Eleven vessels. Three years. More than 3,600 instances of fishing boats apparently switching off their location transponders near West Africa's coastline — and according to a new study out of Florida International University, those same eleven ships accounted for 73 percent of the pattern. The findings point researchers toward a narrow, workable target for an otherwise overwhelming ocean surveillance problem.
The study, published in the journal Applied Geography, was led by Stephen Pires, associate chair of FIU's Department of Criminology & Criminal Justice in the Steven J. Green School of International & Public Affairs and an Institute of Environment researcher, as reported by Miami's Community Newspapers. Pires co-authored the research with Emilie Christiansen, a Ph.D. candidate studying international crime, and Dylan Spencer, a researcher at Georgia Southern University. Using publicly available Global Fishing Watch data, the team examined dark journeys off the coasts of 23 West African countries, including roughly 1,058 journeys involving vessels flying flags of convenience.
Why Flags of Convenience Matter
Flags of convenience are a legal practice in which a vessel is registered in a country other than its own, often because doing so provides economic or regulatory advantages where enforcement and oversight are weaker, per the same account of the study. Pires said flags of convenience and transponder gaps are associated with an increased risk of illegal, unreported and unregulated fishing. The researchers stressed that a transponder gap alone does not prove a vessel is fishing illegally — AIS disabling can also stem from legitimate technical or operational issues, Christiansen noted.
What the data did show is that the same vessels repeatedly went dark in particular locations, a pattern the researchers say mirrors how land-based crime behaves. The team applied crime-science methods typically used to study street crime to the open ocean, reflecting the fact that crime tends to cluster in places with high opportunity and low risk of detection. Dark activity was concentrated around the Gulf of Guinea and near the outer edges of countries' exclusive economic zones, the report's movement-pattern analysis found.
Boundaries Where Enforcement Gets Fuzzy
Convenience-flagged dark journeys began or ended beyond an EEZ boundary in nearly half of all cases studied, according to the research. Those boundary zones carry shifting enforcement responsibilities and limited patrol resources even though they sit near some of the most productive fishing grounds. Convenience-flagged vessels were nearly twice as likely as other vessels to begin or end a dark journey beyond an EEZ, and they averaged about five times as many dark journeys per vessel compared with non-convenience-flagged ships.
Pires said the practical takeaway is that authorities should target and board chronic offenders rather than attempt to inspect every vessel at sea. The study identified 11 such chronic offenders over the three-year period. He suggested authorities could inspect those vessels' documentation and investigate possible labor abuse or trafficking of illicit products, and that vessels found engaging in illegal behavior could be removed from these waters — a step Pires said can lead to a significant reduction in illegal activity.
Deterrents Beyond the Boarding
Pires also pointed to vessel confiscation and loss of insurance coverage as potential deterrents, noting that losing insurance could deprive shipping companies of access to many ports worldwide. He additionally recommended geofencing technologies to monitor vessel movements in and out of high-risk zones, systems that could trigger alerts when a ship's AIS is disabled, when it enters a high-risk area, engages in a potential ship-to-ship encounter, or visits a known port of convenience.
Mike Heithaus, a marine biologist and FIU's vice provost of environment and environmental resilience, said the research may help keep fisheries healthy and operations sustainable, calling illegal fishing one of the biggest threats to ocean health. FIU continually monitors illegal fishing activity in the eastern Pacific Ocean off South America and Mexico as well, with similar research emerging from the university's National Security Research Hub. Illicit fishing carries negative effects on regional economic and political stability and natural resources, beyond the direct toll on fish stocks.
The Scale of the Global Dark-Fleet Problem
West Africa's stakes in this fight are steep: the region suffers an average economic loss of $2 billion annually from maritime resource theft and illegal, unreported and unregulated fishing, according to data from the Economic Community of West African States reported by the UN Food and Agriculture Organization. In November 2025, the European Union allocated 59 million euros, or roughly $68 million, over five years for the West Africa Sustainable Ocean Programme across 13 coastal nations, aiming to curb illegal fishing and restore marine ecosystems; fishing and aquaculture support about 15 percent of ECOWAS GDP, as reported by Mongabay.
The gap FIU's team is studying is not unique to West Africa. A 2024 global study published in Nature by Global Fishing Watch estimated that roughly 75 percent of the world's industrial fishing vessels operate dark, without broadcasting public tracking positions. A separate 2022 analysis by NOAA Fisheries, UC Santa Cruz and Global Fishing Watch examined 28 billion AIS signals and found more than 55,000 instances of intentional transponder disabling, obscuring an estimated 6 percent of all global fishing vessel activity, per NOAA Fisheries.
A Patchwork of Rules on the High Seas
Part of the challenge is regulatory. Under Chapter V, Regulation 19 of the International Maritime Organization's SOLAS Convention, AIS transponders are required on ships of 300 gross tonnage and above engaged on international voyages, and on cargo ships of 500 gross tonnage and above not engaged on international voyages — a rule aimed at collision avoidance rather than fisheries surveillance, according to the International Maritime Organization. That leaves smaller commercial fishing vessels governed primarily by individual coastal nations or regional agreements rather than a single international mandate.
Developed regions have moved to close that gap on their own terms. The European Union has required Class A AIS devices on all EU-flagged fishing vessels over 15 meters since 2014, while the U.S. Coast Guard began requiring AIS on commercial fishing vessels 65 feet or longer in U.S. waters in 2016, according to the USCG Navigation Center. The IMO also expanded its Ship Identification Number Scheme in December 2017 to cover motorized inboard fishing vessels down to 12 meters operating outside national jurisdiction, giving authorities a way to track vessels even when flags or names change.
Pires brings a track record in applying criminology to environmental crime; he previously analyzed 31,270 large-scale U.S. wildlife trafficking incidents and found 83 percent entered the country via air cargo and personal baggage, according to FIU News. FIU was separately ranked No. 1 in the U.S. for its impact on UN Sustainable Development Goal 14, Life Below Water, in the 2026 Times Higher Education Impact Rankings, per the university's Institute of Environment. For now, Pires's core message is that authorities don't need to chase every vessel on the water — they need to identify the chronic offenders and board them to ask questions, request documentation, and check for other crimes.
