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Not So Slick

Oil spills are undeniably terrible. The images we have all seen on the news, of birds and marine animals coated in oil, drowned, poisoned and washed up on the beach are terribly upsetting. However, what you might not be aware of is that spilled oil still affects the environment long after the TV cameras have finished rolling... As oil is less dense than water, it floats on the surface and can wash up on beaches, coat shallow coastal areas and even travel long distances across the ocean. Even when extensive clean-up efforts are made, the small amounts of oil left in the ocean can have serious impacts on the health of marine life, for many years to come. So what was the worst oil spill to date? Why is oil so harmful? And how does it affect sharks?

Scientists are concerned that chemical contaminants from the Deepwater Horizon disaster may affect the health of silky sharks in the Gulf of Mexico for many years to come (Image Credit: DOGAN_KASIKCI / Shutterstock)

In Deep Water

There have been several catastrophic oil spills in human history, but by far the worst was that of the Deepwater Horizon in 2010. On the 20th of April a technical failure caused the rig to explode and sink. Located some 41 miles off the coast of Louisiana in the Gulf of Mexico, The Deepwater Horizon spewing crude oil into the ocean for 87 days. By the time the flow could be capped, 210 million gallons of oil had spread over an area as large as 92,500 miles (Frias-Torresa & Bostater, 2011).

The Big Short

In the short term, the Deepwater Horizon had an impact on certain species of sharks in the Gulf; namely whale sharks (Rhincodon typus). As filter-feeders, whale sharks have intricate filtering organs (known as "gill rakers"), which sift minute ocean life out of the water. Thick oil can clog these filtering organs and lead to starvation. What's more, as the gills are vital for breathing, blockages also lead to asphyxiation. It is not known how many whale sharks were directly impacted by oil pollution in the weeks and months following the Deepwater Horizon disaster (Frias-Torresa & Bostater, 2011).

Oil spills can clog the feeding and breathing organs of whale sharks (Image Credit: DJ Mattaar / Shutterstock)

The Long Game

What may be of even more concern it the long-term impact that this oil can have on sharks in the Gulf... Crude oil is a mixture of hydrocarbons; this means it is made up of chains of hydrogen and carbon atoms. In its raw form, this oil is a mixture of many different hydrocarbon chains of varying lengths. Different lengths have different combustibility and therefore, different uses - jet fuel is not the same stuff that we use to power our cars.

When crude oil finds its way into the marine environment, the long hydrocarbon chains begin to slowly degrade into their shorter constituent parts. In the natural environment, this can be a slow process, so shorter hydrocarbon chains are released into the marine environment over many years (Cullen at al, 2019).

Some of the most dangerous products of this process are two classes of chemicals known as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs). Chronic exposure to even small amounts of these toxins has been linked to developmental abnormalities and cancer in human beings (Jung, 2017; Cullen at al, 2019).

PAHs = polycyclic aromatic hydrocarbons

PCBs = polychlorinated biphenyls

Blacktip sharks have alarming levels of toxic PAHs and PCBs in their system thanks to the Deepwater Horizon oil spill (Image Credit: Albert Kok / WikimediaCommons)

There have been several studies assessing the impact that PAHs and PCBs leached from the Deepwater Horizon have had on sharks. One paper showed that silky sharks (Carcharhinus falciformis) which live in the Gulf of Mexico have significantly higher PAH exposure than normal (Heuter, 2010).

Similarly, research has shown that bull sharks (C. leucas), blacktip sharks (C. limbatus), sandbar sharks (C. plumbeus) and bonnetheads (Sphyrna tiburo) exposed to the Deepwater Horizon oil spill, all have enhanced levels of PCBs and PAHs in their systems (Cullen at al, 2019).

These sharks have accumulated such high levels of these toxins because they are at the top of their food chain; eating smaller animals which had ingested or been exposed to the toxins in the environment. This is known as "bioaccumulation". Scientists are concerned that the levels of PCBs and PAHs these sharks are burdened with could have serious health impacts, such as reduced growth rates, reduced fertility and a weakened immune systems (Heuter, 2010; Cullen at al, 2019).

Scientists are concerned about how toxins from oil spills might affect the health of bonnethead sharks (Image Credit: Eric Kilby / WikimediaCommons)

Never Again!

As it takes time for the long-terms effects of crude oil pollution in our environment to become apparent, we are only just beginning to understand how one event a decade ago is still affecting our marine life... It is very possible that future work will uncover more impacts that are as yet unforeseen (Cullen at al, 2019).

The Deepwater Horizon was the worst oil spill in human history (Image Credit: US Coast Guard / WikimediaCommons)

Put simply, oil spills are devastating. We cannot allow such terrible pollution into the marine environment!

If we are stop such disasters happening in the future, we will obviously need better tighter safety regulations, better engineering and stricter checks on rigs etc. etc. ... but there are also small things that you can do to help... We will not have as many oil spills in our oceans if we reduce our dependance on hydrocarbon fuels! So, if you work towards living more sustainably (using less energy in your home, switching to renewable power sources, reducing your consumption levels and recycling where you can), you will be doing your part to ensure we can move away from our reliance upon damaging fossil fuels. Then, hopefully, one day, we will never have to witness devastation like the Deepwater Horizon tragedy again.


Cullen J, Marshall CD, Hala D (2019). Integration of multi-tissue PAH and PCB burdens with biomarker activity in three coastal shark species from the northwestern Gulf of Mexico. Science of the Total Environment, 650, 1158–1172. Access online.

Frias-Torresa S & Bostater CR (2011). Potential impacts of the Deepwater Horizon oil spill on large pelagic fishes. Proceedings of the Society of Photo-Optical Instrumentation Engineers, Vol. 8175. Access online.

Hueter RE (2010). Effects of the Deepwater Horizon oil spill on epipelagic and large coastal sharks and teleosts of the Gulf of Mexico. Mote Marine Laboratory FIO Block Grants Final Report. Access online.

Jung D (2017) Autoignition and chemical-kinetic mechanisms of homogeneous charge compression ignition combustion for the fuels with various autoignition reactivity. Advanced Chemical Kinetics. Access online.

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This is very worrying. Interesting article.

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