Only 474 out of more than 90,000 oil slicks from ships worldwide were reported to authorities over five years, with hardly any resulting in penalties, according to new findings.
The data, obtained by the Guardian and Watershed Investigations from Lloyd’s List, covers pollution reports between 2014 and 2019. This pales in comparison to a scientific study using satellite images that detected thousands of oil slicks from ships during the same period.
New research from Florida State University reveals that all ship-related oil slicks visible by satellite are illegal because they exceed pollution limits by at least a thousand times. Many of these spills occur when ships deliberately dump oily bilge water to maintain stability.
“The scale of unreported pollution is staggering,” said Dr. Elizabeth Atwood from Plymouth Marine Laboratory. “Some argue that natural oil seeps contribute equally, but recent research proves this isn’t true for most of the world’s oceans.”
Hugo Tagholm of Oceana UK called the findings “appalling,” highlighting both the severe pollution and the shockingly low reporting rates.
Scientists analyzing satellite images from 2014 to 2019 found that 20% of slicks—90,411 in total—came from ships, covering an area roughly the size of Italy. These spills clustered along major shipping routes. In contrast, just 2% came from oil platforms and pipelines, while 6% originated from natural seabed leaks. The rest were from land sources or unidentified vessels.
“This data proves these spills are persistent, widespread violations,” said Ian MacDonald, a retired oceanography professor and co-author of the study.
The research also confirms, for the first time, that properly treated ship discharges leave no visible traces—meaning most spills are deliberate.
“Bilge dumping has been a global problem for decades,” said lead researcher Carrie O’Reilly.
Yet under the international Marpol treaty, only a tiny fraction of these violations face consequences.
“Taking legal action against well-funded shipping companies is costly and time-consuming,” said MacDonald, explaining why so few cases are pursued.
Even in European waters, where pollution laws are stricter and satellite monitoring is used, enforcement remains weak. A March report by the European Court of Auditors found that fewer than half of detected spills are investigated, with pollution confirmed in just 7% of cases.
“Polluting ships still slip through the cracks,” said auditor Nikolaos Milionis, citing gaps in both technology and enforcement.
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Illustration: Guardian Design / Rich Cousins
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Auditors found that ship pollution is a serious issue, with over three-quarters of European seas affected. The EU’s goal of zero pollution to protect health, biodiversity, and fish stocks now seems unattainable.
A recent study of satellite images from 2021-2022 along six West African coasts revealed that 16% of oil slicks—covering an area roughly 28,800 football pitches in size—came from ships. Yet, international marine pollution authorities recorded no incidents during that time off the coasts of Benin, Ghana, Ivory Coast, Sierra Leone, Liberia, or Togo.
Experts warn these ongoing spills threaten marine life.
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“All visible slicks harm marine life, especially since even trace amounts of oil damage plankton, the foundation of the ocean food chain,” said O’Reilly.
“Individually, spills may seem small, but with increasing ship traffic and more frequent slicks, the cumulative impact is significant,” added McDonald.
Much of this pollution comes from bilge dumping. Ships collect oily, potentially toxic water in their bilges—the lowest part of the hull. If not removed, bilgewater can destabilize ships and corrode structures. While large ships must treat bilgewater before release, some bypass the process.
“A poorly maintained ship might constantly fill its bilge,” said an anonymous mariner. “Separating oil takes time, so crews skip it or use broken equipment.”
Disposing of bilgewater at ports is costly. “Shipping is profit-driven,” the mariner explained. “Ports are expensive—fees add up, so minimizing time there saves money.” He also questioned whether port inspections are thorough enough.
An IMO spokesperson stated: “Bilgewater discharges are regulated under Marpol Annex I. Ships must log all oil transfers, and flag and port states enforce compliance.”
Ocean advocates like Tagholm urge stronger action. “Tankers carrying fossil fuels make up a third of shipping,” he said. “Ending our reliance on these fuels—essential to prevent climate disaster—would also stop much of this pollution.”It would also prevent the devastating effects of these spills, which can damage the very base of ocean food chains.
This story was created in collaboration with the Pulitzer Center’s Ocean Reporting Network.
