., 2022). Combining AIS with RFMO authorization lists provides a basis for compliance verification, but no study has used this linked dataset to systematically identify risk activities at a global scale. Event characterization methods have further revealed that transshipment durations follow bimodal distributions reflecting differences in event scale, suggesting potential for estimating transfer volumes and quantifying species-specific activity (Saito et al., 2022). The transition from paper to electronic reporting shows promise for improving data accuracy and timeliness (Zhu et al., 2021), yet stakeholder resistance and data quality challenges remain barriers to adoption. Optimization approaches have also been proposed: cost-based adaptive clustering has been used to identify optimal locations for transshipment monitoring stations using fishing effort data and management zone boundaries (Saxena et al., 2024), representing a shift from reactive detection to proactive surveillance infrastructure design.

Geographical patterns

At-sea transshipment concentrates in regions where productive fishing grounds, weak governance, and strategic proximity to shipping routes converge, a spatial pattern that has remained remarkably stable across multiple independent studies spanning different time periods and data sources.

Global-scale mapping has consistently identified transshipment hotspots in the Northwest Pacific, West Africa, the Indian Ocean, and waters surrounding Southeast Asia, with additional clusters in the Southern Indian Ocean and off South America (Boerder et al., 2018; Miller et al., 2018; Petrossian et al., 2022). Between 2012 and 2017, AIS data identified over 10,500 likely transshipment encounters across two independent studies (Boerder et al., 2018; Miller et al., 2018), of which 51.8% occurred on the high seas and 48.2% within EEZs (Miller et al., 2018), though this distribution is strongly influenced by Russian-waters activity, which accounts for 39% of all EEZ-based encounters; excluding Russia, high-seas encounters rise to 57% (Boerder et al., 2018). Encounter volumes have continued to grow, rising from approximately 5,000 in 2017 to over 11,000 in 2022 involving around 300 vessels annually (Bengtsson et al., 2024). These hotspots align with areas of high fishing effort and major distant-water fleet operations, and they overlap significantly with AIS disabling hotspots, particularly near the EEZs of Argentina, West African nations, and the Northwest Pacific (Welch et al., 2022), suggesting a spatial coupling between transshipment activity and deliberate evasion of monitoring.

Regional and national patterns reveal distinct transshipment dynamics shaped by local regulatory environments and fleet compositions. The Northwest Pacific dominates global transshipment geography, accounting for 44% of all AIS-detected events and an estimated 33% of IUU fishing (Boerder et al., 2018); within the NPFC Convention Area, substantial activity involving 26 carrier vessels and at least 600 documented events is dominated by longliners (36%), saury gear (24%), and squid jiggers (18%) (Saito et al., 2022). In the Western Central Pacific (FAO Area 81), network analysis of repeat encounters identified 30 carriers interacting with 613 fishing vessels between 2015 and 2021, with high concentrations within and immediately outside the EEZs of Papua New Guinea, Kiribati, Nauru, Solomon Islands, and the Marshall Islands (Bichler et al., 2024; Petrossian et al., 2022). Indonesia has served as a natural policy experiment: the 2014 transshipment ban and fishing license moratorium caused production declines exceeding 60% in the longline tuna sector and a 69.6% reduction in fleet size at the Nizam Zachman Oceanic Fishing Port between 2014 and 2019 (Khoerunnisa et al., 2021), while simultaneously triggering adaptive evasion strategies including VMS disabling, use of unofficial landing sites, and effort redistribution to less regulated waters (Satria et al., 2018; Utama et al., 2021). Off West Africa, dark vessel activity is pervasive, with 75% of SAR-detected vessels in Ghana's EEZ lacking AIS equivalents (Kurekin et al., 2019), and illegal at-sea transshipment between industrial trawlers and artisanal canoes has persisted despite formal prohibition, with governance dynamics repositioning the practice under state facilitation rather than eliminating it (Mensah et al., 2025; Owusu et al., 2025).

Transboundary dimensions are a recurring theme. Transshipment landings concentrate in East and Southeast Asian port hubs, including Busan, Zhoushan, Kaohsiung, and Singapore, with Mauritius as a notable Indian Ocean outlier (Petrossian et al., 2024). RFMO convention areas, particularly the WCPFC and NPFC, serve as critical regulatory arenas where detected transshipment events and official records frequently diverge, with under-reporting estimated at over 10-fold in some regions (Saito et al., 2022; Seto et al., 2022). Only limited research has investigated transshipment in protected or disputed maritime areas; the Barents Sea represents one of few case studies where conservation mandates and fishing activity create heightened regulatory complexity (Aanes et al., 2011), and the near-absence of research on transshipment within MPAs represents a significant gap given that AIS disabling and dark vessel activity have been documented in these areas. The spatial distribution of transshipment is not static; regulatory changes trigger adaptive responses including VMS disabling, use of unofficial landing sites, and effort redistribution to less regulated waters (Satria et al., 2018; Utama et al., 2021), yet regional coverage in the literature remains uneven, with the Northwest and Western Central Pacific and the Indian Ocean relatively well studied while the Southeast Atlantic, the Mediterranean, and the Eastern Pacific remain significantly under-examined.

Key actors

The literature reveals extreme structural concentration across all actor categories, carriers, corporate owners, flag states, and ports, creating layered opacity that simultaneously amplifies risk and offers targeted intervention opportunities.

Ownership and fleet concentration

The global reefer fleet comprises approximately 569 vessels, with Russian and Chinese companies controlling 26% and 20%, respectively, and 65% of all reefers flying Russian, Panamanian, or Chinese flags (Bengtsson et al., 2024). Fishing vessels from 47 nations participate in transshipment, yet Russia (26%), China (20%), and Taiwan (15%) account for the majority, with Russian-flagged vessels alone representing 44.7% of all fishing vessels in documented encounters (Boerder et al., 2018; Miller et al., 2018). Network analysis confirms that this structural inequality extends to operational patterns: a subset of only 130 central carriers (22%) accounts for 72% of all transshipment events globally, with these high-volume activities being heavily facilitated by vessels flying open-registry flags such as Panama, Liberia, and Vanuatu (Petrossian et al., 2022). In the Western Central Pacific, highly central FOC-flagged reefers form shadow networks with operationally deficient fishing vessels, creating clusters at elevated IUU risk (Bichler et al., 2024). The convergent finding is that a small number of corporate entities exert disproportionate influence over transshipment logistics, meaning that targeted regulatory engagement with these actors could yield outsized improvements in oversight and traceability.

Flags of convenience

FOC registration pervades the carrier fleet, with 41%−42% of reefer vessels flying flags of convenience (Boerder et al., 2018; Miller et al., 2018), and 169 of 472 carriers registered across five RFMOs flagged to FOC states, with Panama alone accounting for 119 (Ewell et l., 2017). FOC-flagged carriers serve as vehicles for ownership obfuscation and catch laundering through shell companies and offshore registries that conceal beneficial ownership (Telesetsky, 2014). Panamanian carriers dominate encounters with the major distant-water fishing fleets, accounting for 59%−81% of encounters involving Chinese, Chinese-Taipei, South Korean, and Japanese vessels (Miller et al., 2018), yet Panama holds limited involvement in RFMO decision-making (Bengtsson et al., 2024). In the Western Central Pacific, 91% of fishing vessels are Asian-flagged while 53% of carriers use FOC flags (Bichler et al., 2024), creating an asymmetry in which the most visible vessels (fishing) are nationally flagged while the logistical backbone (carriers) operates under reduced accountability.

Ports as critical nodes. Port characteristics significantly predict transshipment activity: higher overall vessel traffic, proximity to high-seas transshipment hotspots, and Chinese government-backed affiliation are all associated with greater FOC-flagged carrier visits (Petrossian et al., 2024). Weaker regulatory enforcement, limited monitoring capacity, and the presence of free port zones make certain facilities disproportionately attractive, with free ports receiving 5.71 times more IUU vessel visits than non-free ports (Petrossian et al., 2015). Transshipment landings concentrate in East and Southeast Asian hubs, suggesting that port-level interventions could represent an efficient enforcement leverage point (Petrossian et al., 2024).

Vessel types and target species

Transshipment participation varies systematically by gear type: trawlers account for 53% of events and operate mainly within EEZs, while longliners (21%) predominate on the high seas, and squid jiggers (13%), pot/trap vessels (7%), and purse seiners (1.2%) contribute smaller shares (Boerder et al., 2018). The target species are predominantly high-value or high-volume: tuna, sharks, billfishes, groundfish, salmon, crustaceans, and squid (Boerder et al., 2018; Petrossian et al., 2022), with regional specificity in Indonesia (tuna longline and purse seine), the NPFC area (Pacific saury, sardine, mackerels, and neon flying squid), and the Barents Sea (Northeast Arctic cod and haddock) (Aanes et al., 2011; Saito et al., 2022; Satria et al., 2018; Utama et al., 2021). A strong correlation exists between transshipment involvement and elevated risks of both labor abuse and IUU fishing, particularly among drifting longliners, trawlers, set longliners, and squid jiggers (Selig et al., 2022). Fuel tankers also play a crucial supporting role, resupplying fleets and extending fishing operations at sea; SNA in the southern Indian Ocean has identified tankers as central actors within transshipment networks (Ford et al., 2018a).

Governance, policy, and compliance monitoring

The governance landscape for at-sea transshipment is characterized by a fundamental structural paradox: the regulatory frameworks designed to control transshipment are systematically undermined by the actors and jurisdictional fragmentation they are meant to address.

RFMO capacity and heterogeneity

RFMOs bear primary responsibility for regulating high-seas transshipment, yet comparative analysis across all 17 organizations reveals significant heterogeneity in monitoring mandates, enforcement capacity, and transparency (Ewell et al., 2017, 2020). Regulatory stringency remains uneven: by 2015, only one RFMO (SEAFO) had imposed a total ban on at-sea transshipment, while five others had adopted partial bans, and the remainder permit transshipment with varying levels of oversight (Ewell et al., 2017; Miller et al., 2018). Thirteen RFMOs mandate vessel tracking and 10 require observers, yet only three [Convention on the Conservation and Management of Pollock Resources in the Central Bering Sea (CCBSP), Northwest Atlantic Fisheries Organization (NAFO), South East Atlantic Fisheries Organization (SEAFO)] require full observer coverage, with NAFO allowing exemptions (Boerder et al., 2018; Ewell et al., 2020). Transparency is critically insufficient: no RFMO publishes full observer reports or mandates human rights violation reporting (Ewell et al., 2020), and governance approaches vary considerably, from the Indian Ocean Tuna Commission (IOTC), which restricts transshipment to large longliners, to North East Atlantic Fisheries Commission (NEAFC), which relies on port state controls (Aanes et al., 2011; Bengtsson et al., 2024).

Verification and transparency deficits

Even comparatively robust frameworks cannot independently verify most detected events. In the WCPFC, 68% of AIS-identified potential transshipments remained unsubstantiated after triangulation with multiple data sources, due to restricted access to VMS and observer data (Seto et al., 2022). The NPFC similarly lacks an onboard observer program and public VMS data, with substantial gaps between reported and AIS-detected transshipment activity, including suspected under-reporting of species such as Japanese sardine and mackerels (Saito et al., 2022). GFW's linkage of AIS encounter data with RFMO authorization lists has begun to address this transparency deficit, and GFW data have enabled systematic assessments of transshipment-related labor abuse, IUU risk, and port-level vulnerability (Petrossian et al., 2024; Selig et al., 2022), though no study has used authorization-list cross-referencing to systematically identify unauthorized transshipment at a global scale. Flag state disengagement compounds these weaknesses, with Panama's limited RFMO participation despite its large reefer fleet exemplifying the disconnect between flag state registration and governance responsibility (Bengtsson et al., 2024; Petrossian et al., 2022).

The prohibition-regulation tension

A fundamental policy divergence separates advocates of a total high-seas transshipment moratorium, who argue that ecological and social costs outweigh economic benefits (Ewell et al., 2017), from proponents of designated, monitored transshipment infrastructure, who contend that outright bans sacrifice efficiency and may generate perverse incentives (Saxena et al., 2024). Empirical evidence from Indonesia illustrates both sides: the 2014 ban reduced the number of longline tuna vessels based at Nizam Zachman port from 280 to 83 between 2014 and 2019, with production declining from 11,768 to 4,637 tons (Khoerunnisa et al., 2021), while simultaneously creating regulatory loopholes and adaptive evasion strategies (Satria et al., 2018). These contrasting outcomes underscore the need for comparative policy evaluation that accounts for enforcement capacity, economic impacts, and the behavioral responses of regulated actors.

National-level governance challenges

Weak enforcement capacity, corruption, and regulatory fragmentation serve as persistent enablers of IUU fishing in countries where transshipment circumvents catch documentation requirements (Khan et al., 2024). Domestic legal frameworks across major fishing nations have not fully criminalized key IUU acts such as flag-hopping and catch laundering, and FOC states demonstrate negligible prosecution capacity (Telesetsky, 2014). In Ghana, closed fishing season policies have achieved some stock recovery gains, including a 35% yield increase from octopus closures, but transshipment regulation has not eliminated illegal practices; instead, governance dynamics have repositioned saiko under state facilitation, with fishing communities perceiving policies as inequitable due to absent alternative livelihoods and the disproportionate burden on artisanal operators (Mensah et al., 2025; Owusu et al., 2025). Preliminary evidence suggests that Port State Measures Agreement ratification may reduce visits by higher-risk vessels to signatory countries, though observed effects remain statistically uncertain (Selig et al., 2022). Scholars have recommended standardized reporting protocols, mandatory observer submissions, enhanced data sharing (Seto et al., 2022), and adoption of electronic monitoring systems (Saito et al., 2022; Zhu et al., 2021), yet compliance monitoring research demonstrates that no single instrument is sufficient in isolation; effective oversight requires integration of electronic reporting, vessel tracking, and independent verification to address systematic underreporting (Aanes et al., 2011; Utama et al., 2021; Zhu et al., 2021).

Risk factors

The literature identifies multiple behavioral, spatial, and regulatory risk indicators for transshipment-related IUU activity, yet these indicators remain fragmented across studies and have not been integrated into a unified assessment framework.

Behavioral indicators cluster around three dimensions. First, offshore loitering, sustained low-speed movement away from port, is the most widely recognized transshipment signal, with classification typically requiring vessels to move below 2 knots for over 8 h at least 20 nautical miles from shore (Miller et al., 2018). While near-port loitering may reflect benign behavior, statistical approaches using generalized additive models (GAMs) have demonstrated that vessels with unusually frequent offshore loitering can be ranked as high-risk with actionable accuracy (Ford et al., 2018b). Second, AIS signal suppression, including intentional shutdown, spoofing, and intermittent transmission, constitutes a distinct risk dimension: disabling clusters near EEZ boundaries, transshipment hotspots, and marine protected areas, with more than 40% of fishing vessels in offshore waters (beyond 50 nautical miles from shore) exhibiting suspected disabling events that collectively obscure 6% (over 4.9 million hours) of fishing vessel activity globally (Bernabé et al., 2024; Sharma et al., 2024; Welch et al., 2022). Third, nighttime operations represent a temporal risk factor, as reduced visibility facilitates concealment; illegal transshipment frequently occurs during unconventional periods with high nighttime probability (Ying et al., 2023), though some activities, such as squid fishing, legitimately occur at night (Jones et al., 2023).

Spatial indicators add geographic specificity. Operating within marine protected areas introduces a spatial risk dimension because these zones restrict extractive activities; stationary and loitering vessels within MPAs have been classified as suspicious across multiple regional studies (Jones et al., 2023; Nascimento et al., 2024). However, research remains limited to specific regions and no global study has assessed high-risk transshipment activity across all MPAs. The spatial co-occurrence of transshipment hotspots with AIS disabling hotspots (Welch et al., 2022) and with weak-governance EEZs (Miller et al., 2018) suggests that spatial risk is not randomly distributed but clusters where oversight capacity is lowest.

Regulatory indicators provide a third risk dimension. RFMO authorization status, observer coverage gaps, and flag state compliance records all contribute to the risk profile of individual transshipment events. Carriers exploit regulatory fragmentation through FOC registration and weak port state compliance (Bichler et al., 2024; Petrossian et al., 2022, 2024), and even RFMOs with observer mandates cannot verify the majority of detected events (Seto et al., 2022). Although GFW has begun linking AIS encounter data with RFMO authorization lists, this dataset has not been used for systematic risk assessment.

The convergent finding is that behavioral, spatial, and regulatory risk factors are mutually reinforcing rather than independent: vessels disabling AIS near EEZ boundaries in regions with limited observer coverage and FOC-flagged carriers operating without RFMO authorization present compound risk that exceeds any single indicator. Yet no study has attempted to integrate these dimensions into a unified global risk assessment framework, a gap that limits the field's ability to prioritize enforcement resources and that represents the most significant analytical opportunity identified in this review.

Social, ecological, and economic impacts of fishery transshipment

At-sea transshipment operates as a dual-function mechanism: it serves a legitimate logistical purpose while simultaneously functioning as the primary enabler of IUU fishing, catch laundering, and associated crimes. The literature documents interconnected economic, ecological, and social impacts, but research remains largely siloed, with ecological studies rarely addressing labor dimensions and social impact research seldom quantifying stock depletion.

Economic impacts

Transshipment enhances fleet efficiency by reducing operational costs and maximizing time in fishing grounds (Ewell et al., 2017; Saxena et al., 2024), but when oversight is weak, it creates channels for the laundering and market entry of undocumented catch, obscuring seafood origins (Boerder et al., 2018; Ewell et al., 2017; Miller et al., 2018). The economic consequences of IUU fishing facilitated by unregulated transshipment are substantial: losses estimated at IDR 101 trillion annually in Indonesia (Khan et al., 2024), USD 10–23.5 billion globally (Agnew et al., 2009; Welch et al., 2022), and disproportionate harm to developing nations, including USD 2.3 billion in losses across West Africa alone (Owusu et al., 2025). These losses fall heaviest on national economies and small-scale fishers, who experience declining legal catches and market access (James et al., 2018; Riskas et al., 2018). The most specific quantification of regulatory consequences comes from Indonesia, where the combined transshipment ban and fishing license moratorium reduced production by 60.6% and fleet size by 69.6% at Nizam Zachman port between 2014 and 2019 (Khoerunnisa et al., 2021), illustrating the economic tension between prohibitive regulation and continued illicit activity. Despite transshipment's central role in enabling these losses, no study has provided a distinct economic valuation of its specific contribution, largely because at-sea transfers obscure both the origin and quantity of landed fish.

Ecological impacts

Unregulated transshipment undermines sustainability by facilitating catch laundering and obscuring the origins of harvested species, which in turn compromises stock assessments and enables overfishing (Bichler et al., 2024; Miller et al., 2018; Seto et al., 2022). The practice intensifies IUU fishing methods, including bottom trawling that severely damages coral reefs and marine biodiversity (Khan et al., 2024), and drives overexploitation of valuable and vulnerable species, particularly tuna, sharks, billfish, salmon, and crabs (Khan et al., 2024; Miller et al., 2018; Seto et al., 2022). Expert elicitation indicates 83% agreement that IUU fishing poses a high threat to sea turtle populations in the Indian Ocean and Southeast Asia, with transshipment enabling extended fishing trips in areas of high bycatch risk (Riskas et al., 2018). Optimization-based approaches suggest that adaptive, well-monitored transshipment hubs could enhance oversight while maintaining operational efficiency (Saxena et al., 2024), but this proposition remains untested in practice.

Social impacts

Port-level labor abuse and IUU fishing risks are strongly correlated (r = 0.897, p < 0.001), with 82% of fishing trips ending in assessed ports linked to labor abuse or IUU activity (Selig et al., 2022). Unregulated transshipment has been associated with human rights abuses, including trafficking and forced labor, and with organized crime such as smuggling of weapons, drugs, and wildlife (Ewell et al., 2017; Ford et al., 2018a; Khan et al., 2024; Petrossian et al., 2022; Telesetsky, 2014). The distributional effects are uneven: port-based transshipment in Tuvalu generates government income and local employment but has displaced small-scale fishers and depleted local catches (James et al., 2018), while in Ghana, regulation has repositioned illegal transshipment under state facilitation rather than eliminating it, with fishing communities perceiving policies as inequitable due to absent alternative livelihoods (Mensah et al., 2025; Owusu et al., 2025).

The cross-cutting finding is that transshipment represents the critical rupture point in seafood supply chain traceability, the stage at which catch enters legal markets without documentation of origin, harvesting conditions, or labor practices (Boerder et al., 2018; Chuaysi and Kiattisin, 2020; Selig et al., 2022; Telesetsky, 2014). Catch laundering undermines stock assessments, which drives overfishing, which depresses legitimate livelihoods, which in turn perpetuates exploitative labor practices; a reinforcing cycle in which economic, ecological, and social harms are causally linked. The costs fall disproportionately on small-scale fishers and coastal developing states, while economic benefits accrue to distant-water fleets and corporate actors. Despite these interconnections, the literature remains largely siloed: ecological studies rarely address labor dimensions, social impact research seldom quantifies stock depletion, and economic analyses focus on aggregate losses without distinguishing transshipment's specific contribution. This disciplinary fragmentation limits holistic policy development and represents a critical opportunity for future interdisciplinary research.

This systematic review of 35 studies reveals that at-sea transshipment sits at the intersection of three underconnected literatures, detection and surveillance, governance and compliance, and socio-ecological impact, and that progress within each has been uneven. AIS-based analytics have enabled large-scale mapping of transshipment encounters and loitering behavior (Boerder et al., 2018; Miller et al., 2018; Welch et al., 2022), and machine learning classifiers now achieve high detection accuracy for vessel behavior and AIS manipulation (Bernabé et al., 2024; Chuaysi and Kiattisin, 2020; Nascimento et al., 2024). Yet these technological gains expose a fundamental governance deficit: the majority of identified encounters occur outside RFMO authorization frameworks, observer coverage remains low and inconsistently mandated (Ewell et al., 2020; Seto et al., 2022), and flag-of-convenience registration concentrates transshipment capacity in states with minimal oversight (Bengtsson et al., 2024; Petrossian et al., 2022). The result is a monitoring architecture that can detect suspicious activity but lacks the institutional mechanisms to act on it.

Three cross-cutting findings emerge. First, cooperative tracking data (AIS/VMS) are simultaneously the field's most productive resource and its most critical vulnerability, since intentional signal disruption clusters precisely where illicit transfers are most likely (Bernabé et al., 2024; Welch et al., 2022). Second, economic, ecological, and social harms are causally linked through catch laundering, yet the literature treats them in disciplinary silos: ecological studies rarely quantify labor dimensions, social research seldom models stock depletion, and no study has isolated transshipment's distinct economic contribution from aggregate IUU losses (Ewell et al., 2017; Khan et al., 2024; Selig et al., 2022). Third, national regulatory interventions produce mixed outcomes; Indonesia's transshipment ban reduced fleet activity substantially but did not eliminate illicit transfers (Khoerunnisa et al., 2021; Satria et al., 2018), while Ghana's closed-season policy repositioned illegal transshipment under state facilitation rather than curtailing it (Mensah et al., 2025; Owusu et al., 2025).

These findings point to three priority research gaps. The spatial relationship between transshipment hotspots and ecologically sensitive zones, including MPAs and EEZ boundaries, remains unexamined despite evidence that encounters concentrate at jurisdictional margins (Boerder et al., 2018; Miller et al., 2018). No study has linked vessel registries, RFMO authorization records, and port inspection databases into an integrated compliance verification system, even though the data infrastructure increasingly exists to do so (Ewell et al., 2020; Petrossian et al., 2024; Seto et al., 2022). Finally, interdisciplinary research that traces the full chain from at-sea transfer through catch laundering to market entry, labor conditions, and stock impacts is entirely absent. Closing these gaps will require standardized data-sharing across national authorities, RFMOs, and technology providers, coupled with mandatory electronic monitoring, to transform transshipment from a persistent regulatory blind spot into a leverage point for sustainable ocean governance.