<P> Both the long - term and short - term effects of the oil spill have been studied . Immediate effects included the deaths of 100,000 to as many as 250,000 seabirds, at least 2,800 sea otters, approximately 12 river otters, 300 harbor seals, 247 bald eagles, and 22 orcas, and an unknown number of salmon and herring . </P> <P> In 2003, fourteen years after the spill, a team from the University of North Carolina found that the remaining oil was lasting far longer than anticipated, which in turn had resulted in more long - term loss of many species than had been expected . The researchers found that at only a few parts per billion, polycyclic aromatic hydrocarbons caused a long - term increase in mortality rates . They reported that "species as diverse as sea otters, harlequin ducks and killer whales suffered large, long - term losses and that oiled mussel beds and other tidal shoreline habitats will take an estimated 30 years to recover ." </P> <P> In 2006, a study done by the National Marine Fisheries Service in Juneau found that about 6 miles (9.7 km) of shoreline around Prince William Sound was still affected by the spill, with 101.6 tonnes of oil remaining in the area . Exxon Mobil denied any concerns over any remaining oil, stating that they anticipated a remaining fraction that they assert will not cause any long - term ecological impacts, according to the conclusions of the studies they had done: "We've done 350 peer - reviewed studies of Prince William Sound, and those studies conclude that Prince William Sound has recovered, it's healthy and it's thriving ." However, in 2007 a NOAA study concluded that this contamination can produce chronic low - level exposure, discourage subsistence where the contamination is heavy, and decrease the "wilderness character" of the area . </P> <P> The effects of the spill continued to be felt for many years afterwards . As of 2010 there were an estimated 23,000 US gallons (87 m) of Valdez crude oil still in Alaska's sand and soil, breaking down at a rate estimated at less than 4% per year . </P>

What were the causes of the exxon oil spill