Oil from the 1989 Exxon Valdez spill is still being ingested by Alaskan wildlife more than 20 years after the disaster, says a team of international biologists.

The research, led by a Simon Fraser University scientist and published in the April issue of Environmental Toxicology and Chemistry, measured a biomarker called CYP1A, which indicates exposure to oil. The scientists measured CYP1A levels in harlequin ducks.

In this April 9, 1989, photo, crude oil from the tanker Exxon Valdez, top, swirls on the surface of Alaska's Prince William Sound near Naked Island. (John Gaps III/Associated Press)"We found (the ducks') CYP1A levels were unequivocally higher in areas oiled by the Exxon Valdez spill than in nearby areas, a conclusion supported by multiple samples and two independent laboratories," SFU wildlife scientist Daniel Esler said in a release.

"We believe it is important to recognize that the duration of presence of residual oil and its associated effects are not limited to a few years after spills, but for some vulnerable species may occur over decades."

The Exxon Valdez spill happened in 1989 when a huge oil tanker hit a reef in Prince William Sound, on the south coast of Alaska. The spill caused 50 million litres of crude oil to be dumped into the water, extending more than 3,300 square kilometres and devastating fish and wildlife.

It is still regarded as one of the most damaging instances of human-caused contamination, and its effects on wildlife and humans have been debated by biologists, ecologists and the oil industry ever since.

Previous research has linked controlled doses of oil in wildlife to reproduction problems, reduced weight gain and increased organ weight.

Linked to invertebrates

The SFU-led team looked at harlequin ducks because they eat invertebrates that live in inter-tidal areas and can't metabolize residual oil very well.

"Our research has shown that oil remaining in the area, particularly in inter-tidal areas, was encountered and ingested by some near-shore animals," said Esler.

In 2006, researchers at the National Marine Fisheries Service in Juneau, Alaska, found oil buried in shoreline sand and silt that is exposed only at extremely low tides.

Limited exposure to weathering and other elements means the oil is less likely to degrade. That's a concern because those areas are prime feeding grounds for sea otters, ducks and other wildlife.