What is an Oil Spill?

Over the past month we’ve watched as the MV Wakashio has leaked approximately 1,000 tonnes of oil into the pristine waters, and surrounding reefs, of Mauritius. The ship, which hit a coral reef on the 25th of July 2020, and subsequent oil spill, has threatened corals, fish, and other marine life, in what some scientists suggest could be the worst ecological disaster that Mauritius has ever seen.

Thames shipping near Canvey Island and the Coryton Oil Refinery. Essex. © Terry Whittaker / 2020VISION

Early estimates from the US analytics company, Ursa Space Systems, as of the 11th of August 2020, found that the spill was covering an area of 27 square kilometres. Given that the ship ran aground in a sanctuary for rare wildlife, and close to a wetlands designated as a site of international importance, this is particularly concerning.

The MV Wakashio oil spill is by no means the first, or worst, of its kind, and it is, unfortunately, unlikely to be the last. Oil spills have reached every ocean globally, including waters closer to home. But more on that in a moment. First, let’s establish some basics.

What are oil spills?

According to Dictionary.com, an oil spill is the “accidental release of oil into a body of water, as from a tanker, offshore rig, or underwater pipeline, often presenting a hazard to marine life and the environment”. Oil spills are, as noted above, generally accidental, however there have been cases historically, as during the Gulf War, where spills were instead intentional. This would, however, generally be considered a rarity.

What causes oil spills?

Accidental oil spills into bodies of water can be caused by human error, equipment breaking or failing, or by natural disasters. Investigations are generally carried out to determine the cause of a spill.

What happens during/directly after an oil spill?

As oil enters the water it will generally float – very heavy oil will occasionally sink in freshwater systems – and begin to spread, forming a thin layer on the surface. We call this an oil slick. We can actively see slicks or sheens on the water, and you are actually likely to have seen smaller versions on road surfaces – they resemble a rainbow.

Atlantic grey seal pup © Manon Chaurtard / CBMWC

As this oil spreads, it can be very harmful to wildlife and communities in its wake.

Why are they a problem for wildlife?

Oil spills can have devastating consequences on marine and coastal wildlife. Particularly those that spend time on the surface of the ocean, or on the shoreline.

Fur-bearing mammals and birds

For fur-bearing mammals, such as sea otters and seal pups, oil can destroy the insulating ability of their fur. Similarly, when coated in oil, birds’ feathers will lose their ability to repel water. In both cases, the affected individuals are completely exposed to the environment, and the ocean around them; and are therefore unable to protect themselves from the elements. Many may suffer from hypothermia, and subsequently die.

Bottlenose dolphin surfacing © Dr Sarah Perry / CBMWC

Dolphins, whales and turtles

Species without fur or feathers, including charismatic animals such as dolphins, whales, and turtles, are also affected. Dolphins and whales may inhale oil as they surface, which can subsequently affect their lungs, immune system, and reproduction. Whilst sea turtles, and a number of other animals, may ingest oil, either by mistaking it for food or attempting to clean themselves. Oil is, perhaps unsurprisingly, poisonous if ingested.

Since the MV Wakashio ran aground last month, at least 17 dolphins have been found dead on the coast of Mauritius.  Postmortem investigations are ongoing to determine the cause of death.


Even organisms which do not directly encounter the slick at the surface, are affected. Adult fish can experience changes in growth rate, heart rate, and reproductive rates, as well as enlargement of the liver. For fish eggs and larvae, the effects can be even more severe, with spills often proving lethal.

Shanny © Dr Sarah Perry / CBMWC

Why are they a problem for coastal communities?

For coastal communities, a reduction in fish stock in this way, is not only a concern ecologically, but also in terms of food supply and the local economy.

Fishermen, ecotourism businesses, and environmental charities, to name a few, can all be negatively affected by the aftereffects of oil spills. Potentially leaving the environment, and economy – depending on the severity of the spill – in a state of disrepair for years to come.

Oil spills in Welsh waters

Historic spills in Welsh waters

Wales has seen its share of oil spills over the years. With perhaps the most infamous being the Sea Empress oil spill in 1996, and the Christos Bitas oil disaster in 1978. Both of these spills took place off the Pembrokeshire coastline and were caused by the ships running aground on rocks, spilling 73,000 tonnes and 5,000 tonnes of oil respectively.

Male Common Scoter © Derek Moore

Data from the time showed that, as a result of the 1978 spill, 1,520 sea birds were covered in oil, of which 68%, a huge 1,035 died, alongside 3 Atlantic grey seals who were residing around Skomer Island.

The Sea Empress spill, which released over fourteen times the amount of oil compared to the Christos Bitas disaster, was equally catastrophic for marine life, with around 1/3 of the local scoter population believed to have been killed in the spill. However, a report from two years later, remarked of how the area was recovering well – partly due to luck (timing, wind direction, oil type), and largely due to the monumental clean-up effort that followed.

And so, oil spills are not necessarily just a catastrophic event that we see in the news, happening in far off places. They will, have, and indeed may happen again, in waters closer to home.

Laura Evans, our Living Seas Wales Project Officer, at one of our 'Memory Pod' events © CBMWC

The Sea and Me

We are interested to hear from anyone who recalls either of the aforementioned oil spills, the Sea Empress (1996) and Christos Bitas (1978), as part of our marine memories project: The Sea and Me. We are looking to record your stories of the marine environment, and marine wildlife, with the hope of being able to use these conversations, to push back historical conservation baselines.

Now what does that mean? It means we will be able to gain a greater understanding of what the marine environment use to look like, what it looks like today, and how it might look going in to the future. There are real applications from this project to shape how we can best protect and conserve our oceans going forward. 

If you are able to contribute, please feel free to submit your memories via our website, or drop an email to our Living Seas Engagement Officer, Beth, at livingseas@welshwildlife.org.

Thank you.

Main Image: "Clean Up After a Big Oil Spill" by NOAA's National Ocean Service, which is licensed under CC BY 2.0.