What do sharks, plankton, and your takeaway coffee cup have in common?
More than you might think.
Every year, more than 460 million metric tons of plastic are produced, most of which never gets recycled or thrown away properly. As a result, somewhere between 19 and 23 million metric tons of plastic waste ends up in our rivers, lakes and oceans each year.
Once plastic is in the water, it harms marine life in several ways. Bigger pieces, known as macroplastics, can trap or injure animals and damage their habitats. As plastic breaks down over time, it turns into microplastics, particles smaller than a grain of rice, that can be ingested by a wider range of organisms and spread harm across the food chain.
Shark populations are already in decline because of shark finning, bycatch, habitat loss and climate change. Their global population has dropped by 71% between 1970 and 2018, and because of their slow life cycle, any additional stress could be the last straw. The effects of microplastics on sharks are still being studied, but from what we know about their effects on fish and other marine organisms, there is no doubt they are a major concern.
Here are seven things to know about plastic pollution and sharks.
1. Ghost gear is the leading cause of plastic entanglement in sharks.
Abandoned, lost, and discarded fishing gear, known as ghost gear, is the main cause of shark entanglement. Fishing nets, lines, and traps can stay in the ocean and continue to entangle sharks and other marine animals for years after being discarded. When sharks become entangled, it can not only cause cuts and injuries (often leading to infections), but also prevent them from breathing, feeding and swimming properly.
Hammerhead shark in a discarded fishing net, Hawaii. Credit: Toby Matthews / Ocean Image Bank
Commercial tuna fisheries often use fish aggregating devices (FADs) made of rafts, floating buoys, ropes and nets. Over time, algae, mollusks and other marine organisms grow on these structures, attracting larger pelagic fish such as tuna and billfish. Once abandoned at sea, these floating devices continue to trap marine life. In the Indian Ocean, discarded FADs kill five to ten times more silky sharks than the same fisheries report as bycatch. This amounts to between 480,000 and 960,000 silky sharks killed each year in the Indian Ocean alone.
2. Ingesting plastic can starve sharks.
Sharks don’t just swim through plastic pollution. Sometimes, they eat it too. Plastic bags, food packaging, and other plastic waste has been found in the digestive system of several shark species.
Once inside their digestive tract, plastic can build up and block the intestines. As a result, sharks may have a false sense of fullness, eat less, and suffer from internal injuries. In some cases, this accumulation of plastic can lead to starvation and even death.
3. Sharks ingest microplastics everywhere.
Microplastics are found in every ecosystem on Earth, including every ocean. These tiny particles have been documented in sharks from the Atlantic, Pacific, and Indian Oceans. Tiger sharks and hammerheads have both been found with hundreds to thousands of microplastic particles in their bodies. From coastal waters, to the open ocean and the deep-sea, no shark species is safe from microplastics.
4. Whale sharks are particularly vulnerable to microplastics.
Filter-feeding species, such as whale sharks and basking sharks, are particularly vulnerable to microplastic pollution. While they filter large amounts of water through their gills to capture plankton and other small prey, they also ingest microplastics suspended in the water column.
Microplastics, mostly synthetic fibers, have been found in whale shark feces. This form of plastic is released from clothing during washing, from ropes and fishing gear, and from other plastic debris as they degrade.
Whale sharks, Philippines. Credit: Vincent Kneefel / Ocean Image Bank
5. Microplastics transport toxic chemicals into shark tissues.
The plastic itself is not the only problem. Microplastic can also carry many harmful chemicals. Additives such as bisphenol A (BPA) and phthalates are often added to plastic during manufacturing to improve its flexibility, durability, or resistance to fire. Once ingested, these chemicals can leach from plastic particles, accumulate in shark tissues, and may prevent the hormonal system from functioning normally.
Microplastics can also adsorb pollutants already present in the surrounding water. Their surface acts like Velcro, allowing chemicals to stick and be ingested along with them. Organic pollutants can be concentrated on microplastic surfaces up to one million times higher than in the surrounding water. This can considerably increase the levels of contaminants sharks accumulate.
6. Microplastics are getting into sharks’ reproductive organs.
Microplastics can also accumulate in reproductive organs. They have been found in the reproductive tissue of 95% of both male and female spadenose sharks during a study along India’s west coast. Males with higher microplastic load tended to have a lower reproductive organ weight relative to their body mass. This indicates that higher microplastic accumulation may be linked to reduced reproductive investment, raising concerns for shark populations.
7. Microplastics reach sharks before they even hatch.
For some sharks, exposure to plastic may begin before birth. Microplastics were recently found inside mermaid purses, the tough egg cases produced by certain egg-laying shark species, including Hasselt’s bamboo shark and the coral catshark examined in this study. 100% of the egg cases contained microplastics, and a correlation was found between yolk size and plastic concentration.
Yolk is the embryo’s only source of nutrition. Microplastics leaching chemicals into an embryo’s environment could pose a serious threat to its development and survival.
Catshark egg case in seagrass, Cornwall, UK. Credit: Shannon Moran / Ocean Image Bank
How You Can Help
Protecting sharks from plastic pollution requires action at every level. We need to reduce plastic production, improve waste management, and prevent fishing gear from being discarded into the ocean.
At the individual level, choices matter too. Using less plastic in your day-to-day, recycling, and participating in local beach cleanups are all great examples of simple actions that can make a difference. You can also support organizations working to reduce plastic pollution in our oceans through education and innovative technologies.
~ By Léane Desrochers
