Nearly four years after BP’s Deepwater Horizon oil spill, evidence of the environmental damage the company caused continues to surface. A new study by scientists at Stanford University and the National Oceanic and Atmospheric Administration (NOAA) reveals how chemicals from the oil spill caused heart failure in fish, in particular, the Atlantic bluefin tuna.

BP’s Deepwater Horizon oil rig blowout and subsequent 5-month-long oil spill occurred in the major spawning area of the western Atlantic bluefin tuna during the fish’s peak spawning season. Tuna embryos and larvae were exposed to crude oil, causing a substantial decline in the population. Scientists believe that other fish spawned in the area, including blue marlin, swordfish, and yellowfin tuna, were also affected. The study is part of ongoing research into the environmental impact of the BP oil spill.

Scientists found that polycyclic aromatic hydrocarbons (PAHs), which are found in crude oil, disrupt normal heart muscle contraction and relaxation in fish. PAHs are particularly dangerous because they can linger for long periods in the environment; most do not break down quickly in the water. In other studies, PAHs have been found to cause tumors and reproductive problems in laboratory animals. They have also been linked to cancer in humans, according to the Environmental Protection Agency (EPA).

“We’ve known from NOAA research over the past two decades that crude oil is toxic to the developing hearts of fish embryos and larvae, but haven’t understood precisely why,” Nat Scholz, co-author of the study and leader of the Ecotoxicology Program at NOAA’s Northwest Fisheries Science Center, said in a Stanford press release. “These new findings more clearly define petroleum-derived chemical threats to fish and other species in coastal and ocean habitats, with implications that extend beyond oil spills to other sources of pollution.”

Exposing tuna heart cells to low amounts of crude oil allowed scientists to understand how oil affects heart functions in embryonic and larval fish. “When we see these kinds of acute effects at the cardiac cell level, it is not surprising that chronic exposure to oil from spills such as the Deepwater Horizon can lead to long-term problems in fish hearts,” Barbara Block, a professor of marine sciences at Stanford, said in the press release.

During the past year, scientists discovered that the BP oil spill has impacted dolphins and whales in the Gulf as well. In August, Ocean Alliance announced that the toxic dispersant Corexit, which was heavily-used by BP to hide spilled oil and disperse it into the ocean, is responsible for toxins found in the Gulf’s sperm whale population. Whale researchers revealed that the sperm whale population in the Gulf “may be the most polluted in the world.”

Recent research also reveals that dolphins living in one of the areas most severely impacted by the 2010 oil spill are suffering from lung problems consistent with oil exposure. Dolphins in the heavily-oiled Barataria Bay exhibited lung damage and other abnormalities not seen before in other dolphin populations. Disease conditions of dolphins in Barataria Bay were “significantly greater in prevalence and severity” than those of dolphins in a control group.

According to the Financial Times, “BP is trying to force the US government to release evidence that the company says shows its 2010 oil spill in the Gulf of Mexico has done much less damage than many people feared.” In a court filing, BP stated that there “is extensive evidence that the environmental harm was far less than the US or others feared” and that “environmental recovery is well under way.”

The company has been trying to negotiate the “penalty phase” of its trial, which will determine the fine it has to pay for violations of the Clean Water Act. BP has also complained that the US government’s assessment of evidence supposedly supporting its claims of minimal environmental damage has been “fundamentally unfair and prejudicial.” BP has also complained that the company has not been properly allowed to defend itself.

Meanwhile, the federal government is offering up more of the already-damaged Gulf to oil and gas exploration. The Bureau of Ocean Energy Management (BOEM) announced on Monday that the Interior Department will offer more than 40 million acres for oil and gas exploration and development in the Gulf of Mexico in March.

Two lease sales will be held in New Orleans on March 19 and will be the fourth and fifth offshore auctions under the Obama administration’s Outer Continental Shelf Oil and Gas Leasing Program for 2012 to 2017, according to a press release.

Alisha is a writer and researcher with Ring of Fire. You can follow her on Twitter @childoftheearth.