The Exxon ValdezTwenty years ago last month, the supertanker Exxon Valdez struck a reef in Prince William Sound and ran aground, releasing 40 million liters {approximately 10 million gallons) into the surrounding sea and onto the beaches. It remains the worst oil spill in US maritime history. In the days that followed, impact inventories revealed the lethal outcome: a quarter of a million sea birds had been killed, along with 22 Orca whales, nearly 3000 sea otters, 300 harbor seals, and unknown millions of fish eggs.

In 1991, the Alaskan and US Governments reached an agreement with Exxon Mobil in a 900 million dollar settlement, almost 200 million of which was set aside for scientific study of the disaster and its impact on the PWS ecosystem. Exxon Mobile also funded its own studies (generating 400 papers and reports) which were frequently in disagreement with the government scientists’ reports and findings.

Twenty years after, the Exxon Valdez spill has become the most studied maritime, industrial disaster ever. A news report in Science Magazine (March 26, 2009) by Lila Guterman (with Jacopo Pasotti reporting) presented some of the scientific findings from the post-spill research.

One of the principle foci of these post-spill studies has been determining the fate of any remaining oil. As late as 2001, a NOAA research team (lead by Jeffery Short) conducted random sample analyses from over 90 beaches in PWS. The team estimated that some 55, 000 liters of oil remain—spread out over 11 hectares of shoreline and beaches (note: NOAA states that only 2% of all the spilled oil remains). This figure was originally criticized as too high by Exxon Mobile scientists, who later nonetheless came to be in agreement with it. Still, industry employed scientists continue to doubt any significant, negative impact on wildlife from the remaining oil.

However, in a NOAA follow up study in 2005 of the same beaches tested in 2001, analysis showed that the remaining oil was decaying at a maximum of just 4% annually (with some samples showing zero decay). The prognosis: oil residue will persist for up to a century. In a second follow up study by Short’s team (published in the journal Marine Environmental Research, in 2007), results showed that bio-active contaminants in these ecosystems were predominantly from the oil spill. Short also believes that “bio-markers” (such as the presence of certain enzymes in animal livers) indicate repeated exposure
of organisms to the oil.

More debate and questions remain about the Exxon spill.

NOAA scientists inspect and collect samples from the Exxon Valdez oil spillWhat caused two Orca whale pods (observed in the oil slick in 1989) to lose 40% of their members? One of these pods is slowly recovering, but the second, originally composed of 22 members, has now lost all of its female members, making the survival of the pod impossible. Government scientists assert that the sudden and coinciding pod declines were caused by the oil spill—either through breathing the fumes or eating contaminated prey. However, Exxon scientist assert that the pod declines can not be conclusively linked to the spill.

Sea otter populations were also heavily impacted. While most of these populations around the sound have rebounded, many populations that inhabited the worst stricken areas back in 1989 remain notably low. According to a US Geological Survey report, oil remains in the shallow, intertidal zones of many of these beaches, and that digging in these zones by otters (an activity comprising 18% of sea otter dives) continually exposes them to oil residues. To what extent this exposure has prevented these populations from rebounding (perhaps due to hydrocarbon impacts on otter fertility cycles) may be a question that can never be answered satisfactorily.

One issue that scientist on both sides do agree on: the Pacific herring population has declined dramatically (by 85%). But was this due to the oil spill? This is difficult to determine since deeper water animals (like fish) may not feel the impact of such spills immediately; the herring population did not crash until 1993, one year following a particularly weak plankton bloom, which may have left the fish hungry and compromised their immune systems. Using this information, researchers from  the British Columbia Ministry of Agriculture and the University of Alaska have developed a model that faithfully replicates the “busts” and “booms” of the herring population for the past 15 years. However, Richard Thorne, of the Prince William Sound Science center, had conducted a hydro-acoustic monitoring survey, using sonar to count fish. His results matched well with the aerial surveys of herring spawn which have been conducted every year for the past thirty. The comparisons indicate that the collapse began in 1989, the same year as the spill. Thorne believes that the spill, followed by three years of unchecked fishing, have caused the herring decline. But due to a lack of pre-spill data on this fish population, the mystery as to why the herring are not coming back remains.
For the future, marine scientist are focusing their efforts on understanding how such a spill combines with other impacts (disease, predation, over-fishing and climate change) to cause animal declines. One route to species recovery is simply to  protect the species from fishing and other impacting factors. One idea here is to allow targeted fishing of a the Pacific Herring’s natural predators (pollack). Another is to construct hatcheries for the herring. Both of these bio-remediation strategies carry risks, known and unknown, but, it is believed, they can hardly be worse for these ecosystems than the original spill. There is one other approach: simply waiting ; (although unsuitable for commercial and industrial purposes) many declining species recover over time, as long as threats to that recovery do not multiply.

Video: solver solution for cleanup of the remaining oil from the Exxon Valdez Spill:

Image Credits: EPA, Environment Canada, UNEP, NASA, NOAA,

Exxon Valdez Oil Spill Trustee Council, State Of Alaska on