Sharks Under Threat: Understanding Pollution's Impact

Sharks are top predators in marine and freshwater ecosystems and are highly susceptible to environmental pollution. A wide range of pollutants, from plastic waste to oil spills, pose a significant threat to the health and survival of sharks. While the specific effects of these pollutants on sharks are still being studied, initial findings suggest that plastic ingestion and entanglement are particularly harmful to these marine creatures.

Sharks, as apex predators, are vulnerable to bioaccumulation and biomagnification of pollutants, leading to a build-up of toxic chemicals and heavy metals in their bodies. The impact of pollution on shark reproduction, immune function, and overall survival is a growing area of research, with scientists working to understand the true extent of human-generated pollution on marine life.

Plastic ingestion

Sharks often mistake microplastics for food, as they look like plankton or algae, and end up consuming them. This can lead to intestinal damage, as the sharp edges of microplastics can cause physical damage to the digestive tracts and internal tissues, potentially resulting in internal injuries and even death. Blockages and injuries from ingested plastic have been recorded as the cause of death for whale sharks. In addition, plastic ingestion can lead to starvation, as it gives sharks a false sense of fullness, reducing their consumption of actual food. This can result in malnutrition and, eventually, death.

The presence of plastic in their diet can also alter migration patterns. Sharks often follow migratory routes in search of food, and disruptions in their sensory or navigational abilities due to plastic ingestion may lead to deviations from their typical migration paths.

Furthermore, microplastics can act as vectors of toxins, absorbing toxic additives and chemicals and releasing them into the shark's body. This can lead to increased toxin levels and toxicological effects, such as a decrease in fitness and overall health.

The impact of plastic ingestion on sharks is a growing area of research, and it is important to note that not all ingested plastics cause immediate harm. Some smaller pieces may pass through the intestinal tract without causing damage. However, the potential for long-term harm, especially from nanoplastics, is a significant concern.

Entanglement in plastic

Sharks and rays in the open ocean and those living on the seafloor appear more likely to get entangled. Materials such as nets loaded with dead fish sink to the seafloor and attract predators, which then become stuck. Migration patterns and body shape also play a role in the risk of entanglement. Species that cover long distances are more likely to encounter plastic waste, and sharks seem to be at greater risk than rays. Species with unusual features, such as manta rays, basking sharks, and sawfish, are also more susceptible to entanglement.

The impact of plastic entanglement on sharks can be severe, causing pain, suffering, and even death. For example, an adult shortfin mako shark in the Pacific Ocean became entangled in a fishing rope that tightened around its body as it continued to grow, damaging its spine and causing scoliosis. In another case, a tiger shark entangled in a plastic strap for at least nine months experienced severe trauma and anatomical deformities, and removing the strap likely resulted in post-release mortality.

The growing production of plastic materials, coupled with poor waste management, is contributing to the increasing presence of plastic waste in the oceans. Plastic waste does not disappear over time but breaks down into smaller and smaller pieces, creating microplastics and nanoplastics that are impossible for animals to avoid. While plastic ingestion often results in the fragments passing through the intestinal tract without causing damage, it can also lead to blockages, internal injuries, and malnutrition. Additionally, ingested plastics may offload pollutants into the sharks' tissues, potentially suppressing reproduction and causing other health issues.

Toxic chemicals and heavy metals

Sharks, as top predators in marine and freshwater ecosystems, are highly susceptible to environmental pollution. Pollutants tend to bioaccumulate in sharks' bodies, and biomagnification occurs when sharks ingest prey that have already accumulated these pollutants. This has led to high concentrations of toxic pollutants, including organic substances such as PCBs, DDTs, and organochlorines, and inorganic substances like heavy metals (e.g., mercury), being found in sharks and rays.

Research on the direct impact of these pollutants on sharks is still ongoing, but studies on similar marine life, such as marine mammals and teleost fishes, have found neurological disorders, structural damage to organs and gills, reduced fertility, developmental issues, and cancers at similar levels of pollutant exposure. While shark physiology may differ, these findings are concerning as shark species continue to be exposed to increasing pollutant loads.

Sharks can also inadvertently transfer pollutants to their developing pups. Studies have shown that adult female common threshers transferred a significant percentage of the mercury and organic contaminants in their bodies to their embryos. Similarly, high levels of organochlorines have been detected in young white shark pups, likely transferred from their mothers' tissues. These elevated levels of pollutants in juvenile sharks indicate an increased future risk of adverse health effects, as they will continue to bioaccumulate contaminants throughout their lifetimes.

In addition to the direct health consequences, the presence of toxic chemicals and heavy metals in sharks poses a risk to humans who consume shark meat and other products. High concentrations of pollutants, especially mercury, have been found in shark meat, exceeding recommended daily intake limits. Exposure to other pollutants found in shark tissue, such as PCBs and dioxins, has been linked to cancer, liver and kidney damage, immunosuppression, reproductive issues, and endocrine disruption.

Addressing the Issue

To address the issue of toxic chemicals and heavy metals in the ocean and their impact on sharks, several measures need to be taken:

• Reducing the use of disposable plastics and improving waste management practices to prevent these materials from ending up in the ocean.
• Educating the public about the issue of plastic pollution and its impact on marine life, including sharks.
• Implementing more rigid regulations and punishments for companies that produce or use plastic materials, particularly single-use items.
• Encouraging responsible consumption and disposal practices to reduce the amount of toxic chemicals and heavy metals that enter the ocean.
• Supporting research efforts to better understand the impact of these pollutants on shark health and to identify safe thresholds for pollutant concentrations.

Oil spills

The worst oil spill in history was the Deepwater Horizon disaster in 2010. The rig exploded and sank, spewing crude oil into the Gulf of Mexico for 87 days. This resulted in 210 million gallons of oil spreading over an area of 92,500 square miles. The impact of this spill on the Gulf of Mexico's shark populations is well-documented.

Whale sharks, which are filter feeders, have intricate filtering organs called "gill rakers" that sift minute ocean life out of the water. The thick oil from the spill can clog these organs, leading to starvation and asphyxiation. Whale sharks are not the only species affected. Blacktip sharks, for example, have been found to have alarming levels of toxic polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) in their systems due to the Deepwater Horizon spill.

PAHs and PCBs are dangerous products of the degradation of crude oil in the marine environment. They are linked to developmental abnormalities and cancer in humans and other animals. Several studies have assessed the impact of these chemicals on sharks. Silky sharks, bull sharks, blacktip sharks, sandbar sharks, and bonnetheads exposed to the Deepwater Horizon spill have shown enhanced levels of PCBs and PAHs in their systems.

These sharks accumulate toxins because they are at the top of the food chain, a process known as "bioaccumulation." Scientists are concerned that the levels of these toxins could lead to serious health issues such as reduced growth rates, decreased fertility, and weakened immune systems. The long-term effects of oil spills on marine life are still being uncovered, and future research may reveal even more impacts.

Impact on human health

The impact of ocean pollution on shark health is a growing concern, and it also has implications for human health. With high concentrations of pollutants, including toxic chemicals, heavy metals, and inorganic substances, found in sharks, human health is at risk when consuming shark meat and other derivatives.

Sharks are apex predators, and their diet is a significant pathway for the bioaccumulation of toxins. As a result, around two billion people living within 100 km of the coast are exposed to these toxins when consuming seafood, especially meat and other products derived from sharks.

The US Food and Drug Administration and the Environmental Protection Agency have advised against consuming shark meat due to the high levels of mercury found in these species. A single serving of shark meat can expose adults and children to over three times the maximum recommended daily mercury intake. High mercury levels have been documented in various shark species, including blue sharks, silky sharks, and hammerheads.

In addition to mercury, other pollutants found in shark tissue, such as PCBs and dioxins, pose significant health risks. Exposure to these contaminants has been linked to cancer, liver and kidney damage, immunosuppression, reproductive defects, and endocrine disruption. Pregnant women and young children are especially vulnerable to these adverse health effects.

The impact of plastic pollution on shark health is also a growing concern. Microplastics and nanoplastics, which are ingested by sharks, can accumulate in their organs and cells, potentially transferring toxic chemicals to humans who consume shark products.

To mitigate these risks, regular testing of shark products by food safety agencies is essential. Additionally, consumers need to be made aware of the potential health hazards associated with consuming shark meat and other derivatives. Local authorities should consider implementing a pause on the sale and export of shark products if pollutant levels exceed safe limits, creating an incentive to identify and reduce pollution inputs.

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