(CNN) Oil pollution has been detected in thousands of fish in the Gulf of Mexico, including higher levels in popular seafood choices like yellowfin tuna, tilefish and red drum, according to a new study.

The research was carried out between 2011 and 2018, sampling more than 2,500 individual fish that belonged to 91 species living in 359 different locations in the Gulf. All of them contained oil exposure.

When the Deepwater Horizon explosion occurred 10 years ago, millions of gallons of oil spewed into the Gulf of Mexico's waters over 87 days. The BP oil spill became the largest accidental oil spill in US history.

After the explosion, researchers, like those at the University of South Florida, raced to study the spill and its environmental effects in real time.

Weeks later, BP made a 10-year, $500 million commitment to fund research. While the funding comes to a close this year, research done over the last 10 years is being released.

The oil pollution in the fish included levels of polycyclic aromatic hydrocarbons, known as PAHs. The toxic crude oil component was found in the bile of the fish. Fish bile, found in their livers, stores waste and aids digestion.

As researchers learned during and after the spill, not all oil floats. Instead, it finds its way to the seafloor as well.

While high levels of PAHs were expected in tilefish, which live in seafloor burrows where oil and PAHs are still found, the levels have been increasing over time.

The researchers noted high levels in species of grouper, specifically yellowedge grouper. The concentration of oil pollution in their liver tissue and bile increased more than 800% between 2011 and 2017.

And high levels of PAHs appeared in fish that don't call the seafloor home. The study published last week in the journal Scientific Reports

"We were quite surprised that among the most contaminated species was the fast-swimming yellowfin tuna as they are not found at the bottom of the ocean where most oil pollution in the Gulf occurs," Erin Pulster said in a statement. Pulster, the lead study author, is a researcher in the University of South Florida's College of Marine Science. "Although water concentrations of PAHs can vary considerably, they are generally found at trace levels or below detection limits in the water column. So where is the oil pollution we detected in tunas coming from?"

During the study, thousands of fish were weighed and their fin clips removed for genetic analyses.

The highest levels of oil pollution were found in fish living in the northern Gulf of Mexico, where the Deepwater Horizon spill occurred. Storms and currents stir up oily sediments, which keeps exposing fish living toward the bottom of the Gulf over and over.

The oil even attached itself to plankton, which landed on the seafloor.

Coastal runoff from heavily populated areas, like Tampa Bay, can also contribute to these hotspots of oil pollution in the ocean.

Oil and gas activity is high in the northern Gulf. Oil and gas platforms can release it, as well as boats and airplanes. Even the seafloor can leak natural oil.

"This was the first baseline study of its kind, and it's shocking that we haven't done this before given the economic value of fisheries and petroleum extraction in the Gulf of Mexico," Steven Murawksi said in a statement. Murawksi is lead of the international research effort and professor of fisheries biology at the University of South Florida. "Literally all the fish that we've tested have some level of hydrocarbon; there are no pristine fish in this system."

The study reinforced that fish from the Gulf of Mexico are tested for contaminants to ensure that they're safe to eat, and levels in fish flesh are below the recommended levels.

But the levels of PAHs are causing a decline in fish health, especially their livers.

This Gulf-wide study of oil pollution and its effects was funded by a $37 million grant from the Gulf of Mexico Research Initiative to establish the Center for Integrated Modeling and Analysis of Gulf Ecosystems. The international research effort includes professors, postdoctoral academics and students from 19 different institutions.

Overall, researchers have sampled 15,000 fish and taken 2,500 sediment cores from the Gulf.

Over time, they saw increased levels of skin lesions on fish, which decreased over time.

But fish populations near the site of the explosion have decreased between 50 and 80% since 2010. And as the fish have declined, so have their predators , including squid, blue crabs, bottlenose dolphins, oysters, red snapper and southern hake.

Steve Murawski has spent 10 years testing for oil pollution in the Gulf of Mexico.

In the years after the spill, as many as 75% of dolphin pregnancies failed in the area, endangered whale populations decreased by 22%, and 800,000 birds and 170,000 sea turtles died, according to Oceana, a nonprofit ocean conservation organization.

It published a new report for the 10th anniversary with more findings.

More decadal insights can be found in a review published Monday in the journal Nature Reviews Earth & Environment

Contamination can be passed on through fish eggs, according to the researchers. They're currently conducting a 15-year study to track fish eggs and determine the long-term effect on fish spawning.

"We've been pulling together all of these critical pieces of research and incorporating them into larger ecosystem-wide modeling studies. This is the basis for making predictions of how the Gulf will respond to future spills and deep blowouts," Murawski said.

The effects of the Deepwater Horizon spill could linger even longer into the future. The researchers also discovered, by studying sites of previous oil spills, that oil can remain in seafloor sediment four decades later

"Long-term monitoring studies such as these are important for early warning of oil pollution leaks and are vital for determining impacts to the environment in the case of future oil spills," Pulster said.