Fishes, crustaceans, squid show persistent declines and even chemical compounds in their tissues More than 16 years after one of the deadliest oil spills in history, a highly comprehensive study has found that life in the waters of the Gulf of Mexico remains severely in decline.

The Deepwater Horizon oil rig, operated by BP off the coast of Louisiana, exploded on April 20, 2010 — killing 11 people and injuring 17 others. During the three months that followed, as much as 4.9 million barrels of oil gushed into the Gulf, according to a U.S. government task force. The environmental catastrophe had a devastating impact on marine life, which continues today.

"A lot of things died right off the bat," said Tracey Sutton, a biological oceanographer at the Guy Harvey Oceanographic Research Center of NOVA Southeastern University whose team has been studying the impacts of the spill for years.

Their research found that some species declined by 60 to 85 per cent in the immediate three and a half years after the spill. Sutton's latest report, combining another eight years of detailed scanning and sampling, was just published in the Proceedings of the National Academy of Sciences. Time hasn't helped the situation, according to the authors.

"That low state has just maintained low," Sutton said, explaining that the dense patches of prey that predators need are missing, resulting in a "decline in things like deep-diving whales and dolphins" as well as fishes, shrimps and squids.

The immediate aftermath of the Deepwater Horizon spill killed lots of marine life, and the authors of a new study say that decline has persisted. (Charlie Riedel/The Associated Press) Lingering in the deep The most likely cause of this decline, the study suggests, is the Deepwater Horizon disaster, not least of which because their sampling is still finding remnants of all that spilled oil.

"What we've seen is the persistence of some of the worst components in the oil … polycyclic aromatic hydrocarbons, or PAHs. That's the really bad stuff," Sutton told CBC News.

The study found these oil-derived compounds inside the muscle tissue and eggs of deep-sea fishes and squid, potentially affecting health and reproduction. The authors also suggest these lingering PAHs can affect some male fishes' sense of smell, which they use to find mates.

Deepsea experts who weren't involved with the research say the study accounted for the natural factors that might affect fish abundance, such as temperature, availability of food and oxygen.

"They eliminate any other factors," said Fabio De Leo, senior scientist at Ocean Networks Canada and a professor at the University of Victoria. "And so the probable cause here is the 87 days of gushing oil through the Gulf for Mexico." Parsing out environmental factors is a sign of the report's strength, says Kelly Robinson, an associate professor at the School of Biological Sciences at the University of Louisiana at Lafayette.

"Sometimes when you work on data, it can show you a different side," said Robinson. "But if it keeps showing you the same message, that's probably what's happening.”

More than a decade later, researchers say chemical compounds from the oil spill have made their way into fish tissue. (Rich Matthews/The Associated Press) Diverse and essential Experts say that the negative effect on species has a larger effect for the whole Gulf ecosystem and its role in helping capture carbon in its depths.

The deep pelagic zone, where Sutton and his team surveyed sampled fishes, crustaceans and cephalopods (such as squid) for years, is an umbrella term for a range of depths — anywhere from around 200 metres to 1,500 metres below the surface.

While sunlight stops relatively high up in that zone, the biodiversity doesn't. Lanternfish, stingrays, sperm whales, krill and many more aquatic species are all normally found throughout these depths and regularly migrate between them.

"You have to remember that these animals feed at the surface," De Leo said. “They migrate downwards and then they respire at that [level]. So the carbon dioxide gets dissolved into the water and there are nutrients that they regenerate back into the water."

"They are a train that's moving energy from the surface to the deep very efficiently," Robinson adds. "And so if we see a decline in that, it's like taking cars away from a train, right? All of a sudden you're not moving as much material."

These species effectively act as a "biological pump" for carbon, and it's one of the ways that oceans help absorb carbon dioxide from the surface, including the excess greenhouse gases humans are pumping into the atmosphere. The waters still dissolve CO2 on their own, but this study shows how pollution can weaken this service that the oceans provide.

Oil booms like this were deployed to try and contain the oil spill, but the volume proved too much and many failed.

Risk of future offshore drilling The Trump administration's promise to "drill, baby, drill" has led to a push for the U.S. to extract more oil and natural gas, including new areas of offshore oil exploration.

In Canada, economic uncertainty has led to the Carney government's support for Pacific Link, a proposed oil pipeline from Alberta to B.C's coast, destined for overseas markets. Despite promised ocean protections, experts fear a long-lasting impact if an oil tanker were to be damaged and leak.

"There's more discussion about increasing oil transport in these areas of the Pacific," De Leo told CBC News from Victoria.

"We're unlikely to have something of [Deepwater Horizon’s] magnitude but there's potentially more risk if there's more traffic," De Leo said.