Five years after the BP well explosion, there is no single, conclusive answer to how the Gulf of Mexico is doing, but there are many questions. Here are some of them:
WHAT DO SCIENTISTS SAY?
To assess the health of the Gulf of Mexico, The Associated Press surveyed 26 marine scientists about two dozen aspects of the fragile ecosystem to see how this vital waterway has changed since before the April 2010 spill. On average, the researchers graded an 11 percent drop in the overall health of the Gulf.
The surveyed scientists on average said that before the spill, the Gulf was a 73 on a 0 to 100 scale. Now it's a 65. In the survey, scientists report the biggest drops in rating the current health of oysters, dolphins, sea turtles, marshes, and the seafloor.
WHAT HAPPENED TO DOLPHINS?
Common bottlenose dolphins have been dying at a record rate in northern parts of the Gulf of Mexico since the BP spill, according to NOAA and other scientists who have published studies on the figures. From 2002 to 2009, the Gulf averaged 63 dolphin deaths a year. That rose to 125 in the seven months after the spill in 2010 and 335 in all of 2011, averaging more than 200 a year since April 2010.
That's the longest and largest dolphin die-off ever recorded in the Gulf. But the number of deaths has started to decline, said Stephanie Venn-Watson, a veterinary epidemiologist at the Marine Mammal Foundation and a lead author of studies on the dolphin mortality.
In its report on the Gulf five years after the spill, BP said necropsies of dolphins and "other information reveal there is no evidence to conclude that the Deepwater Horizon accident had an adverse impact on bottlenose dolphin populations."
WHAT HAPPENED TO TURTLES?
The endangered Kemp's Ridley sea turtle used to look like a success story for biologists. It was in deep trouble and on the endangered list, but a series of actions, such as the use of turtle excluder devices, had the population soaring and it was looking like the species soon would be upgraded to merely threatened, said Selina Saville Heppell, a professor at Oregon State University.
Then, after the spill, the number of nests dropped 40 percent in one year in 2010. "We had never seen a drop that dramatic in one year before," Heppell said. The population climbed in 2011 and 2012 but then fell again in 2013 and 2014, down to levels that haven't been that low in nearly a decade, she said.
There is not enough data or research to blame the oil spill with scientific rigor, "but it's a remarkable coincidence, isn't it?" Heppell said. BP in its report said: "The changing nesting trends could be due to many factors including natural variability and record cold temperatures."
WHAT HAPPENED TO FISH?
University of South Florida marine scientist Steve Murawski sees problems — tumors, lesions and oil traces in internal organs — in key fish such as red snapper, kingsnake eels and especially tilefish. Carcinogenic chemicals associated with oil appear to have gotten through the skin of these bottom-dwelling fish, he said.
"Their livers have fresh Macondo oil in them," said Samantha Joye of the University of Georgia.
BP's report said commercial catches for finfish "continue to exceed immediate pre-spill levels."
WHAT HAPPENED TO BIRDS?
There have been at least two surveys of bird populations in Barataria Bay, the scene of the heaviest oiling and an important stopping place for numerous migratory bird species. Those surveys of shore birds and migratory birds found no obvious problems. But a recent study of native seaside sparrows in Barataria has found bird counts down.
BP said "analysis and field observations conducted to date indicate any impacts on bird populations and nesting were limited and were followed by a strong recovery."
HOW ARE THE MARSHES?
Oil hit about 620 miles of Louisiana's marshland. A lot of science has gone into studying the spill's effects on the marsh, in particular in the Barataria Bay area. And Barataria is not a pretty picture. Tar balls and mats are routinely found here. Fishing remains closed in parts of the bay.
An entire mangrove island, an important bird colony, has nearly disappeared under the water. Satellite imagery shows that about a foot of marsh has been eaten away along many shorelines here. In the plants and animals scientists have identified oil contamination and they are tracking its progression in fish, birds, mice, dolphins and insects.
BP said by 2014, "only 0.7 miles remained heavily oiled."
HOW ARE THE BEACHES?
After an intense focus on cleaning up the Gulf's beaches, traces of the spill are hard to find along the sugar white sands of Florida, Alabama and Mississippi. But there are places, in particular at the extremities of south Louisiana, where large oil mats are resting, getting churned up by waves and engrained with sand deposits and the fragile delta ecosystem already stressed by sea level rise, hurricanes and a host of other man-made harms.
WHERE DID THE OIL GO?
"It's not all gone," said former U.S. Geological Survey chief Marcia McNutt. Her team calculated that most of the oil evaporated, dissolved or dispersed. Two peer-reviewed studies by separate respected teams in 2014 and 2015 found that up to 10 million gallons of oil is left on the seafloor; one of them compared it to a bathtub ring. BP disputed those figures.
WHAT DON'T WE KNOW?
National Oceanic and Atmospheric Administration chief scientist Richard Spinrad said the government hopes to finish its five-year assessment on the health of the Gulf by the end of the year, so it is too early to make any real conclusions. Some problems may show up later. It was not until 10 years after 1989's Exxon Valdez spill that scientists noticed a dramatic crash in the vital herring population.
Borenstein reported from Washington, Burdeau from New Orleans. Reporter Stacey Plaisance contributed from New Orleans.