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An internationally binding treaty known as the (PSMA) has made it harder for vessels fishing outside national waters to avoid port inspections for illegal catches, but inconsistent standards across foreign versus domestic fleets could undermine its effectiveness, according to a new study.

Illegal fishing costs countries billions of dollars in lost revenue each year due to the diversion of fish from legitimate markets. It also threatens millions of coastal livelihoods, imperils food and nutrition security, and undermines the environmental sustainability of fisheries.

"Given the potential of the PSMA to reduce illegal fishing, we wanted to explore how vessels fishing in international waters have behaved differently since it entered into force," said Elizabeth Selig, lead author of the study Sept. 5 in Science Advances and the managing director of the Stanford Center for Ocean Solutions. "Understanding patterns of vessel behavior can help countries that have ratified the PSMA identify what they might need to do to strengthen implementation."

All seafood must pass through a port to enter the market. Countries and territories that have ratified the PSMA, also known as Parties to the PSMA, are required to designate certain ports for foreign vessels to land their fish, conduct standardized inspections of arriving vessels, and deny entry of illegal catches.

Article 20 of the PSMA stipulates that inspections of domestic vessels should be as effective as measures applied to foreign vessels, but it does not specify whether the two approaches should be the same. The PSMA entered into force in 2016. As of September 5, 2025, there were , including the European Union, which signed as one party.

Every fishing vessel is registered to a single country known as its "flag state," which determines who has jurisdiction over the vessel and applicable laws onboard. A vessel is considered domestic if it offloads its catch in the same country as its flag state, and foreign if it offloads its catch in a different country.

Signs of progress

The co-authors looked at how fishing vessel behavior changed in the five years before and after the PSMA entered into force, based on satellite data curated by the nonprofit Global Fishing Watch. Their analysis focused on vessels larger than 300 gross tons, which account for the majority of fishing vessels in international waters, also known as the high seas.

The researchers estimated the size of catches delivered to ports within and outside of Parties to the PSMA by analyzing a vessel's engine power consumption and hours spent fishing. Their analysis revealed that the proportion of estimated catches landed in Parties to the PSMA doubled from 2016 to 2021.

The increase corresponds with more countries ratifying the PSMA, thereby lengthening the distance vessels must travel to land their catches in non-PSMA countries. From 2016-2021, the co-authors found that fishing vessels had to travel nearly twice as far to reach a non-PSMA country.

"These results indicate it's getting harder for fishing vessels to avoid landing in ports where countries have adopted the PSMA," said co-author Jim Leape, the William and Eva Price Senior Fellow at the Woods Institute for the Environment and co-director of the Center for Ocean Solutions.

However, the co-authors also found that fishing vessels landed a growing share of their estimated catches at domestic ports, rising from 31% in 2016 to 46% in 2021, warranting a closer look at domestic fleets.

Domestic vessels dominate

In 2021, domestic vessels fishing on the high seas accounted for 66% of port visits globally, compared to 45% of port visits in 2015, the year before when the PSMA entered into force.

"The dominance of domestic vessels in port visits globally highlights an opportunity for PSMA Parties to strengthen implementation of Article 20 by extending similar port state measures for their domestic fleets. In many cases, doing so equitably will require targeted support to bolster the human, technical, and financial capacity of port states," said co-author Colette Wabnitz, lead scientist at the Center for Ocean Solutions.

Greater attention to domestic vessels is also important because fishing vessels can opportunistically change their flags to act or operate as domestic vessels.

The researchers looked at fishing vessels that switched to a new flag state in the five years before and after the treaty entered into force. After the PSMA entered into force, they observed a 30% increase in port visits to PSMA Parties by vessels that had changed their flags, compared to the 2010-2015 baseline period. By switching to a domestic flag, fishing vessels could circumvent PSMA inspections.

"We hope this study encourages more attention on domestic fleets and consistent implementation of port state measures across both foreign and domestic fleets," said Selig. "Monitoring and inspecting vessels at port is logistically easier and more cost-effective than piecemeal enforcement at sea and is one more tool to deter illegal fishing."