Updated: Jul 26
Pollution in our oceans is becoming an increasingly serious problem. From chemicals released into the marine environment, to litter, to noise pollution, there are many ways in which human activity can significantly damage animals living in our oceans. We are only just beginning to understand the long-term effects of some of these pollutants, which means that the full impact may not yet be clear. Over the course of their long lives, large-bodied animals like sharks, can be especially vulnerable to pollutants, as they can accumulate within their bodies over time. So on this world clean-up day, take a moment to lean what pollution is damaging to sharks... Maybe there is even something you can do to help clean up our oceans!
Ocean pollution can be in the form of a major incident, like an oil spill or a chemical accident, which immediately kills marine animals. On the other hand, pollution can be on a smaller scale, like litter or run-off, which accumulates in the environment, and cumulatively and insidiously degrades the habitat...
Major Incidents of Pollution
When we witness tragic events like an oil spill, the initial aftermath commonly shows seabirds and marine mammals glued down in oil or poisoned by chemicals. This is undeniably horrendous, but after the initial mortality from the oil itself, sometimes we forget that the pollution does not go away after the event is forgotten in the news. You might assume that an oil spill does not have an effect on sharks, but on the contrary, the chemicals remain in the marine environment and can potentially have an impact for decades afterwards.
Scientists have attempted to understand how the Deepwater Horizon oil spill might have affected sharks by analysing the polycyclic aromatic hydrocarbon aka PAH (chemicals that occur naturally in crude oil) levels in their bodies. This can be achieved by measuring the levels of the chemicals themselves or by measuring the levels of enzymes that are responsible for digesting these chemicals (Heuter, 2010).
When comparing individual silky sharks (Carcharhinus falciformis) and stingrays (Pteroplatytrygon violacea) from the northeastern Gulf of Mexico to sharks from other areas, scientists found that those which lived nearby the Deepwater Horizon site had elevated levels of PAHs and associated enzymes. There is concern that these contaminants can create metabolic stress and altered immune system functioning, which could reduce the overall health of the sharks. This could affect their growth and reproduction, and over a long-time period, could have population-level impacts (Heuter, 2010). To learn more about how oil spills affect sharks, check out Not So Slick.
“It’s just one plastic bottle”, said 8 billion people
One of the most significant examples of pollution which affects sharks is ocean litter. According to a report from the Secretariat of the Convention on Biological Diversity and the Scientific and Technical Advisory Panel (2012), fatal entanglement in marine trash and ingestion of debris by marine animals has increased by as much as 40% since the beginning of the new millennium! Sharks can either suffer immediate mortality as a result of entanglement or ingestion of ocean litter, or small pieces of trash can accumulate within their bodies and have an effect on their long-term health.
One of the most distressing and dramatic forms of ocean litter which affects sharks is “ghost fishing”. Ghost nets are lost or discarded fishing gear which drift in the ocean unsupervised and continue to entangle and kill marine life. In 2018, a particularly upsetting incident was reported from the Caribbean, where hundreds of sharks and other marine animals had been killed in an enormous ghost net. Whilst not always on such a huge scale, ghost fishing is killing sharks in our oceans all the time, but as no-one is watching these ghosts nets, it is very difficult to quantify the scale of the problem (Stelfox et al, 2016). To learn more about ghost fishing, you can check out Ghosts in the Ocean.
Yet it is not just large pieces of trash that can kill sharks. Every-day plastics, like water bottles and wrappers are finding their way into our oceans in alarming quantities! Filter-feeding animals can ingest large quantities of small pieces of litter over the course of their lives, which can seriously damage their health. For example, studies of whale sharks (Rhinocodon typus) and manta rays (Mobula alfredi), have shown significant levels of plastics in their faeces, and recent autopsies of whale shark carcasses have shown that trash which has blocked their gastrointestinal tract, may have caused death by starvation (Germanov et al, 2019). Not only is this a conservation issue for this threatened species, but an animal rights issue… It is simply a terrible and cruel way for an animal to die… and it is entirely our fault!
Pollution in the Future
In recent years it has become apparent that even tiny pieces of litter can cause significant damage to sharks. Plastic items which are very slow to biodegrade, often erode into microscopic pieces, known as “microplastics”, which can remain in the marine environment for a very long time. As this issue has only been recently discovered, we are still learning what long-term impacts may arise as a result.
In a recent study, scientists sought to understand how microplastics affect different species of sharks in the north-east Atlantic. After performing necropsies, researchers found that microplastics had been ingested by spiny dogfish (Squalus acanthias), starry smooth-hounds (Mustelus asterias), and small- and greater- spotted catsharks (Scyliorhinus canicula and S. stellaris respectively). The most common form were fibres made from synthetic cellulose and polyester. Therefore, they concluded that the pieces had probably broken off from fishing gear, been shed from car tyres or from textiles used in clothing. The larger the animal the more plastics it had eaten, suggesting the problem is only compounded as the shark ages (Parton et al, 2020). To learn more about plastic pollution, check out The Worst... Beware of the Plastics!
Similarly, even very tiny, but regular injections of chemical pollutants into the marine environment can have a devastating impact upon sharks, as they accumulate within the body over their lifetimes. Even when they are used far inland, fertilisers and pesticides which are used in agriculture, are eventually washed into the oceans by water run-off. For example, polychlorinated biphenyls (PCBs) and organochlorine pesticides (DDTs) have been banned in many countries since the 1970s, when it became clear that their impact was far more wide-reaching than the farm they were being used on. Once they reach the marine environment, these chemicals can increase in effect and detriment as they advance up the food chain; the chemicals are absorbed by animals at the bottom of the food chain and are then absorbed into the animals on higher “trophic levels” as they eat the animals below. This is known as “bioaccumulation” or “biomagnification”. As predators, sharks are very vulnerable to bioaccumulation of chemical contaminants (Storelli et al, 2005).
In order to determine whether DDT and PCBs are still affecting sharks, a research group has been assessing the levels in the bodies of sharks in different European waters. In the Mediterranean, they reported high levels in blue sharks (Prionace glauca) and kitefin sharks (Dalatias licha), but they found even higher levels in blackmouthed dogfish (Galeus melastomus), smooth hammerheads (Sphyrna zygaena), gulper sharks (Centrophorus granulosus) and longnose spurdogs (Squalus blainvillei) from the Adriatic Sea.
These chemicals have been associated with immune system dysfunction, which can lead to ill-health and even death, and multiple reproductive issues, including hormone disruption and birth defects (Storelli et al, 2005).
These studies occurred between 2001 and 2005… over 30 years since the use of these pesticides was banned in neighbouring countries. This shows us that the effects of chemical pollutants can take years to present themselves. Therefore, the affects of substances that have been used in the past may still not have reached their peak and the effects which may happen as a results of today’s pollution, are potentially yet to be seen!ons
I hope I have depressed you with all this, because that means you might feel inspired to make a change! It has never been more important to clean up our oceans and every single one of us has the power to make things better! In order to participate in World Clean-Up day or just to reduce the impact you are having on ocean pollution every day you can:
Use recyclable products and make the effort to recycle everything you can, especially plastics.
Ideally avoid single-use plastics completely and replace these items with something you keep and reuse or swap for a recyclable option.
Try to only use organic products wherever you can. This is an especially good idea when it comes to food because it will be good for your health too!
Choose to avoid toiletries with microplastic beads, which are often found in exfoliative products. It can be tricky to figure out whether a product is ok, so it might be easier to do a bit of research on the internet and choose a brand that is eco-friendly.
Avoid buying clothing with synthetic fibres, which might be washed out into the ocean. You can find the fabric content in the care label. Cotton, bamboo, linen, wool, silk and other natural fibres are good, but polyester and other synthetics are bad. If you’re not sure, google the name of the fabric for more information.
DO NOT LITTER! I can’t stress this enough and it should be a given! Anywhere that you drop a piece of trash, it has the potential to kill a whale shark. This is especially true when you are near rivers and oceans. So, always tidy up after yourself and leave the beach pristine. Not only is this good for the environment, but its much nicer for other people who want to enjoy the area after you.
Support clean-up events near you. You can find events on social media or through conservation organisations, but it need not be a big production… if you see a piece of litter on the ground, pick it up! The tiniest actions will have a huge impact if we all do it!
Germanov ES, Marshall AD, Hendrawan IG, Admiraal R, Rohner CA, Argeswara J, Wulandari R, Himawan MR & Loneragan NR (2019). Microplastics on the menu: Plastics pollute Indonesian manta ray and whale shark feeding grounds. Frontiers in Marine Science, 6. 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.
Parton KJ, Godley BJ, Santillo D, Tausif M, Omeyer LCM & Galloway TS (2020). Investigating the presence of microplastics in demersal sharks of the North-East Atlantic. Scientific Reports, 10, 12204. Access online.
Secretariat of the Convention on Biological Diversity and the Scientific and Technical Advisory Panel—GEF (2012). Impacts of Marine Debris on Biodiversity: Current Status and Potential Solutions, Montreal, Technical Series No. 67. Access online.
Stelfox M, Hudgins J & Sweet M (2016). A review of ghost gear entanglement amongst marine mammals, reptiles and elasmobranchs. Marine Pollution Bulletin, 1:111, 6-17. Access online.
Storelli MM, Storelli A & Marcotrigiano GO (2005). Concentrations and hazard assessment of polychlorinated biphenyls and organochlorine pesticides in shark liver from the Mediterranean Sea. Marine Pollution Bulletin, 50, 850–855. Access online.