Fish Deaths: Understanding The Impact Of Water Pollution

Fish kills, also known as fish die-offs, refer to the mass death of fish populations, which may be linked to the more general mortality of aquatic life. The most common cause of fish kills is low oxygen levels in the water, which may be due to a variety of factors, including harmful algal blooms, overpopulation, drought, and a sustained increase in water temperature. Infectious diseases, parasites, and toxicity from human activity are also causes of fish kills. Toxicity is less common but is often associated with man-made water pollution, such as the release of chemicals and waste.

Low oxygen levels in water

There are several factors that influence the DO levels in water bodies. Firstly, temperature plays a significant role. Warmer water holds less dissolved oxygen than colder water, and during the summer months, water temperatures increase, leading to reduced DO levels. This is further exacerbated by the metabolic rate of fish, which accelerates in warmer water, resulting in higher oxygen consumption by the fish. Additionally, weather conditions can impact DO levels. Cloudy days can slow down or halt oxygen production through photosynthesis, while still and windless days limit the circulation of water and surface diffusion of atmospheric oxygen.

Another factor contributing to low DO levels is the presence of decaying organic matter, such as aquatic plants and algae. As these organisms decompose, they consume oxygen, reducing the amount available for fish. This is particularly significant during algal blooms, which are often fuelled by fertilizers and excess nutrients like nitrogen and phosphorus. The decomposition process, driven by bacteria, further decreases DO levels as the bacteria require oxygen to break down the organic matter.

Low DO levels can also be attributed to poor water circulation or flushing, dredging, and sudden weather changes. For example, a heavy rainstorm or cold front can cause the top layer of water to cool and mix with the deeper, oxygen-deficient water, leading to a pond-wide oxygen depletion.

The impacts of low DO levels on fish are significant. Fish initially react by moving and eating less. As oxygen levels continue to drop, they exhibit labored breathing and rapid gill movements as they struggle to obtain enough oxygen. Eventually, they may begin gasping at the surface, known as "piping," and if levels remain critically low for an extended period, fish death can occur.

Toxicity from human activity

Fish kills, or fish die-offs, refer to a localized mass die-off of fish populations. While toxicity is a less common cause of fish kills than factors such as reduced oxygen levels in the water, it is often associated with man-made water pollution. Human activity, such as agricultural runoff, sewage, surface runoff, chemical spills, and hazardous waste spills, can all lead to water toxicity and subsequent fish kills.

One example of a toxic compound that can cause fish kills is pesticides. Pesticides are toxic to waterborne insects, which are a source of food for fish. If the pesticide does not kill the insect, it is transferred when a fish eats it. Over time, the pesticide builds up in the fish until it reaches a fatal level.

Another way that human activity can cause toxicity in the water is through the introduction of nutrients such as nitrogen and phosphorus. These nutrients enter rivers, lakes, and oceans through runoff, such as rain washing excess fertilizer from a lawn into a lake, or through the direct discharge of processed sewage into a river by a sewage treatment plant. As these excess nutrients build up in a body of water, they cause plants and algae to grow at accelerated rates, leading to plant overgrowth and harmful algal blooms. When plants die, the decay process lowers the dissolved oxygen level in the water to a level too low for fish to survive.

Additionally, some species of algae produce toxins that can accumulate in the bodies of fish that ingest them and are passed on to other fish that eat them. An example of this is the algal bloom of Karenia brevis, a microscopic marine dinoflagellate common in the waters of the Gulf of Mexico. This algal bloom produces a toxin that paralyzes the central nervous system of fish, causing them to be unable to breathe.

Waterways laced with traces of endocrine-disrupting synthetic chemicals have also been shown to exhibit gender-bending in fish, with male fish taking on the appearance and behavior of females and some possessing both male and female organs. Antidepressants have also been found to affect fish behavior.

Natural causes of low oxygen

Fish require oxygen to survive, and they obtain it in the form of oxygen gas dissolved in the water. The concentration of oxygen in water is often reported in mg/L or as a percentage of saturation. The amount of oxygen in a water body varies naturally, both seasonally and over time.

One natural cause of low oxygen levels in water is stratification in the water column. This occurs when less dense freshwater from an estuary mixes with heavier seawater, restricting the supply of oxygen from the surface waters to the more saline bottom waters, leading to hypoxic conditions in bottom habitats.

Another natural factor influencing oxygen levels is the weather. On cloudy days, the production of oxygen through photosynthesis is slowed or halted. Still, windless days limit the circulation of water and the surface diffusion of atmospheric oxygen. During the hot summer months, deeper ponds experience stratification as the water near the surface warms and becomes less dense than the cooler water near the bottom. As the season progresses, the cool water near the bottom becomes stagnant and depleted of oxygen.

Algae play a significant role in oxygen levels in water bodies. During the daytime, algae produce oxygen through photosynthesis, but at night, they consume oxygen through respiration. When algae and other organisms die, the organic matter left behind and the bacteria consuming it also consume oxygen. Excessive levels of algae can cause dissolved oxygen levels to drop rapidly, leading to fish suffering and death.

Disease and parasites

Fish are susceptible to various pathogens, including bacteria, viruses, and fungi, as well as parasites like protozoans, flukes, worms, and crustaceans. These pathogens and parasites are naturally occurring in many water bodies, and stressed fish, due to factors like spawning or poor water quality, become more vulnerable to them.

Some common indicators of disease in fish include sores, missing scales, discolouration, abnormal growths, visible parasites, and abnormal behaviour, such as laziness, erratic movements, gasping for air at the water surface, or floating in an abnormal position. For instance, a bacterial infection may cause bloody lesions, tumours, excess mucus production, discolouration, loss of scales, and bulging or sunken eyes. Viral infections, on the other hand, do not affect humans, but some bacterial and parasitic infections can be transmitted to people.

Parasites can also be influenced by and benefit from the presence of pollutants. Endoparasitic protozoons, such as myxozoons, microsporans, and haematozoons, increase in prevalence and intensity when interacting with pollutants. Parasites may also enhance their host's susceptibility to pollutants, making the fish more vulnerable to the contaminated environment.

In addition, certain parasites have complex life cycles, involving multiple hosts. For example, the Henneguya parasite infects juvenile salmon as they migrate to the ocean, and the fish carry the parasite until the next spawning cycle. The myxosporean parasite, which causes whirling disease in trout, has a similar life cycle.

To manage diseases and parasites in fish, strategies such as vaccine development and improving genetic resistance have been employed. The ParaFish Control project, for instance, aims to control parasitic diseases in European fish farms.

Climate change and temperature rise

Climate change and rising temperatures pose a significant threat to fish populations, with freshwater fish species being particularly vulnerable. Freshwater habitats cover only 0.8% of the Earth's surface, yet they host around 15,000 fish species, accounting for approximately half of the known global fish diversity. These unique ecosystems are facing substantial declines in biodiversity due to human activities and environmental changes.

One of the primary ways in which rising temperatures affect fish is by reducing the oxygen levels in the water. Warmer water holds less oxygen, and this oxygen loss has already led to mass die-offs of fish around the world. For example, in New Zealand, a rapid surge in water temperatures caused marine salmon farms to dump over a thousand tonnes of suffocated fish. Additionally, warmer waters tend to favour sub-tropical zooplankton, which are poor food for juvenile salmon and the fish they prey on, further reducing their chances of survival.

The effects of rising temperatures are exacerbated by other factors related to climate change. For instance, the construction of dams and urban development have hindered the ability of salmon to migrate upstream to reproduce. The combination of warming waters, reduced oxygen levels, changing stream flows, and increased stressors such as runoff, pollutants, and land degradation pose a significant threat to freshwater ecosystems.

Moreover, climate change is predicted to cause a six-fold increase in the frequency of fish mortality events by 2100 based on local water temperature projections. This increase is expected to be even more severe, with a 34-fold increase, when considering local air temperature projections. These projections highlight the urgent need to address climate change and its impact on fish populations.

Research also suggests that climate change could lead to the extinction of up to 60% of all fish species in the oceans by the year 2100. This is a significant concern as fish are an important source of nutrition for humans, and their loss could have far-reaching consequences for both ecosystems and human communities. To protect fish populations, it is crucial to limit climate change by reducing the burning of fossil fuels and mitigating other human activities that contribute to environmental degradation.

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