Skytruth Finds Ship Oil Slicks Cover 1.6 Million Square Kilometers

Oil slicks from ships and offshore platforms cover nearly 1.6 million square kilometers of the ocean surface annually, an area slightly larger than Mongolia, according to a report by the U.S.-based nonprofit SkyTruth. Mongabay reported that the organization used satellite imagery, machine learning, and vessel tracking data to trace global ocean oil pollution between 2023 and 2025.

Satellite Imaging Exposes Hidden Sea Discharges

The bottom of a vessel’s hull accumulates this contamination during operations as bilge waste, a combination of water and oil. John Amos, CEO of SkyTruth, told Mongabay by phone that vessel operators previously felt confident that nobody would notice illegal dumping in the middle of the open ocean. Tankers also routinely wash out their cargo holds and flush crude oil residue straight into the sea. Amos described the discharged substance as nasty oily dirty gunk that crews want to get rid of cheaply.

SkyTruth modeling determined that vessels were responsible for 81% of the total slick area, primarily cargo ships and tankers. Offshore oil platforms and other marine infrastructure accounted for 18% of the pollution. Most of these routine discharges violate MARPOL, the international treaty governing ship pollution. Under a 1973 MARPOL agreement, ships are allowed to release bilge water as long as the concentration does not exceed 15 parts of oil per million parts of water.

Strait of Malacca Waters Show Heaviest Concentrations

Indonesian waters recorded the highest number of verified slicks in the report at 823, with neighboring Malaysia registering 214. Both Southeast Asian nations share the Strait of Malacca, which ranks among the busiest shipping lanes in the world. Amos stated that the sheer volume of large cargo ships and oil tankers transiting those waters is largely responsible for the high tally.

SkyTruth successfully located ownership records for only 9 of the 20 vessels flagged as repeat polluters. Two of those identified ships belong to Indonesia’s PT Waruna Nusa Sentana, while another two are registered to Nigeria’s Sea Transport Services Nigeria Ltd. Mongabay reached out to both shipping companies for comment but received no response prior to publication.

Economic Pressures Drive Illegal Dumping

Amos indicated that the global estimate represents an undercount because proper waste disposal is costly. Operating a large cargo ship costs tens of thousands of dollars per day, creating a strong financial incentive to avoid port detours for legal waste management. Operators can either pay for proper disposal facilities or wait until they reach remote ocean areas where they believe surveillance is absent.