The question of whether fish live in sewers often arises from popular culture, but the reality for sustained fish life in these environments is generally not possible. While isolated incidents of fish entering sewer systems may occur, the conditions within these networks are not conducive to their long-term survival.
Understanding Sewer Environments
Sewer systems are engineered to transport wastewater, creating a specialized environment distinct from natural aquatic habitats. They are characterized by a complete absence of light, leading to perpetual darkness. The water often contains a complex mixture of organic waste, household chemicals, and industrial discharges. Temperatures can fluctuate, and the flow can be turbulent.
Sewer water has extremely low dissolved oxygen (DO) content, frequently approaching anoxia, meaning a near-complete absence of oxygen. This is due to the decomposition of organic matter by microorganisms, a process that rapidly consumes available oxygen. Sewers also contain high concentrations of various bacteria and other microorganisms.
How Fish Might Enter Sewers
Fish can occasionally find their way into sewer systems through several accidental routes. One common scenario involves fish being flushed down toilets, particularly smaller aquarium pets.
Storm drains also serve as entry points, especially during heavy rainfall. When urban areas experience significant precipitation, storm drains can become inundated, sometimes connecting to combined sewer systems. Fish from nearby ponds, rivers, or flooded areas might be swept into these drains, which can then lead into the sewer network.
Survival Challenges for Fish
Sewer systems lack the specific environmental conditions fish need to survive. A significant challenge is the severe lack of dissolved oxygen. Fish gills extract oxygen from water, requiring a minimum of 5-6 parts per million (ppm) for healthy growth and activity; levels below 2 ppm are lethal for most species. The decomposition of organic waste in sewers rapidly depletes oxygen, making respiration impossible.
Sewer water is also a toxic mixture. It contains chemicals like chlorine and chloramines from treated tap water, which are harmful to fish and can damage their gills. Household products contribute pharmaceuticals, cleaning agents, and heavy metals, while industrial discharges add other pollutants. Exposure to these substances can cause gill damage, organ failure, and reproductive impairment.
The absence of a viable food chain also poses a problem. Without sunlight, photosynthesis cannot occur, meaning there are no primary producers like algae to form the base of an aquatic food web. While organic matter exists, it is often contaminated and unsuitable as a food source. The high concentration of disease-causing bacteria, viruses, and parasites in sewage also presents a constant threat, leading to infections and increased metabolic stress.
Other Organisms in Sewers
Other organisms are well-adapted to sewer conditions. Various types of bacteria thrive in these environments, including anaerobic bacteria that do not require oxygen and break down organic matter. Genera like Arcobacter, Acinetobacter, and Aeromonas are common. Fungi also contribute to decomposition.
Invertebrates like Tubifex worms, also known as sludge worms, are frequently found in sewers. These segmented worms tolerate low dissolved oxygen levels and feed on decaying organic materials. Larvae of certain insects, like drain flies, also inhabit sewer pipes, feeding on the microbial films and organic sludge that accumulate there. Larger scavengers, including rats, are common sewer dwellers, benefiting from shelter and abundant organic waste. Occasionally, amphibians and reptiles like frogs, lizards, and alligators may use storm drains and sewer systems for passage or temporary shelter, though they do not typically reside there permanently.