It has been five years since the worst oil spill in U.S. history, a tragedy that killed 11 people and choked local ecosystems with millions of barrels of oil. The Gulf of Mexico seems to be doing OK now, given the circumstances, and a 2015 report by BP even boasts "strong signs of environmental recovery."

The Gulf has proved resilient overall, but a recent spate of wildlife declines is raising doubts about the depth of its recovery. In 2014, for example, dolphins turned up dead along Louisiana's coast at four times the historical average, and research has shown that dolphins living near the spill site are five times more likely to suffer from lung disease than dolphins living farther away in Florida.

The spill also killed about a third of all laughing gulls in the northern Gulf, along with 12 percent of brown pelicans. Coral reefs still show signs of oil damage, and scientists recently found an oil "footprint" staining 9,200 square miles (2,400 square km) of seabed around the spill site. Last month, the National Wildlife Federation (NWF) identified at least 20 species still reeling from the 2010 spill.

One of the most troubling declines, however, is that of the Kemp's ridley sea turtle. The critically endangered reptile tumbled near the brink of extinction last century, battered by human activities such as egg collection, beach development, ocean pollution and "bycatch" in fishing gear. Conservation efforts have helped the species claw back over the past 30 years — from a record-low 702 Kemp's ridley nests counted in 1985 to about 21,000 in 2009 — averaging 15 to 18 percent annual growth.

But things took a turn for the worse in 2010, with nest numbers abruptly falling by 35 percent at primary nesting beaches. 2011 and 2012 saw slight increases, although not at the pre-spill pace, and now the number of nests is falling again. The 2014 nest total was the lowest in eight years, according to the U.S. National Oceanic and Atmospheric Administration (NOAA), dipping below even the 2010 total.

The graphs below show the number of Kemp's ridley nests at the species' three main nesting beaches from 1966 to 2013, followed by the average hatchlings per nest during the same period:


Kemp's ridley sea turtle hatchlings


It's unclear whether this is related to the 2010 spill, especially since sea turtles of all kinds still face a barrage of everyday dangers like bycatch and ocean plastic. And Kemp's ridleys are vulnerable even by sea turtle standards: While other species are known to range around the planet, they're almost entirely limited to the Gulf of Mexico and the U.S. Atlantic Seaboard. They also tend to keep their eggs in relatively few baskets, nesting in large congregations known as "arribadas" that squeeze 90 percent of their entire species into a handful of beaches in Mexico and Texas. 

Some researchers suggest the decline may be driven by factors beyond the oil spill. Wild weather of recent winters could have shocked the cold-blooded animals with cold water temperatures, for example, a common problem for sea turtles in general. Kemp's ridleys might even be victims of their own success, having rebounded too quickly in recent decades for the embattled Gulf ecosystem to sustain them.

Yet the speed of the drop hints at something big and traumatic, and Kemp's ridleys had ample exposure to oil during and after the spill. "Research has found that critical sea turtle foraging areas and migration routes overlap significantly with areas affected by oil from the spill," NOAA points out. This has led many experts to suspect oil is responsible — and to worry if the worst is yet to come. Kemp's ridleys don't start reproducing until around age 10, so it could be years before the spill's full impact is known.

"The recovery of the Kemp's ridley, which once seemed inevitable, may now be in doubt," the NWF warns in its new report. "Scientists are currently trying determine whether the decrease in nests is due to increased mortality alone, or if adult females may be less healthy and therefore less able to reproduce. This health effect could have been caused by exposure to oil or by a reduction in the available food supply, such as blue crabs. Preliminary studies indicate that the Kemp's changed foraging habitat in 2011 and 2012, but the significance of this change is not well understood."

Kemp's ridley sea turtle

This oiled Kemp's ridley sea turtle was spotted near the BP oil spill site on June 13, 2010. (Photo: Kate Sampson/NOAA)

The species' outlook may become a little clearer later this year, New Scientist reports, with new status reviews expected from NOAA and from the International Union for Conservation of Nature.

For now, though, many female Kemp's ridleys in the Gulf have something more urgent on their minds: nesting season, which begins in May. If all goes well, they'll lay two to three clutches of about 100 eggs each, which will take roughly two months to incubate. A torrent of tiny hatchlings will then dodge various predators as they run back home to the sea, where they'll hopefully thrive for the next decade before the females eventually return to nest on the same beach sometime around 2025 or 2030.

The video below — from 2010, of all years — shows a group of newborn Kemp's ridleys scrambling to the sea with human help. They may face an ocean of natural and man-made dangers once they get there, but any animal that can endure this kind of gauntlet so soon after birth, over and over for millions of years, has more grit than we realize. And as long as we share the ocean with them, they'll need it.

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Russell McLendon ( @russmclendon ) writes about humans and other wildlife.