There's something amiss with iconic marine animals in the Gulf of Mexico: sea-turtle populations are in retreat, dolphins are in poor shape and whales are avoiding their usual hunting grounds.
Such long-term effects seem to linger five years on from the largest oil spill in US history, which followed the explosion of the Deepwater Horizon rig on 20 April 2010, killing 11 workers and wreaking havoc on the region's wildlife.
Following the spill in the north of the Gulf of Mexico, there was a reverse in a sustained two-decade recovery of the world's most endangered sea turtle – the Kemp's ridley, which neared extinction in the 1980s.
To what extent the oil disaster is to blame is still under debate, but the matter is shrouded in mystery partly because ongoing litigation over compensation means that few scientists are prepared to discuss their data publicly.
The answer is important because, like other iconic and long-lived marine animals such as sperm whales and dolphins (see box, below), sea turtles are at or near the top of the food chain. What happens to them is therefore a bellwether for potential impacts of the spill on the rest of the marine ecosystem.
Changing fortunes
Most Kemp's ridleys lay their eggs on beaches in the Tamaulipas region of north-eastern Mexico. A joint Mexican and US conservation programme launched in 1978 had put the turtles on the route to recovery: the number of nests rose by 15 per cent per year on average, from a record low of 702 in 1985 to 21,000 in 2009, with growth accelerating to 19 per cent in the late 2000s.
Then, in 2010, it all started to unravel (see graphic, above). "Suddenly, the number of nests counted at the primary nesting beaches plummeted by nearly 40 per cent," says Selina Heppell of Oregon State University in Corvallis. Although nest numbers rose back up to 2009 levels in 2011 and 2012, they did not resume the increasing trajectory. "Now, the number of nests is declining, with 2014 showing the lowest number since 2006," says Heppell.
Was the 2010 collapse and slowdown in recovery caused by the spill, or was it a coincidence driven by other factors?
Kimberly Reich and her colleagues at Texas A&M University in Galveston presented data at a February meeting in Houston, Texas, showing that the turtles stopped foraging on the seabed in areas contaminated by oil. But there's no proof that this affected survival and precipitated the collapse of nesting numbers.
Benny Gallaway, president of LGL Ecological Research Associates in Bryan, Texas, and a consultant who has previously helped compile stock reports on Kemp's ridley turtles, speculates that the collapse was driven by an exceptionally cold year in the northern Gulf. In this area, the turtles forage and gain strength to prepare for their long migration in spring to the breeding beaches in Mexico.
Another possibility is that the turtle population has outgrown the capacity of the gulf to support it.
But Heppell says that such a sudden collapse in 2010 is not consistent with this explanation. "Nest counts alone are not enough to point the finger at BP, as there are a number of things that could affect nest numbers," she says. However, we've never seen such a dramatic drop in one year as in 2010, she adds. Also, the recovery came to an abrupt halt, and didn't slow gradually as we might expect to happen if it was coming to some sort of environmental carrying capacity.
Status reviewed
Some answers may be forthcoming with the publication later this year of an updated five-year status review for Kemp's ridleys by the National Oceanic and Atmospheric Administration's National Marine Fisheries Service, and a Red List Assessment report by the marine-turtle specialist group of the International Union for Conservation of Nature.
BP issued a pre-emptive five-years-on assessment last month. This claimed that the clean-up operation has been a success and that the impacts on wildlife have not been as damaging as some may have anticipated, with the Gulf of Mexico "rebounding".
The report glossed over the effect on the turtles, claiming that changing trends in nesting "could be due to many factors including natural variability and cold temperatures".
Conservation groups such as the US National Wildlife Federation have published reports that paint the opposite picture.
And the Natural Resource Damage Assessment Trustees, including those from NOAA say it is inappropriate and premature for BP to reach conclusions about impacts from the spill before the completion of the official evaluations by the trustees.
"There are a number of plausible explanations for the turtle decline, and long-term research is critical to finding out the real answer," says Gallaway. But with cuts in funding from US Fish and Wildlife Service's for turtle research and secrecy amid ongoing legal cases against BP, this may prove hard to do. "No one can be forthcoming [with regard to data] because they need permission from whichever side is funding them," says Gallaway. "A lot of information is therefore not available because of confidentiality agreements."
Dolphins and whales under scrutiny
Turtles aren't the only large animals that seem to have been affected by the 2010 Gulf of Mexico oil spill.
In February at the 2015 Gulf of Mexico Oil Spill & Ecosystem Science Conference in Houston, Texas, several teams presented data on the fate of large mammals, and although most were unwilling to talk to New Scientist before full publication of their results, abstracts made public at the meeting hint at their findings.
Lori Schwacke of the National Oceanic and Atmospheric Administration in Charleston, South Carolina, and her colleagues compared blood and other samples from bottlenose dolphins caught between 2011 and 2014 in Barataria Bay, Louisiana – close to the spill zone – with samples from dolphins caught in Sarasota Bay in Florida, which was not affected by the spill.
The dolphins caught a year after the spill in Louisiana were five times as likely as the Florida ones to have lung disease, and a quarter of them had poor body condition. They also had a higher prevalence of inflammation, liver disorders and iron levels, although follow-up studies in 2013 and 2014 revealed that the scale of abnormalities has gradually been decreasing.
Sylvain De Guise of the University of Connecticut in Mansfield and his team measured and compared white blood cells from dolphins in the same two populations between 2011 and 2014. Those from the spill zone had immune-system changes that left them more vulnerable to bacterial infections, especially Brucella, which is linked with die-offs of newborn dolphins in the same area.
And Bruce Mate of Oregon State University in Newport and his team fitted six whales with tags, tracking them between 2010 and 2013. They found a region of the seabed that covered 4000 square kilometres and included the spill site where the whales no longer foraged, when compared with an earlier survey that tracked whale locations between 2001 and 2005.
Mate's team proposes that sperm whales no longer forage there because contamination has reduced populations of bottom-dwelling fish and the squid that feed on them, which are in turn prey for whales. If this leads to a long-term loss of habitat, they say, there should be more concern about such effects from potential subsequent spills.
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