Sharks Under Threat: Understanding Pollution's Impact

Pollution is a pressing issue that affects all marine life, and sharks are no exception. From plastic waste to oil spills, the impact of human activities on the ocean has far-reaching consequences for these predators. As apex predators, sharks are particularly vulnerable to the accumulation of toxins in their bodies, which can have detrimental effects on their health and survival. With their position at the top of the marine food chain, sharks are exposed to high levels of pollutants that bioaccumulate over time. This, coupled with the long-lasting nature of pollutants such as plastic, poses a significant threat to shark populations.

Ingesting plastic and microplastics

Sharks are vulnerable to ingesting plastic and microplastics due to their position at the top of the marine food chain. They feed on smaller fish, crustaceans, marine turtles, and marine mammals, all of which have been found to contain plastic and microplastics.

Microplastics are tiny fragments of plastic that are less than five millimetres in size. They are formed by the weathering of larger plastics, intentionally created as raw materials for other plastic products, or used in other products, such as cosmetics. They are very lightweight and can be easily dispersed by wind, water, and humans.

Sharks accumulate microplastics in their bodies through two primary pathways: direct 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 look like plankton or algae, they mistakenly do so. Whale sharks, for example, could swallow hundreds of microplastics in just an hour. Indirect ingestion occurs when sharks consume other fish that have microplastics in them. Since some shark species are at the top of the food chain, they tend to feed on almost all other fish in the ocean, and thus, they indirectly consume microplastics. Not all sharks are pelagic; some are demersal and may also ingest microplastics directly from the sediment on the seafloor.

The ingestion of microplastics could potentially have far-reaching consequences for sharks. Firstly, the sharp edges of microplastics can cause physical damage to the digestive tracts and other internal tissues in sharks, potentially leading to internal injuries, complications, and death in cases of long-term exposure. Secondly, microplastics act as vectors of toxins as they adsorb toxic additives and chemicals and release them into the shark's body. These may lead to toxicological effects such as a decrease in fitness and overall health. Thirdly, 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 over time as the ingested microplastics offer no nutritional value, thereby causing starvation and, in the long term, death. Fourthly, microplastics may contain chemical additives that, when absorbed by the shark's body, can act as endocrine disruptors. Endocrine-disrupting chemicals can interfere with hormonal balance and signalling systems, potentially influencing the shark's behaviour. Fifthly, behavioural changes induced by the presence of microplastics might extend to alterations in migration patterns. Sharks often follow migratory routes in search of food, and any disruption in their sensory or navigational abilities due to the presence of microplastics could lead to deviations from typical migration paths. Sixthly, microplastics can cause tissue damage, oxidative stress, and changes in immune-related gene expression and sharks' antioxidant status. Lastly, microplastics' endocrine-disrupting effects on sharks may interfere with their reproductive processes. This could lead to reduced reproductive success, impacting the population dynamics of shark species and contributing to global population declines.

Entanglement in plastic debris

Plastic pollution is a significant threat to marine ecosystems, and the continued production of plastic materials means this issue is likely to persist. Plastic debris in the ocean, such as discarded fishing gear and nylon nets, poses a severe risk of entanglement for sharks, a top marine predator.

Sharks can become entangled in plastic debris, leading to physical trauma and abnormal anatomical development. In some cases, the removal of plastic straps from entangled sharks has resulted in post-release mortality, indicating that strap removal may not always be effective in ensuring the survival of these creatures. The shape of the plastic material, such as circular straps, is believed to play a crucial role in the entanglement process.

Tiger sharks, for example, have been observed with circular plastic straps causing severe trauma in 3% of sampled individuals. This entanglement led to abnormal anatomical development, highlighting the detrimental consequences of plastic pollution on this species. The removal of an embedded plastic strap from a tiger shark is suspected to have resulted in its death after release.

Sharks are also affected by plastic pollution through ingestion, which can cause internal injuries and blockages, leading to malnutrition or even death. The presence of microplastics and nanoplastics further exacerbates the problem, as these tiny plastic particles can penetrate the organs and cells of sharks, with unknown long-term effects.

The impact of plastic pollution on sharks underscores the urgent need for addressing this issue. Mitigation strategies, such as eliminating the circular integrity of plastic materials, can help reduce the incidence of shark entanglement. Additionally, phasing out disposable plastics and improving waste management practices are crucial steps towards protecting sharks and the marine ecosystem as a whole.

Exposure to heavy metals and other toxins

Sharks are highly susceptible to environmental pollution, particularly to heavy metals such as mercury, lead, cadmium, and arsenic. These toxins are often released into the ocean through industrial waste, runoff from agricultural and urban areas, and even from winds depositing atmospheric waste. Once in the ocean, heavy metals can be ingested by sharks, leading to bioaccumulation—where the amount of toxins in the shark's body grows faster than its ability to excrete them—and biomagnification, where sharks ingest toxins that have accumulated in their prey.

High concentrations of toxic heavy metals have been found in shark species such as the blue shark, white shark, shortfin mako, common thresher, and whale shark. Research suggests that the presence of these pollutants could lead to neurological disorders, structural damage to organs and gills, reduced fertility, developmental effects, and cancers.

Sharks can also inadvertently transfer pollutants to their developing pups, with studies finding that adult female common threshers transferred up to 54% of the mercury and organic contaminants in their bodies to their embryos. The elevated levels of pollutants in young sharks point to a heightened future risk of health issues, as they will continue to bioaccumulate contaminants throughout their lifetimes.

In addition to heavy metals, sharks are also exposed to organic pollutants such as PCBs, DDTs, and organochlorines. These toxins can have detrimental effects on shark health, including reproductive defects and endocrine disruption. The consumption of shark meat that contains high levels of these pollutants also poses significant health risks to humans, especially pregnant women and young children.

Plastic pollution is another major source of toxins affecting sharks. Microplastics, defined as plastic particles or fibers smaller than 5mm, are ubiquitous in the ocean and are often ingested by sharks. A study on demersal sharks in the North Atlantic Ocean found that 67% of the sampled sharks contained microplastics, with the highest number found in an individual bull huss, which had 154 polypropylene fibers in its stomach and intestines. While the direct health impacts of microplastics on sharks are not yet fully understood, it is known that microplastics can negatively influence feeding behavior, development, reproduction, and lifespan in other marine organisms.

Furthermore, plastics can break down into even smaller particles called nanoplastics, which are smaller than 1 micron in size. Nanoplastics can penetrate the organs and cells of sharks and are believed to have long-term toxic effects, although further research is needed to fully understand their impact.

Overall, exposure to heavy metals and other toxins poses a significant threat to shark health and can have detrimental effects on their populations. The accumulation of these toxins in sharks also has implications for human health, particularly for those who consume shark meat and other products.

Oil spills

The Deepwater Horizon oil spill in 2010 was the largest accidental spill in history. It released 750 million litres of oil, covering over 180,000 km2 of surface waters, and contaminating surrounding deep-water areas. The spill affected over 80 species of sharks and rays in the Gulf of Mexico. Whale sharks, giant manta rays and scalloped hammerheads were the most susceptible. Whale sharks and manta rays are filter feeders, and oil can clog their intricate filtering organs (gill rakers) and lead to starvation and asphyxiation.

The long-term impact of oil spills on sharks is also a concern. Crude oil is a mixture of hydrocarbon chains of varying lengths, which degrade at different rates. 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 these toxins has been linked to developmental abnormalities and cancer in humans. Studies have shown that several shark species exposed to the Deepwater Horizon spill had enhanced levels of PCBs and PAHs in their systems. These toxins bioaccumulate in the sharks, and scientists are concerned about the potential health impacts, such as reduced growth and fertility, and weakened immune systems.

Climate change and ocean acidification

Ocean Warming

Ocean warming is causing sharks to alter their behaviour and migrate to new areas. As apex predators, these changes in movement can have a huge effect on oceanic ecosystems. For example, whale sharks have been spotted in mainland Europe (Portugal) for the first time, and tiger sharks are being caught off the coast of Canada. At the same time, some tropical waters are becoming uninhabitable for sharks, and cooler-water species are losing habitat as their environments warm.

Warmer ocean temperatures also affect shark physiology. As the water gets hotter, sharks' metabolic rates increase, meaning they have to swim faster to deliver sufficient oxygen to their bodies and eat more to meet their energy needs. This leaves them with less energy for growth and reproduction.

Ocean Acidification

Ocean acidification is another consequence of the ocean absorbing excess carbon dioxide from the atmosphere. This process reduces the ocean's pH level, making it less basic and more acidic. This decrease in pH reduces the amount of calcium carbonate in the water, which many marine animals use to build their skeletons and shells. While sharks may show some physiological tolerance to elevated carbon dioxide levels, acidification can still negatively affect their growth and metabolism and reduce their ability to locate food through olfaction.

Ocean acidification also poses a threat to coral reefs, an important habitat for many shark and ray species. As the reefs degrade, shark populations that rely on them become more vulnerable.

The Future

Sharks are slow to evolve, with long generation times and slow growth rates. This makes it difficult for them to adapt to the rapid changes in the marine environment caused by climate change. As climate change continues, the impacts on shark populations are expected to worsen.

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