Seabirds' blood can hold vital clues about the long-term ecological impacts of oil spills, researchers suggest.
Gulls in affected areas had a higher level of pollutants in their blood
Scientists collected samples from gulls in north-west Spain, close to where the Prestige tanker sank in November 2002.
Seventeen months after the disaster, concentrations of toxic compounds in the birds' blood were, on average, 120% higher than normal.
The team hopes its findings will offer a way to collect data about oil spills' accumulative effect on wildlife.
Writing in the American Chemical Society's journal Environmental Science and Technology, the Spanish scientists explained why seabirds could be considered good "bioindicators" of lasting pollution.
"Life history characteristics of seabirds make them particularly vulnerable to oil pollution because they spend much of their lives on the ocean's surface, and because their populations concentrate in habitats prone to high oil exposure."
They added that the birds also were also high up the food chain, making them "good candidates to monitor the marine ecosystem".
Between May and June 2004, the researchers collected blood samples from 61 adult yellow-legged gulls (Larus michahellis) breeding in seven colonies along the north-west Spanish coast.
An estimated 300,000 birds died as a result of the Prestige spill
Four of the colonies were chosen because the sites were in the path of the Prestige spill, which dumped about 60,000 tonnes of oil into coastal waters and has been described as one of Europe's worst wildlife disasters.
Samples from birds within the affected colonies had concentrations of polycyclic aromatic hydrocarbons (PAHs) that were on average 120% higher than those taken from gulls at the oil-free sites.
PAHs are compounds linked to carbon-rich fuels, such as wood, coal and oil. The presence of the compounds in the environment cause concern because of their known toxic and bio-accumulative effects in animals.
"The major sources affecting the presence and distribution of PAHs in the environment are anthropogenic," the team wrote.
"In the marine environment, these include large oil spills, oil discharges from ships, and activities associated with offshore oil and gas exploration."
Building an understanding
While the immediate aftermath of an oil spill are highly visible and well documented, the researchers said the non-lethal chronic effects had rarely been studied.
"Very few studies have monitored PAH concentrations in bird tissues and these studies were mainly based upon examinations of birds found dead or sacrificed.
"Scarcity of data probably reflects the view that vertebrates are not good models to assess oil contamination because of their great ability to absorb and metabolise PAHs."
Dead birds only reveal part of the picture (Image: WWF-Canon/Jorge Sierra)
However, measuring the pollutants in blood samples offered a greater degree of accuracy, they explained.
"Since blood cells are continuously being produced and have a lifespan of several weeks, the presence of PAHs probably indicates a recent incorporation."
From this data, the researchers suggested that it would be possible, over a period of time, to build up a better understanding of how an area was affected by an oil spill, and measure its recovery.
For example, the team took further blood samples from gulls in the affected colonies 12 months later and found that PAH concentrations had decreased by about a third.
"To our knowledge," they wrote, "this is the first field study in which levels of PAHs were measured non-destructively in a vertebrate with the purpose to monitor oil pollution.
"Overall, our study provides reliable support to the potential use of seabird blood as a monitoring tool for oil exposure."