Brian Barton
Water pollution is a pressing issue that poses a significant threat to aquatic life, particularly fish. Fish are highly susceptible to the detrimental effects of pollutants, which can lead to mass mortality events known as fish kills. These incidents are often caused by various human activities and natural factors, resulting in the contamination of waterways with toxic substances such as agricultural pesticides, fertilizers, and industrial waste. The impact of pollution on fish is twofold: it can directly poison and kill fish or indirectly disrupt their ecosystem, leading to oxygen depletion and the destruction of their food sources. Understanding the complex interplay between pollution and its deadly consequences for fish is crucial for devising effective strategies to mitigate this pressing issue.
| Characteristics | Values |
|---|---|
| Frequency | Hundreds of fish kills occur in Minnesota every year, mostly in lakes and ponds. |
| Cause | Pollution can directly kill fish or harm them by changing their surroundings, killing their food sources, or causing plant or algae overgrowth that starves them of oxygen. |
| Type of Pollution | Nitrogen, phosphorus, pesticides, manure, fertilizers, high-temperature wastewater, stormwater discharges, toxic spills, heavy metals, hydrocarbons, crude oil, used oil, oilfield brine, sewage, chlorinated water, etc. |
| Impact | Fish may die from starvation, suffocation, disease, or gas bubble disease. |
| Prevention | Reducing the amount of pollution entering waterways, matching fertilizer applications to crop needs, treating sewage effluent, and controlling atmospheric emissions from industry and transport. |
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Pesticides and fertilisers
Pesticides are chemicals used to kill or control pests such as insects, weeds, fungi, and other pests. They are often used to protect crops and increase yields. However, pesticides are toxic to invertebrates that fish feed on, and can be transferred to fish when they eat these invertebrates. Over time, pesticides can build up in fish until they reach fatal levels. Pesticides can also contaminate water bodies through rainfall and runoff, especially after heavy rainfall events. The movement of pesticides into water bodies can occur through runoff in areas that have been sprayed with pesticides or through soil erosion.
Fertilizers are another contributor to water pollution that can harm fish. Lawns and plants are often unable to absorb all the water-soluble fertilizers in chemical fertilizers, leading to water pollution. As algae grow out of control, it reduces the clarity and visibility of the water, hindering photosynthesis by oxygen-producing aquatic plants, and thus reducing the oxygen available for fish. Repeated algae blooms can create green-colored lakes with low oxygen levels, leading to fish kills or depleted water habitats.
To minimize the impact of pesticides and fertilizers on fish, proper land and water management practices are crucial. This includes smart application techniques, such as avoiding applications during rainfall or on saturated soil, staying clear of steep slopes and water bodies, and following application guidelines. Additionally, using water-insoluble fertilizers, such as organic fertilizers, can help prevent the leaching of fertilizers into water bodies.
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Pharmaceuticals
The presence of pharmaceuticals in the water has been shown to have detrimental effects on fish populations. For example, studies have found that exposure to even small amounts of synthetic estrogen from oral contraceptives can lead to smaller fish populations and a higher ratio of female to male offspring. This is because estrogen causes male fish to develop female characteristics, resulting in intersex fish with both male and female traits. Additionally, common drugs like non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen, aspirin, and naproxen, have been found to negatively impact the organ functions of fish, causing cytotoxic and genotoxic damage, high oxidative cell stress, and detrimental effects on growth, reproduction, and behavior.
The impact of pharmaceuticals on fish populations is a growing concern, and it highlights the need for better wastewater treatment methods and the reduction of pharmaceutical pollution entering water bodies.
Furthermore, pharmaceuticals in the water can also have indirect effects on fish populations by impacting their food sources and the surrounding environment. For example, pesticides used in agriculture can contaminate water bodies, leading to the death of invertebrates that fish rely on for food. Additionally, nitrogen and phosphorus pollutants can cause excessive plant and algae growth, reducing oxygen levels in the water and creating harmful conditions for fish survival.
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Low oxygen levels
Fish require untainted food, suitable habitat, and adequate oxygen to survive. Pollution can directly kill fish, but it can also change the makeup of their surroundings, killing their food sources or causing plant or algae overgrowth, which starves the fish of oxygen.
In addition, slow-moving or still water may have low oxygen levels due to a lack of turbulent aeration. Slow currents can also hamper the delivery of oxygen to organisms. Reduced water volume can further concentrate fish into pools or other refugia where respiration exceeds oxygen renewal. Water volume can be reduced by removal for irrigation or other uses, seasonal changes in rainfall, or loss of suitable habitat due to episodic pollution, temperature increases, or other factors.
Agricultural practices can also reduce oxygen concentrations in nearby surface waters by contributing nutrients, pesticides, and organic matter. Channel alteration, such as stream channel straightening, can reduce turbulence and alter curves and riffles, deepening the channel and reducing the surface-to-volume ratio, which decreases diffusion and aeration.
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Disease
Fish require untainted food, a suitable habitat, and adequate oxygen to survive. Any element, chemical or natural, that upsets this balance is considered water pollution. While water pollution can directly kill or harm fish, it can also indirectly affect fish survival by changing the makeup of their surroundings. For example, pollution can kill off sources of food or cause plant or algae overgrowth, which starves the fish of oxygen.
Fish are also susceptible to various diseases caused by water pollution. For example, pesticides can contaminate waterways and result in fish kills. Lower levels of pesticides and other pollutants may not immediately kill fish but can result in an accumulation of toxins in their bodies. This can lead to immunosuppression, making them more susceptible to diseases such as epidermal papilloma, fin/tail rot, gill disease, hyperplasia, liver damage, neoplasia, and ulceration.
In addition, fish can be weakened by disease and then killed by stressful environmental situations caused by pollution, such as low-oxygen concentration and temperature extremes. For instance, when fish move from cold water to much warmer water, bubbles may form in their tissues, leading to gas bubble disease and death. Similarly, when the oxygen concentration in the water falls below the level at which fish can survive, they suffocate. This can occur due to eutrophication, which is the artificial stimulation of plant growth by pollution with fertilizers, sewage, or atmospheric fallout. As the excess plant growth decays, it lowers the oxygen concentration in the water.
Furthermore, bacterial, viral, and parasitic infections can affect fish populations, especially when their immune systems are compromised due to pollution. While these infections typically affect only a single species, they can still have a significant impact on fish populations. Human activities that release toxic chemicals, such as pesticides, fertilizers, and sewage, can also contribute to the spread of diseases among fish populations.
Overall, while the direct link between adverse water quality and fish diseases has not been proven, it is clear that water pollution plays a significant role in weakening the immune systems of fish and making them more susceptible to various diseases and environmental stressors.
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![Comparison of simulation models used in assessing the effects of power-plant-induced mortality on fish populations G.L. Swartzman, R.B. Deriso, C. Cowan, Center for Quantitative Scie [Leather Bound]](https://m.media-amazon.com/images/I/61IX47b4r9L._AC_UY218_.jpg)
Extreme weather
In Minnesota, for example, the state's weather is becoming warmer and wetter, with more frequent transitions from extreme wet to dry conditions. These weather patterns can increase the risk of fish kills in streams. Intense storms can wash away valuable nutrients and pollutants and send them downstream, polluting waterways. Low-flow streams during hot and dry periods are more dramatically impacted by polluted runoff since there is not enough water to dilute the pollution.
Additionally, extreme weather can affect water temperatures, which can stress fish and make them more susceptible to parasites and diseases. Warmer water holds less dissolved oxygen, and during extreme heat and drought, the demand for oxygen by fish can exceed the available oxygen, leading to fish kills. Unusually cold temperatures can also be harmful, with water temperatures below 40°F for extended periods potentially killing fish, especially if they are unable to acclimate and move to warmer water.
Pollution can exacerbate the effects of extreme weather by decreasing dissolved oxygen levels in water, changing the pH, and introducing toxic chemicals. Heavy metals, pesticides, and nanoparticles can cause severe destruction to aquatic organisms, including fish. Climate change is also emerging as a main environmental stressor in aquatic ecosystems, with modern aquatic ecosystems increasingly altered and subjected to daily pollution events and other human-driven stressors.
To minimize the impact of extreme weather and pollution on fish kills, various actions can be taken, such as proper waste disposal, smart application of manure and pesticides, and the use of retention ponds and filtration systems to treat runoff before it enters waterways.
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Frequently asked questions
Fish require untainted food, a suitable habitat, and adequate oxygen to survive. Pollution upsets this balance and can directly kill or harm fish. For example, pesticides are toxic to waterborne insects, which are a food source for fish. If the insect is not killed by the pesticide, it is transferred to the fish when eaten, and the pesticide builds up in the fish until it reaches a fatal level. Pollution can also cause oxygen depletion in water, leading to fish suffocation.
In trout streams, fish kills are often related to the discharge or runoff of pollutants such as toxic spills, manure, pesticides, or fertilizers. For example, in southeast Minnesota, hundreds of fish kills occur every year, mostly in lakes and ponds. This is due to the region's karst landscape, which makes groundwater more susceptible to pollution.
Many fish kills could be prevented by reducing the amount of pollution entering waterways. This includes properly managing the application of fertilizers, treating sewage effluent, and controlling atmospheric emissions from industry and transport.
