1. Pollution:
* Organic Waste: Excess organic matter (like sewage, food scraps, animal waste) decays in water. This decay consumes oxygen, creating a condition called hypoxia. Fish suffocate when oxygen levels are too low.
* Chemicals: Industrial wastewater, pesticides, fertilizers, and even household cleaners can contain toxic chemicals that poison fish directly.
* Heavy Metals: Heavy metals like mercury, lead, and cadmium accumulate in fish and can cause organ damage, neurological problems, and death.
* Pharmaceuticals: Antibiotics, hormones, and other medications are entering waterways through sewage and can disrupt fish physiology, causing reproductive problems, growth abnormalities, and disease susceptibility.
2. Nutrient Overload:
* Eutrophication: Excess nutrients (mainly nitrogen and phosphorus) from agricultural runoff, sewage, and industrial waste cause algal blooms. These blooms block sunlight, deplete oxygen when they decompose, and release toxins that harm fish.
3. Habitat Degradation:
* Sedimentation: Runoff from construction, agriculture, and urban areas carries sediment into waterways. This sediment can bury fish eggs, reduce spawning habitat, and impair water quality.
* Temperature Changes: Wastewater discharge, particularly from power plants, can raise water temperatures, making it harder for fish to breathe and increasing their vulnerability to diseases.
4. Disease Spread:
* Pathogens: Wastewater can carry pathogens like bacteria and viruses that infect fish, causing disease and mortality.
5. Physical Barriers:
* Dams and other barriers: These structures can alter water flow patterns, reduce oxygen levels downstream, and fragment fish habitats.
In summary, waste pollution directly impacts fish health by:
* Reducing dissolved oxygen: Makes it difficult for fish to breathe.
* Introducing toxins: Directly poisons fish.
* Disrupting natural ecosystems: Leads to disease outbreaks and habitat loss.
It's important to note that the specific ways waste affects freshwater fish depend on the type of waste, the concentration, the water chemistry, and other environmental factors. However, the general principle remains: waste pollution poses a significant threat to freshwater fish populations globally.