Four years after the largest oil spill in U.S. history, wildlife in the Gulf of Mexico is still struggling to recover, according to a recent National Wildlife Federation report.

The report focused on 14 species affected by the spill and used data from independent scientists and the National Ocean and Atmospheric Administration.

It found that many species, particularly bottlenose dolphins and sea turtles, are dying in record numbers, and the NWF says the animals' deaths are connected to the Gulf oil spill.

"Four years later, wildlife in the Gulf are still feeling the impacts of the spill," Doug Inkley, senior scientist for the NWF, said in a teleconference. "The science is telling us that this is not over."

The spill began after the Deepwater Horizon oil rig exploded on April 20, 2010, killing 11 people and spilling more than 200 million gallons of oil into the ocean.

Thousands of animals were slicked with oil and died within the first few months of the spill, but scientists say it takes years to track the most profound environmental damage.

Researchers point to Alaska's 1989 Exxon Valdez oil spill as an example. After 25 years, some species still haven't fully recovered.

Various government agencies, as well as nonprofits like the NWF, have been studying the Gulf region since the spill and tracking the impacts of the oil.

Findings from the NWF's report include the following:

  • More than 900 bottlenose dolphins have been found dead or stranded in the area of the spill since April 2010, which is more than scientists have seen in the past decade. Area dolphins are also underweight and anemic and show signs of liver and lung diseases.
  • About 500 dead sea turtles have been found in the region annually since 2011. All five species of sea turtles in the Gulf are listed as threatened or endangered.
  • Nearly 1,000 brown pelicans have been collected since the spill and half of them have died.
  • Other coastal birds have increased concentrations of toxic oil compounds in their blood.
  • Sperm whales that swam near the spill have higher levels of DNA-damaging metals, such as nickel and chromium, in their bodies than they did before the spill.
Despite these findings, BP, the British oil company responsible for the spill, says the report "is a piece of political advocacy — not science."

"The report conveniently overlooks information available from other independent scientific reports showing that the Gulf is undergoing a strong recovery," BP said in a statement provided to National Geographic.

The Gulf oil spill launched thousands of lawsuits, as well as a federal trial that's still ongoing.

BP pleaded guilty to 11 felony counts of misconduct and negligence related to the oil rig deaths, as well as misdemeanors under the Clean Water Act and the Migratory Bird Treaty Act.

The oil giant has agreed to pay a $4 billion fine, making it the largest corporate criminal penalty in U.S. history.

The third and final phase of BP's trial is scheduled for January 2015.

Related on MNN: