Kiera Jefferson
Pollution is a critical threat to sharks, with marine debris, microplastics, oil spills, and chemical contaminants all contributing to the endangerment of shark species. As apex predators, sharks are particularly susceptible to the harmful effects of pollutants, which accumulate in their tissues as they consume prey. The impact of pollution on sharks is twofold: firstly, through entanglement in plastic waste and discarded fishing gear, and secondly, through the ingestion of plastics and release of toxins, which can cause internal injuries and organ damage. With shark populations already vulnerable due to indiscriminate hunting, understanding the consequences of pollution is critical for developing effective conservation strategies to protect these ecologically important species.
| Characteristics | Values |
|---|---|
| Microplastics | Microplastics are tiny fragments of plastic that are less than 5mm in size. They are formed by the weathering of larger plastics, intentionally created as raw materials, or used in other products. They can be ingested by sharks, leading to potential health issues. |
| Oil spills | Oil spills from offshore drilling platforms or tanker accidents can have detrimental effects on sharks and their relatives. |
| Algae blooms | "Red Tides," toxic blooms of Karenia spp. dinoflagellates associated with nutrient runoff from agriculture, can lead to mass deaths of sharks. |
| Chemical contaminants | Industrial chemicals, pesticides, and other chemical pollutants can end up in the ocean, impacting the health and reproduction of sharks. |
| Marine debris | Human activities contribute to marine debris, including plastic bottles, straws, and fishing gear, which can entangle and harm sharks. |
| Neurological disorders | Pollutants can cause neurological issues, structural damage to organs and gills, and developmental effects in sharks. |
| Reproductive issues | Ingested plastics can release endocrine-disrupting chemicals, impacting the reproductive processes and success of sharks. |
| Bioaccumulation | As apex predators, sharks are susceptible to bioaccumulation of toxins and pollutants from lower trophic levels in the food chain. |
| Extinction risk | Shark populations are already declining due to various factors, and pollution increases the risk of extinction for some species. |
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What You'll Learn
Microplastics and nanoplastics in the ocean
Microplastics and nanoplastics are tiny fragments of plastic that measure less than 5 millimetres in size. They are formed by the weathering and breakdown of larger plastics, or are intentionally created as raw materials for other plastic products. These plastics are pervasive in our environment, from personal care products and synthetic clothing to plastic bags and bottles. They are lightweight and can be easily dispersed by wind, water, and humans, eventually making their way into the ocean.
In the ocean, microplastics pose a significant threat to aquatic life, including sharks. Whale sharks, for example, are filter feeders that can inadvertently swallow hundreds of microplastics in an hour. This direct ingestion occurs when sharks mistake microplastic particles for prey. Indirect ingestion also happens when sharks consume other fish that have microplastics in their systems. Demersal shark species, such as smooth hound and spotted gully, may also ingest microplastics directly from the sediment on the seafloor.
The ingestion of microplastics can lead to intestinal damage in sharks, as the sharp edges of these plastics can cause physical trauma to the digestive tracts and internal tissues. Additionally, microplastics can disrupt normal absorption processes in the gastrointestinal tract. While the full extent of the impact of microplastic ingestion on shark health is still being studied, there are concerns about reduced reproductive success and population declines. Climate change may further amplify these issues by altering prey distribution and increasing the release of microplastics into the marine environment.
Nanoplastics, an even smaller form of plastic pollution, are also a concern. Due to their minuscule size, nanoplastics can pass from the stomach and intestines into the bloodstream, accumulating in various organs and cells. The long-term effects of nanoplastic accumulation are still unknown, but they could potentially transfer toxic chemicals to the shark's body. Given that sharks are apex predators, the presence of nanoplastics in their systems can have broader implications for the entire oceanic food web.
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Entanglement in fishing nets and plastic waste
Marine debris, such as discarded fishing nets and plastic waste, poses a significant threat to sharks and other marine life. Sharks can become entangled in these materials, leading to severe consequences.
Fishing nets, particularly those made of synthetic nylon, are designed to be durable and challenging for marine animals to detect. When discarded in the ocean, they can drift over large distances with ocean currents, entrapping and killing animals for many years. This "ghost gear" commonly entangles migratory shark species, such as silky sharks, whale sharks, white sharks, and giant manta rays, which feed in frontal zones where vast amounts of rubbish accumulate.
Plastic waste, in the form of microplastics and larger items, also poses a significant entanglement risk to sharks. Microplastics, tiny plastic fragments smaller than 5mm, can be ingested or physically entangle sharks, leading to internal injuries and anatomical deformities. Larger plastic items, such as circular plastic straps, can trap sharks, causing severe trauma and even death.
The impact of plastic pollution on shark populations is a growing concern. Sharks, as apex predators, play a crucial role in maintaining the balance of marine ecosystems. The accumulation of plastic and toxins in their bodies through ingestion or entanglement can have broader implications for the entire oceanic food web.
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Ingesting plastic and toxins
As apex predators, sharks are particularly vulnerable to plastic and toxin ingestion. They accumulate microplastics in their bodies through two primary pathways: direct ingestion and indirect ingestion. Direct ingestion occurs when shark species that filter-feed consume microplastics directly from polluted water. They do not intend to feed on these plastics but because microplastics resemble plankton or algae, they mistakenly ingest them. For example, a study found that whale sharks, which are filter feeders, could swallow hundreds of microplastics in just an hour. Demersal sharks, which dwell close to the ocean floor, may also ingest microplastics directly from the sediment.
Indirect ingestion occurs when sharks consume other fish that have microplastics in their systems. As top predators, sharks feed on almost all other fish in the ocean, and thus, they indirectly consume microplastics by eating these fish. A study in 2013 found that the rate of plastic ingestion by the Brazilian sharpnose shark was 33%. Another study found that basking sharks, which feed on zooplankton, are particularly vulnerable to the harmful chemicals that come from microplastics as they are indirectly ingesting those toxins when they feed on their prey. High levels of MEHP and DEHP, which are toxic chemicals, were found in zooplankton samples, which is alarming as zooplankton is the only food source for basking sharks.
The ingestion of plastics and toxins can have severe consequences for sharks. Firstly, it can cause intestinal damage and physical injuries to the digestive tracts and internal tissues, potentially leading to internal complications and even death. Secondly, microplastics act as vectors of toxins, absorbing and releasing toxic additives and chemicals into the shark's body. These toxins can alter hormone levels, affecting growth, development, metabolism, and reproductive functions. Additionally, the sharp edges of microplastics can cause mechanical damage to the intestinal mucosa through chafing.
Furthermore, plastic ingestion can lead to starvation in sharks. When sharks mistakenly fill their stomachs with microplastics, they may experience a false sense of fullness, leading to reduced consumption of actual food. This can result in malnutrition and, ultimately, starvation. A blocked digestive tract due to plastic ingestion can also contribute to starvation. While direct ingestion of plastics may not be a significant threat to sharks as they can invert their stomachs to release inedible substances, the accumulation of toxins and mechanical damage caused by microplastics pose a more insidious danger.
The impact of plastic and toxin ingestion on sharks is made worse by climate change. Rising sea temperatures and changing ocean currents affect the distribution and abundance of prey species, leading sharks to search for food in different areas and potentially increasing their exposure to microplastics. Climate-related events, such as extreme weather and storm surges, can also enhance the plastic weathering process, releasing more microplastics into the marine environment and increasing the likelihood of shark encounters. While plastic pollution is not the biggest threat to sharks, with unsustainable overfishing and bycatch being more significant factors, it is a growing concern that cannot be overlooked.
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How pollutants move around the ocean
Marine pollution is a combination of chemicals and trash, most of which comes from land sources. The majority of this waste (80%) comes from human land-based activity, although marine transportation also contributes significantly. Marine debris is mainly discarded human rubbish that floats on or is suspended in the ocean.
There are five gyres in the ocean: one in the Indian Ocean, two in the Atlantic Ocean, and two in the Pacific Ocean. Garbage patches of varying sizes are located in each gyre. Due to winds and currents, garbage patches are constantly changing size and shape. The debris making up these garbage patches can be found from the surface of the ocean to the ocean floor.
Plastic is the leading source of ocean pollution. Plastic waste makes up 80% of all marine debris and around 8 to 10 million metric tons of plastic end up in the ocean each year. Plastic bags, bottles, food containers, cutlery, wrappers, synthetic rope, and fishing items are the most common items found in the ocean. Plastic is particularly problematic as it is long-lasting and does not fully degrade, instead breaking down into microplastics. These microplastics are then ingested by marine life, causing serious harm.
Another pathway of pollution is through the atmosphere. The ocean has long been affected by the passage of chemicals from the atmosphere, such as nutrient sources and pH influences. Wind-blown dust and debris, including plastic bags, are blown seaward from landfills and other areas.
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The impact of mercury and other contaminants
Shark meat and other products often contain high levels of mercury and other pollutants. These pollutants are transferred to humans who consume shark products, posing significant health risks. A study found that the average mercury concentration in sharks exceeded the recommended daily limit of 980 ng g-1 by 66%. Certain shark species, such as the great hammerhead, were found to have even higher mercury levels, surpassing legal limits.
Sharks are apex predators that feed on contaminated prey, leading to the bioaccumulation of mercury and other toxins in their bodies. While sharks may not suffer immediate harmful effects from mercury consumption, these toxins can be transferred to their developing pups. A study on Common Threshers found that an adult female transferred up to 54% of the mercury in her body to her near-term embryos. This transfer of pollutants can have detrimental effects on the health and development of the pups, increasing their vulnerability to illnesses and other health issues.
In addition to mercury, sharks are exposed to other contaminants such as microplastics, crude oil, and marine debris. Microplastics, tiny fragments of plastic less than 5 millimetres in size, can be ingested by sharks and cause internal injuries and gastrointestinal obstructions. They can also absorb and accumulate toxins, including mercury, which are then ingested by sharks. Climate change may exacerbate the impact of microplastics on sharks by altering ocean currents and prey distribution, potentially increasing the sharks' exposure to contaminated food sources.
The ingestion of contaminated prey and the accumulation of toxins can have significant health consequences for sharks. Studies on marine mammals and teleost fishes with similar pollutant exposure levels as some shark species have shown neurological disorders, structural damage to organs and gills, reduced fertility, developmental issues, and cancers. While shark physiology may respond differently to pollutants, the presence of high levels of contaminants in their bodies is concerning and warrants further investigation to ensure the health and sustainability of shark populations.
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Frequently asked questions
Plastic pollution has been found to harm sharks in a variety of ways, including entanglement, ingestion, and the transfer of pollutants to their offspring. Plastic waste can entangle sharks, causing grievous wounds and eventual death. Sharks can also ingest plastic, which can cause internal injuries and release toxins. Microplastics, tiny fragments of plastic less than 5mm in size, can be ingested by smaller species and accumulate in the bodies of sharks as they consume their prey.
Marine debris, such as discarded fishing gear and plastic waste, can entangle and harm sharks, leading to animal welfare concerns and even death. Lost or discarded fishing gear, also known as "ghost nets," can trap and kill sharks over many years.
Pollution, including microplastics and other toxins, can reduce the reproductive success of sharks, impacting their population dynamics and pushing them closer to extinction. Climate change may amplify these effects by altering the distribution and abundance of prey species, potentially increasing sharks' exposure to pollutants.
As top predators, sharks are particularly susceptible to the harmful effects of pollutants. Pollutants accumulate as you move up the food chain, so sharks are more likely to have higher concentrations of harmful chemicals in their tissues. These compounds can cause neurological disorders, structural damage to organs and gills, reduced fertility, developmental issues, and cancers.
The ocean is polluted by various sources, including plastic waste, oil spills, industrial chemicals, pesticides, and microplastics. Microplastics are formed from larger plastics breaking down due to wave action and UV breakdown and can be found in personal care products, synthetic clothing, and plastic bottles. These tiny particles are easily dispersed by wind, water, and human activities, eventually making their way into the ocean and the food chain.
