Sharks In Polluted Waters: Understanding The Impact

Water pollution is a pressing issue that poses a significant threat to marine life, including sharks. With the vast array of pollutants, such as plastics, oil spills, and chemical contaminants, finding their way into our oceans, it is crucial to understand the impact they have on these apex predators. Sharks, occupying the top of the marine food chain, are highly susceptible to the accumulation of toxins and pollutants in their bodies. The consequences of water pollution on sharks are wide-ranging and detrimental, from physical injuries and intestinal damage to endocrine disruption and reduced reproductive success. Understanding these effects is vital for developing effective conservation strategies to protect shark populations and maintain the delicate balance of marine ecosystems.

Microplastics ingestion

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, for example, in the production of cosmetic microbeads. Microplastics can be found everywhere in our environment, from personal care products and synthetic clothing to degraded plastic bags and bottles. They are very lightweight and can be dispersed easily by wind, water, and humans. These tiny materials find their way into the ocean through various pathways, including river and stream runoff, beach littering, atmospheric deposition, fishing and aquaculture activities, wastewater treatment plants, and the weathering of plastics that are already in the ocean.

Sharks are particularly vulnerable to the risk posed by microplastics. They are apex predators at the top of the marine food chain, and they have a long lifespan of about 20 to 100 years. They reproduce slowly and don't give birth to many pups, and they exhibit slow growth rates, which allows them to accumulate microplastics in their bodies for a very long time. Sharks 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 feed on them. For example, a study found that whale sharks, which are filter feeders, could mistakenly 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.

The impact of microplastics on sharks is significant and includes:

• Intestinal damage: 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 even death in cases of long-term exposure.
• Increased toxin levels: Microplastics act as vectors of toxins as they adsorb toxic additives and chemicals and then release them into the shark's body. This may lead to toxicological effects such as a decrease in fitness and overall health.
• Starvation: 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 ingested microplastics offer no nutritional value.
• Endocrine disruption: Microplastics may contain chemical additives that, when absorbed by the shark's body, can act as endocrine disruptors. These chemicals can interfere with hormonal balance and signaling systems, potentially influencing the shark's behaviour and reproductive processes.
• Altered migration patterns: Behavioural changes induced by the presence of microplastics might lead to alterations in migration patterns. Sharks often follow migratory routes in search of food, and disruptions in their sensory or navigational abilities could lead to deviations from typical migration paths.
• Interference with immune system responses: Microplastics can cause tissue damage, oxidative stress, and changes in immune-related gene expression, as well as alter the shark's antioxidant status.
• Reduced reproduction: The endocrine-disrupting effects of microplastics on sharks may interfere with their reproductive processes, leading to reduced reproductive success and contributing to global population declines.

Climate change may amplify the impact of microplastics on sharks. It causes a rise in sea temperatures and changes in ocean currents, affecting the distribution and abundance of prey species that sharks rely on. This may lead sharks to search for food in different areas, potentially increasing their exposure to microplastics. Additionally, climate-related events, such as extreme weather and storm surges, could enhance the plastic weathering process, releasing more microplastics into the marine environment and making it more likely for sharks to encounter and ingest these particles during their normal feeding activities.

Entanglement in plastic debris

One of the most affected shark species is the tiger shark, Galeocerdo cuvier, from the western South Atlantic Ocean. Circular plastic straps have caused severe trauma in 3% of all tiger sharks sampled, leading to abnormal anatomical development. This indicates a worrisome incidence and consequence of plastic entanglement in this species. The process of removing the plastic strap from an entangled shark can also be detrimental, potentially resulting in post-release mortality. This suggests that, in some cases, strap removal may not be effective in ensuring the shark's survival.

The shape of plastic materials plays a crucial role in shark entanglement. For example, circular plastic straps have been identified as a significant cause of cryptic mortality in tiger sharks. By eliminating the circular integrity of plastic materials at first use, such as by cutting or puncturing, the impacts of plastic pollution on marine megafauna can be mitigated to some extent.

Sharks, as top predators, are highly susceptible to the harmful effects of plastic pollution. The long-term presence of plastic straps can lead to severe trauma and anatomical deformities, impacting the shark's ability to survive and reproduce. Additionally, the issue of ghost nets, or lost or discarded fishing gear, poses a significant entanglement risk to sharks and rays. The synthetic nylon nets used in ghost fishing are designed to be hard for ocean wildlife to detect, and they can trap and kill animals for many years.

The impact of plastic entanglement on sharks is not limited to physical trauma but also includes the transfer of toxic chemicals. Plastics can adsorb and concentrate chemical pollutants, such as PCBs, DDT, PAHs, and heavy metals, and release them into the shark's body upon ingestion. This can lead to toxicological effects and further compromise the health and survival of these iconic predators.

Toxin bioaccumulation

Sharks' position at the top of the food chain means they consume prey that has already accumulated toxins, and these toxins are passed on to them. This process is known as biomagnification, and it results in sharks having higher concentrations of toxins in their bodies than the organisms lower in the food chain. As sharks are long-lived, slow-growing species, they have ample time to accumulate toxins in their bodies.

Sharks can also inadvertently transfer pollutants to their developing pups. A study on common threshers found that an adult female transferred 29-54% of the mercury and organic contaminants in her body to her embryos. Similarly, high levels of organochlorine have been found in young white shark pups, presumed to be transferred from their mother's tissue. These pollutants can have detrimental effects on the health and reproductive success of sharks, impacting their population dynamics.

Sharks are also susceptible to bioaccumulation of microplastics, which are tiny fragments of plastic less than five millimetres in size. Microplastics can enter the ocean through various pathways, such as river runoff, beach littering, and fishing activities. Sharks can ingest microplastics directly from polluted water, as they resemble plankton or algae, or indirectly by consuming other fish that have microplastics in their bodies. Microplastics can cause intestinal damage, increase toxin levels, lead to starvation, disrupt endocrine systems, alter migration patterns, and interfere with immune system responses in sharks.

In addition to microplastics, sharks can also bioaccumulate toxic chemicals and heavy metals, such as mercury, PCBs, DDTs, and organochlorines. High concentrations of these pollutants have been found in various shark species, including blue sharks, white sharks, and shortfin makos. While research on the impact of these pollutants on sharks is still ongoing, studies on similar marine organisms have found neurological disorders, structural damage to organs and gills, reduced fertility, developmental effects, and cancers at similar levels of pollutant exposure.

The bioaccumulation of toxins in sharks is a significant concern, as it not only affects the health and survival of these apex predators but also has potential implications for human health. With sharks occupying such a crucial position in marine ecosystems, addressing and mitigating the impact of toxin bioaccumulation is essential for maintaining the balance of marine life.

Oil spills

The Deepwater Horizon oil spill in 2010 was the largest accidental spill in history. It released 750 million litres of oil into the Gulf of Mexico, affecting over 2,100km of coastal habitats and contaminating surrounding deep-water areas. This spill impacted several shark species in the Gulf, particularly the whale shark. Whale sharks are filter feeders with intricate filtering organs called "gill rakers", which sift minute ocean life out of the water. Thick oil can clog these organs, leading 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, and these chains degrade at different rates. The degradation process releases toxic chemicals such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs). These toxins have been linked to developmental abnormalities and cancer in humans.

Several studies have found elevated levels of PAHs and PCBs in shark species following the Deepwater Horizon spill. Silky sharks, bull sharks, blacktip sharks, sandbar sharks, and bonnetheads all showed enhanced levels of these toxins in their systems. These sharks accumulated high levels of toxins through bioaccumulation, as they are at the top of the food chain. Scientists worry that these toxins could lead to reduced growth rates, decreased fertility, and weakened immune systems in these shark populations.

Chemical contaminants

Sharks, as top predators in marine ecosystems, are highly susceptible to the accumulation of chemical contaminants. The process of bioaccumulation occurs when the amount of a chemical in a shark's body increases at a faster rate than its ability to excrete it. This is further compounded by biomagnification, where sharks ingest contaminated prey, leading to a buildup of toxins in their tissues. The long lifespan and slow reproductive rate of sharks make them particularly vulnerable to the effects of chemical contaminants.

One of the primary ways sharks are exposed to chemical contaminants is through direct ingestion of polluted water. Filter-feeding shark species, such as whale sharks, inadvertently consume microplastics and other pollutants as they feed on plankton and algae. The ingested microplastics can cause intestinal damage, increase toxin levels in their bodies, and lead to malnutrition and starvation. Additionally, the chemicals associated with microplastics can act as endocrine disruptors, interfering with hormonal balance and potentially influencing shark behaviour and reproductive processes.

Indirect ingestion is another pathway for chemical contaminants to enter shark populations. Sharks, being apex predators, feed on various fish species that may have accumulated toxins in their bodies. As a result, sharks consume these contaminated prey items and inadvertently ingest the chemicals they contain. This transfer of pollutants up the food chain can have significant impacts on the health and reproductive capacity of shark populations.

Furthermore, chemical contaminants can also affect shark health through physical abrasion and oxidative stress. Microplastics, due to their small size and lightweight nature, can come into direct contact with the skin, scales, and protective coatings of sharks, causing physical damage and increasing their vulnerability to infections and other environmental stressors. Additionally, the presence of these contaminants can induce oxidative stress and alter immune-related gene expression, compromising the shark's immune system.

The impact of chemical contaminants on shark populations is a growing area of research, and the full extent of their effects is yet to be fully understood. However, it is clear that these contaminants pose a significant threat to the health, reproduction, and overall survival of shark species. As top predators, any disruption to shark populations can have cascading effects on the entire oceanic food web. Therefore, addressing chemical water pollution and reducing the release of toxic substances into marine environments is crucial for the conservation and long-term survival of shark species.

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