Fish Resilience In Polluted Waters: Survival Strategies

Fish are incredibly resilient creatures, but they are facing an increasing threat from manmade pollution. Water pollution can directly kill or harm fish, or change their environment, killing off their food sources or causing plant or algae overgrowth that starves fish of oxygen. Fish in polluted waters must work harder to survive, using up to a third of their energy to filter out harmful contaminants. While some fish species are more sensitive to pollution than others, all fish are impacted by pollutants, and their survival is made much more difficult.

Fish require more energy to survive in polluted water

A study by Graham Scott, a biologist at McMaster University in Hamilton, Ontario, found that fish living downstream from a waste treatment plant used at least 30% more energy to survive. The fish closest to the plant, at just 50m remove, used 36% more energy than those in clean water, while those at 830m away still used 30% more energy. This is because their bodies must use energy to filter out the harmful contaminants found in the water.

The energy expended to remove pollutants is energy that could have been used for hunting food, escaping predators, or sourcing a mate. The contaminants may also be affecting the fish in unknown ways, such as causing sex-changing effects. For example, a study found that elevated oestrogen levels in rivers were having a transformative effect on the gender of fish living there.

Some fish species may be better adapted to deal with pollution. Killifish, for example, have been found to survive in highly contaminated sites with pollution levels up to 8,000 times the lethal dose for most killifish. Scientists suspect that these fish are molecular mutants, with genetic changes that help them tolerate pollution without altering their physical appearance.

However, even for species that are more tolerant of pollution, there may be trade-offs. For example, the pollution-tolerant killifish lost some of their genetic diversity, which could make it harder for them to cope with future environmental stressors. Overall, while some fish can survive in polluted waters, it is clear that pollution makes survival much more challenging and energy-intensive for them.

Contaminants in fish are passed on to humans through consumption

Fish are exposed to contaminants in various ways, including through polluted water, sediments, and prey. These contaminants can then be passed on to humans through the consumption of contaminated fish.

Water pollution can occur due to industrial and municipal discharges, agricultural practices, stormwater runoff, and even rainfall washing chemicals from the land or air into water bodies. As a result, fish living in these contaminated waters may absorb pollutants such as mercury, PCBs (polychlorinated biphenyls), PBDEs, dioxins, chlorinated pesticides, and PFAS (per- and polyfluoroalkyl substances).

Mercury, a metal present in the environment due to natural and human-related activities, is a common contaminant in fish. It enters water bodies through improper disposal, industrial and household waste, and coal burning and mining activities. Mercury is converted to methylmercury or methlymercury by bacteria in the water, which is then consumed by small creatures that are eaten by fish, leading to bioaccumulation. This means that larger, older fish tend to accumulate higher levels of mercury in their tissues and organs.

PCBs, though no longer manufactured, can still be found in old electrical equipment and capacitors. They are fat-soluble, increasing the likelihood of entering and remaining in the bodies of fish and other organisms. Other contaminants, such as PFAS, are widespread in the environment and can persist for long periods. They are used in a variety of consumer products, including non-stick cookware and clothing.

Consuming contaminated fish can lead to the build-up of these toxins in the human body, resulting in adverse health effects. Long-term exposure to contaminated fish, even at low levels, can impact the nervous system and muscle coordination. Mercury exposure is particularly harmful to pregnant women, as it can affect the development of the fetus's brain and nervous system. PCBs have been linked to liver damage, skin irritation, reproductive issues, and cancer in animal studies. PFAS exposure has also been associated with various health concerns, although research in this area is ongoing.

It is important to note that not all fish are equally contaminated, and certain species tend to have lower levels of contaminants. Additionally, preparing and cooking fish properly, such as broiling, grilling, or baking, can help reduce the risk of consuming contaminants.

Some fish have evolved to survive in polluted waters

Fish are increasingly threatened by manmade pollution, with some chemicals making their way through even the most advanced wastewater treatment systems and into streams and waterways. While pollution makes survival harder for fish, some fish have evolved to survive in polluted waters.

Killifish, sometimes called mud minnows or mummichog, are abundant in the brackish waterways and marshes along the Atlantic coast. They are typically sensitive to pollution and are used as an indicator species to gauge the health of an ecosystem. However, killifish have been found to survive in some highly contaminated sites, tolerating levels of pollution up to 8,000 times the lethal dose for most killifish.

Genome sequencing has revealed that these pollution-tolerant killifish have a common set of mutations that help deactivate or turn off a molecular pathway responsible for much of the cellular damage caused by chemicals. These mutations are rare among fish living in unpolluted areas, as they do not provide a survival advantage in clean waters. The genetic diversity of killifish, higher than that of any other vertebrate measured so far, including humans, has allowed them to quickly adapt and survive in polluted habitats.

While this local adaptation serves an immediate purpose, it may come at a cost. A loss of genetic variability could make it harder for killifish to cope with future environmental stressors. Additionally, the energy expended in filtering out harmful contaminants can be a disadvantage, as it takes away from energy that could be used for hunting food, escaping predators, or sourcing a mate. Despite their remarkable resilience, the presence of pollution-tolerant killifish does not mean that other fish will evolve similar adaptations.

Water pollution can directly kill or harm fish

The impact of water pollution on fish health and survival was observed in a study conducted on the Cutler Reservoir in northern Utah. The reservoir was listed as water quality impaired due to high levels of phosphorus and low dissolved oxygen levels. Despite this, a diverse range of fish species was found to be thriving in the reservoir. This suggests that some fish possess adaptations or mutations that allow them to tolerate poor water quality and low oxygen levels.

However, it is important to note that water pollution can still have detrimental effects on fish populations. Even if the contaminants themselves do not directly cause mortality, they can put the fish at a significant disadvantage. For instance, fish exposed to pollutants may need to expend more energy on filtering out harmful substances, leaving less energy for essential activities such as hunting, escaping predators, and reproduction. This can impact their growth potential and reproductive abilities, ultimately affecting the overall fish population.

Additionally, water pollution can have indirect effects on fish survival by impacting their habitat and food sources. Pollutants can alter the aquatic environment, making it less suitable for certain fish species. For example, changes in water temperature, pH levels, and the presence of toxic substances can create stressful conditions for fish, making it difficult for them to thrive and increasing their susceptibility to diseases.

While some fish, like killifish, have been found to evolve and develop molecular mutations that allow them to survive in highly polluted waters, this is not the case for all species. The ability to adapt to polluted environments may depend on factors such as genetic diversity and the presence of specific molecular responses. As a result, water pollution remains a significant threat to many fish populations, and it is crucial to address this issue to ensure the health and sustainability of aquatic ecosystems.

Pollution can cause plant overgrowth and harmful algal blooms, starving fish of oxygen

Water pollution is a growing concern for fish survival, as contaminants are increasingly infiltrating streams and waterways due to inadequate wastewater treatment and agricultural runoff. While fish can technically survive in polluted waters, their struggle to survive becomes significantly harder.

One of the ways pollution harms fish is by causing plant overgrowth and harmful algal blooms, which subsequently deplete oxygen levels in the water. Eutrophication, or the enrichment of water bodies with excessive nutrients, is a primary cause of harmful algal blooms. These blooms can occur when agricultural runoff, laden with nitrogen- and phosphorus-rich fertilizers, pollutes waterways. Warm temperatures and reduced water levels further contribute to the ideal conditions for algal blooms.

The excessive growth of algae can have detrimental effects on the ecosystem. Firstly, algal blooms can block sunlight from reaching other organisms, hindering their ability to photosynthesize and create energy. Additionally, the rapid growth of algae consumes available nutrients in the water, leaving other organisms deprived of essential resources. As the algae decompose, they further deplete oxygen levels through nightly respiration and bacterial decomposition.

The resulting oxygen depletion, or hypoxia, severely impacts fish populations. Aquatic insects, fish, and other animals need dissolved oxygen (DO) in the water to breathe. When DO levels drop too low, fish populations may be forced to migrate, fail to reproduce, or even die. This phenomenon is known as a "summer fish kill."

To address this issue, it is crucial to reduce nutrient pollution in water bodies. Properly managing fertilizer use and maintaining septic systems can help prevent the excessive growth of algae. By taking such preventive measures, we can help alleviate the oxygen depletion caused by plant overgrowth and harmful algal blooms, ultimately improving the survival chances of fish in polluted waters.

Frequently asked questions