US scientists say sea otters in the area where the Exxon Valdez oil spill occurred showed appreciable harm a decade after the accident.

The tanker went aground in Prince William Sound, Alaska, in March 1989, spilling an estimated 42 million litres of crude oil.

By September that year, almost 1,000 dead otters had been found in the spill area, and in some oiled areas even now otter densities remain at about half their densities before the spill.

And the researchers say otters in the western part of the sound "have experienced significant long-term negative effects from the spill".

They say: "While lingering effects of acute oil exposure may account for much of the longer-term spill effects, less direct impacts are also likely to have occurred, due either to maternal influences or to continued exposure to oil residues."

And they believe that much of the apparent recovery in oiled areas can be explained by the arrival of otters from unaffected parts of the sound.

Assessment difficulties

The authors are from the US Geological Survey's Alaska Biological Science Center, the University of California, Santa Cruz, and the US Fish and Wildlife Serive.

Their findings are published in the Proceedings of the National Academy of Sciences.

The researchers say the chronic effects of the Exxon Valdez spill and similar environmental accidents have been difficult to prove, partly because of a lack of reliable data on the density and distribution of animal populations before the accidents.

They tried to overcome this problem by inferring mortality patterns from the age distribution of otters found dead in the vicinity of the spill each year between 1976 and 1998.

They used a simple demographic model of age-specific survival rates over time, and a statistical method known as maximum likelihood, to predict the observed age distributions of sea otter carcases collected from beaches each year after the spill.

Cautious optimism

In this way, they found that otters in the area had decreased survival rates in the years after 1989, and animals born after the spill continued to be adversely affected for almost a decade.

They write: "These findings demonstrate that a combination of population data, demographic modelling techniques, and statistical analyses can demonstrate otherwise difficult-to-detect long-term impacts of environmental disasters.

"The same general approach might be used to study lingering effects of other environment accidents on different animal populations."

The authors conclude: "While our findings document continuing demographic effects of the Exxon Valdez oil spill, we also show that these effects have gradually dissipated with time - largely because of the death of cohorts most affected by the spill.

"This finding suggests that cautious optimism is warranted concerning the gradual return of the ecological communities of western Prince William Sound to pre-spill conditions."