Air Pollution's Impact On Seals: Understanding The Danger

Seals are under threat from human activity and the byproducts of human activity, such as air pollution. While seal hunting is now better regulated, seals are still affected by the disappearance of sea ice due to climate change, and the presence of plastics in the oceans. Microplastics, which are tiny pieces of plastic, are ingested by seals and other marine life, causing bioaccumulation of toxins and endocrine disruption. Seals also face the threat of entanglement in plastic waste and abandoned fishing gear.

Microplastics in the food chain

Microplastics are a widespread pollutant of the marine environment. They can be inadvertently consumed by zooplankton, fish, and even larger filter-feeders, such as whales. Recent research has shown that microplastics can transfer up the food chain from fish to top predators, such as seals. This process is known as trophic transfer.

Trophic transfer occurs when prey containing microplastics are consumed by predators, causing the synthetic particles to move up the food web. While this process has previously been observed in animals lower down the food chain, such as mussels and crabs, the recent study by the Plymouth Marine Laboratory, University of Exeter, and the Cornish Seal Sanctuary is the first to provide evidence of it occurring in marine mammals.

In the study, researchers analysed the faeces of captive grey seals and the digestive tracts of the wild-caught Atlantic mackerel that the seals were fed. The results showed that one-third of the mackerel and half of the scat samples contained microplastics, confirming the occurrence of trophic transfer. This finding is significant as it demonstrates that microplastics can be passed from fish to marine top predators, which has important implications for understanding the impact of plastic pollution on marine ecosystems.

The potential effects of microplastics on seals and other marine organisms are a cause for concern. Research has shown that the presence of microplastics in the digestive system can reduce nutrition absorption, energy reserves, and reproduction, potentially having a negative impact on the animals. Additionally, chemicals and organic substances that attach to the plastic in the water may also have detrimental effects on the health of seals and other marine life.

The issue of microplastics in the food chain is not limited to marine ecosystems. Microplastics have also been found in other foods, such as honey, sugar, and beer. Contamination has been detected in bottled water, with higher levels of microplastics found in disposable plastic bottles compared to glass bottles or beverage cartons. This highlights the pervasive nature of plastic pollution and the potential for human exposure to microplastics through various sources.

Overall, the presence of microplastics in the food chain, including in top predators such as seals, is a growing concern. Further research is needed to fully understand the extent and impact of microplastic ingestion by wild animals and the potential risks it poses to ecosystems and human health.

Entanglement in man-made debris

Marine debris, including plastic waste, is a significant threat to seals and other marine life. Seals can become entangled in man-made debris such as nets and plastic packing straps, which can cause serious injuries and even death. For example, a study in Kaikoura, New Zealand, found high entanglement rates for New Zealand fur seals, with green trawl nets and plastic strapping tape being the most common types of debris. Similarly, Hawaiian monk seals have been found entangled in derelict fishing gear and other plastics, resulting in lethal injuries and drowning.

The impact of man-made debris on seal populations is difficult to ascertain, as many seals die at sea and are not found. However, some studies have estimated entanglement rates of up to 7.9% for certain pinniped populations. Entanglement can affect the dynamics of seal colonies, with rates as low as 0.4% having a significant impact. In addition to entanglement, ingestion of plastic is also a concern, as it can lead to toxic chemicals entering the bodies of seals and other marine life.

Efforts to reduce marine debris and disentangle seals are crucial for mitigating these threats. For instance, a multi-agency program in the Northwestern Hawaiian Islands has successfully removed hundreds of metric tons of ocean plastics over the past 25 years, leading to a substantial reduction in seal entanglement rates. Such initiatives are essential for protecting seal populations and preserving marine ecosystems.

The accumulation of marine debris, including plastic fishing gear and other plastic waste, poses a significant threat to seals and other marine life. Seals can become entangled in man-made debris, such as nets and plastic packing straps, leading to injuries and even death. This issue is particularly prevalent in areas with expanding human populations and industries, such as tourism and fishing, that contribute to the increase in marine debris.

The impact of entanglement on seal populations is challenging to determine, as many incidents may go unnoticed when seals die at sea. However, studies have reported entanglement rates of up to 7.9% for certain pinniped species. Even relatively low entanglement rates can significantly affect the dynamics of seal colonies. For example, a rate of 0.4% is considered high enough to impact colony dynamics.

In addition to physical entanglement, seals are also at risk from ingesting plastic debris. Plastic litter does not biodegrade but instead breaks down into smaller pieces that enter the food chain. These microplastics can be ingested by seals, leading to the accumulation of toxic chemicals in their bodies. The presence of microplastics has been detected in the feces of grey seals, indicating the transfer of plastic pollutants through the food chain.

To address the issue of entanglement in man-made debris, it is crucial to implement measures such as releasing marine animals from entanglement, cleaning up derelict fishing gear, and raising public awareness about the impacts of marine debris. By reducing the amount of marine debris and assisting entangled seals, we can help mitigate the threats posed by human activities to seal populations and the broader marine ecosystem.

Bioaccumulation of toxins

Seals are considered a sentinel species, indicating the health of the marine environment. If the seals are sick, it is a sign that the waters are contaminated. Seals' thick layers of blubber absorb and retain pollutants from industrial waterways.

Bioaccumulation refers to how pollutants enter a food chain and is related to the accumulation of contaminants in biological tissues by aquatic organisms from sources such as water, food, and particles of suspended sediment. It occurs when an organism absorbs a substance faster than it can be lost or eliminated by catabolism and excretion. The longer the biological half-life of a toxic substance, the greater the risk of chronic poisoning, even if environmental levels of the toxin are low.

Another example of bioaccumulation in seals is the presence of microplastics in their feces. Microplastics are tiny pieces of plastic, usually between 0.3 and 5 millimeters in diameter. They can come from larger pieces of plastic debris, such as fishing gear and packaging, or they can be "nurdles," pellets used to make plastic products. Microplastics can be ingested by seals directly or indirectly through their prey. While the seals can excrete the plastic, the toxins that latch onto the plastic particles can cause endocrine disruption and alterations to the immune system if ingested.

In addition to microplastics, seals are also affected by the bioaccumulation of heavy metals and organic pollutants such as polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs). These contaminants can have toxic effects on the seals, including impaired brain development and behavior. The loading of toxic metals in Pacific harbor seals in parts of Puget Sound has been found to increase over their lifetimes, indicating bioaccumulation.

Biomagnification is a related process to bioaccumulation, where the concentration of a chemical or metal increases as it moves up the food chain. This can lead to higher concentrations of toxins in top predators like seals, even if the contaminants are only found in small amounts at the lowest levels of the food chain.

Climate change

Seals are semi-aquatic marine mammals that spend part of their lives on solid ground. They are carnivores and most species eat fish. Seals are under threat from human activity and the byproducts of human activity, such as greenhouse gases, which contribute to climate change.

The impacts of climate change continue to threaten several seal species. Environmental changes, including warming oceans, rising sea levels, ocean acidification, and an increase in the frequency and intensity of extreme weather events, are already affecting the distribution and health of many marine species. Seals that depend on specific environmental conditions at different times of the year are particularly vulnerable.

There are several species of ice-associated seals, including bearded seals, ringed seals, spotted seals, ribbon seals, harp seals, and hooded seals, that rely on sea ice for at least part of the year. They use sea ice to rest, breed, nurse and rear pups, molt, and avoid predators. However, the warming climate is changing the reliability, quality, and extent of sea ice, negatively impacting the health and behavior of these seals. Warmer spring temperatures and earlier ice break-up could force pups into the water before they are independent from their mothers and able to feed on their own. If they are not strong enough to survive in open water, they may be killed and eaten by marine predators or drown.

From 2007 to 2018 in the Bering Sea, sea ice decreased by 47,000 square kilometers each year on average, and scientists observed declines in the body condition of ribbon and spotted seal pups. This was likely due to poor foraging conditions for their mothers during pregnancy and while nursing. In addition, the terrestrial habitats of Hawaiian monk seals, one of the most endangered seal species, are being lost due to sea-level rise, erosion, and more frequent and stronger storms caused by climate change.

Guadalupe fur seals are also vulnerable to warming waters, ocean acidification, and harmful algal blooms, which are increasing in frequency due to climate change. They primarily feed on squid and may be unable to adapt their diet if prey availability changes. In recent years, marine heatwaves in the Pacific Ocean have negatively impacted prey distribution and abundance, leading to higher numbers of stranded animals, including an Unusual Mortality Event from 2015-2021 involving more than 700 seals, particularly younger animals, with signs of malnutrition and weakened immune systems.

Sewage and inorganic marine debris

Seals are considered a sentinel species, indicating the health of the marine environment. Harbour seals, in particular, are affected by pollution because they do not migrate but live in the same region year-round. Their thick layers of blubber absorb and retain pollutants from industrial waterways.

Inorganic marine debris, such as derelict fishing gear and trash, is dangerous to seals and other marine life. Every year, countless animals, fish, and seabirds get entangled in marine debris, resulting in injury or death. Seals and seal pups are especially vulnerable to gill nets, and small scraps of net and line are the most common causes of entanglement.

Additionally, plastic litter does not biodegrade but photodegrades into minute pieces of plastic that enter the food chain. These plastic polymers attract PCBs and other toxic chemicals, creating hormonal imbalances and affecting the reproductive abilities and brain activity of marine life. Microplastics have been found in the feces of grey seals, demonstrating how plastic contamination can move up the food chain.

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