Predators' Pollution: Toxic Chemicals In The Food Chain

Predators such as sharks, rays and chimaeras are at risk from a range of pollutants, including microplastics, DDT, PCBs and other chlorinated pesticides. These pollutants can be found in animals from nearly every trophic level in the region. Despite increased interest in elasmobranch toxicology, there is still much to learn about bioaccumulation in sharks, especially large pelagics that are relatively long-lived and feed higher on the food chain.

Microplastics

Trophic transfer plays an important role in the exposure of predators to microplastics. Laboratory experiments have shown that trophic transfer of microplastics can occur up to the third trophic level. Predators in aquatic environments can be exposed to microplastics directly through water and indirectly through prey. In fact, predators have been found to ingest more than 200 times more microplastics through prey than directly from water. This indirect pathway of microplastic ingestion may represent a major route of exposure for marine organisms.

A study published in the journal Environmental Pollution found that microplastics have made their way to predators even higher up the food chain. The researchers analysed the faecal matter of captive grey seals and found microplastics present. This finding provides evidence that microplastics can be transferred to predators higher up the food chain, endangering all marine life, from fish to top predators.

Endangered, filter-feeding whale sharks are also at risk from the microplastics in the seas. Researchers are investigating how these sharks are scooping up microplastics as they feed, as they are endangered and vulnerable to the effects of these pollutants.

DDT (dichlorodiphenyltrichloroethane)

Southern California is one of the largest DDT (dichlorodiphenyltrichloroethane) hotspots in the world due to historic dumping in the region by large manufacturing companies. To this day, high levels of DDT and other organic contaminants such as PCBs (polychlorinated biphenyls) can be measured in animals from nearly every trophic level in the region.

DDT is an organochlorine contaminant, and it is one of the major contaminant groups affecting predators. It is important to compare concentrations of organochlorine contaminants among different species at each major life history point (i.e. young, juvenile, sub-adult and adult) and to compare contaminant bioaccumulation trajectories among species that occupy different trophic levels. This can help determine the contribution of the major contaminant group to the total contaminant load and compare levels to other studies that have documented negative effects of the specific chemical of interest.

Elasmobranch toxicology research in southern California has increased in recent years and has produced some sentinel papers on maternal offloading of contaminants and on bioaccumulation in stingrays. Despite this growth in knowledge, we still do not have a good grasp on bioaccumulation in sharks, especially large pelagics that are relatively long-lived and feed higher on the food chain.

In addition to DDT, other pollutants that can be found in predators include PCBs and other chlorinated pesticides. These contaminants can have negative effects on the species and further exploration and study are needed to examine their effects.

PCBs (polychlorinated biphenyls)

PCBs are man-made chemicals that were once widely used in industrial and commercial applications. They are highly persistent in the environment and can accumulate in the tissues of living organisms, including predators. Over time, PCBs can build up to harmful levels in the bodies of these organisms, a process known as bioaccumulation.

Bioaccumulation of PCBs in predators can have significant ecological consequences. For example, in Southern California, there has been an increase in research on elasmobranch toxicology, with several sentinel papers published on maternal offloading of contaminants and bioaccumulation in stingrays. Despite this, there is still a lack of understanding of bioaccumulation in large pelagic sharks, which are relatively long-lived and feed higher on the food chain.

The effects of PCBs on predators can vary depending on the species and the level of exposure. In general, PCBs are known to have toxic effects on the nervous system, immune system, and reproductive system of organisms. They can also interfere with hormone production and regulation, leading to developmental and reproductive issues.

Research on the effects of PCBs in predators is ongoing, with studies focusing on comparing contaminant bioaccumulation trajectories among species that occupy different trophic levels. By understanding the contribution of PCBs to the total contaminant load in these species, scientists can make inferences on the potential effects that could be occurring and identify areas for further exploration.

Chlorinated pesticides

DDT is an organochlorine contaminant, which means it is a type of chemical that is toxic to the nervous system. Organochlorine contaminants can be found in the environment as a result of industrial and agricultural activities. They are persistent organic pollutants (POPs), which means they do not easily break down in the environment and can accumulate in the food chain.

The Save Our Seas Foundation is studying the change in concentration of organochlorine contaminants in individuals within and among three shark species over a large size range. They are also modelling bioaccumulation curves for each species to understand how these contaminants are accumulating in the bodies of sharks.

Bioaccumulation is the process by which contaminants accumulate in the bodies of organisms over time. It occurs when an organism absorbs more of a contaminant than it excretes, leading to an increase in the concentration of the contaminant in the organism's body. Maternal offloading of contaminants is another process by which contaminants are passed from mother to offspring.

Legacy organic contaminants

DDT, or dichlorodiphenyltrichloroethane, is a highly toxic pesticide that was widely used in the mid-20th century. Due to its persistence in the environment and its ability to bioaccumulate in the food chain, DDT has become a significant contaminant in many ecosystems. In southern California, high levels of DDT can be measured in animals from nearly every trophic level, including predators such as sharks.

PCBs, or polychlorinated biphenyls, are another type of legacy organic contaminant. PCBs were commonly used in industrial and commercial applications until they were banned in the 1970s due to their toxic effects on human health and the environment. However, PCBs are highly persistent in the environment and can still be found in high concentrations in many ecosystems, including marine environments.

Other chlorinated pesticides, such as chlordane and dieldrin, are also considered legacy organic contaminants. These pesticides were widely used in agriculture and pest control until they were banned or restricted due to their toxic effects. However, like DDT and PCBs, these pesticides persist in the environment and can accumulate in the tissues of predators, leading to potential health risks for both the predators and the humans who consume them.

The presence of legacy organic contaminants in predators is a cause for concern due to their potential impacts on the health and reproductive success of these animals. These contaminants can interfere with hormonal and reproductive systems, leading to reduced fertility and birth defects. They can also cause immune system disorders and increase the risk of cancer. Additionally, these contaminants can be passed from mother to offspring, further exacerbating their impacts on predator populations.

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