Oil slicks from ships and offshore oil platforms cover nearly 1.6 million square kilometers of the ocean surface annually, an area slightly larger than Mongolia, according to a report by Skytruth. The U.S.-based nonprofit used satellite imagery, machine learning, and vessel tracking data to trace global ocean oil pollution between 2023 and 2025, revealing a scale of maritime contamination that was previously difficult to track in remote waters.
Satellite Data Exposes Hidden Ocean Pollution
Historically, illegal discharges at sea went largely unnoticed. John Amos, CEO of Skytruth, noted that vessel operators could previously act with the confidence that no one was watching them in the middle of the ocean. Satellite monitoring and vessel-tracking technology have altered that dynamic, making visible what was once hidden beneath the horizon.
Vessels account for 81% of the total slick area identified in the study, with cargo ships and tankers making up the vast majority of that total. Offshore oil infrastructure, such as drilling platforms, accounts for 18% of the recorded slicks.
Bilge waste and tanker cleanouts violate MARPOL treaty
The majority of vessel-derived oil slicks originate from bilge waste—a mixture of oil and water that collects naturally in the bottom of a ship’s hull during routine operations. In addition to bilge accumulation, tankers frequently wash out their cargo holds and flush residual crude oil directly into the sea. Amos described this waste as nasty, oily gunk that operators want to remove cheaply.
Under international law, these practices violate the MARPOL treaty, the primary international agreement governing marine pollution from ships. Adopted in 1973, MARPOL permits vessels to discharge bilge water only if it contains no more than 15 parts of oil per million parts of water. The routine flushing of heavy crude residue, however, far exceeds these legal thresholds.
Economic Pressures Fuel Illegal Dumping Practices
Compliance with international environmental rules carries a steep financial cost for shipping companies. Retaining oily waste onboard requires vessels to make costly detours to ports equipped with proper disposal facilities. Because a single day at sea costs a large cargo ship tens of thousands of dollars, operators frequently choose to discharge waste directly overboard in remote areas.
Skytruth’s modeling suggests that the current global estimates represent an undercount of actual pollution. Many operators continue to risk illegal dumping to avoid port fees and schedule delays.
Global Hotspots and Repeat Polluters
Geographically, Indonesian waters recorded the highest number of verified slicks in the report at 823, followed by Malaysia with 214 slicks. Both nations border the Strait of Malacca, which ranks among the busiest shipping lanes in the world. Amos attributed these high figures to the sheer volume of large cargo ships and oil tankers transiting through the region.
Tracing responsibility for these discharges remains difficult due to opaque ownership structures. Skytruth successfully matched ownership records for only 9 of the 20 vessels flagged as repeat polluters. Among those identified, two vessels belonged to Indonesia’s PT Waruna Nusa Sentana and two to Nigeria’s Sea Transport Services Nigeria Ltd. Mongabay reached out to both companies for comment prior to publication but received no response.
What technology and laws govern oil slick detection?
What specific technology did Skytruth use to detect ocean oil slicks?
Skytruth utilized satellite imagery combined with machine learning algorithms and vessel-tracking data to trace and map oil pollution between 2023 and 2025.
How much oil per million parts of water does the MARPOL treaty permit?
The 1973 MARPOL treaty allows ships to discharge bilge water containing a maximum concentration of 15 parts of oil per million parts of water.
Which companies were identified as operating repeat-polluting vessels in the report?
Out of 20 vessels flagged as repeat offenders, ownership records were found for 9; among those, two belonged to Indonesia’s PT Waruna Nusa Sentana and two to Nigeria’s Sea Transport Services Nigeria Ltd.