The idea of fish swimming through the subterranean network beneath our cities is a persistent urban legend. The immediate answer to whether fish live in the sewer system is generally no, as the vast majority of species cannot survive in the harsh environment of a sanitary sewer. The complex reality depends entirely on the type of drainage infrastructure involved. Understanding the difference between sewer types and the unique biology of wastewater reveals why this common myth is largely inaccurate.
The Chemical and Physical Reality of Sewer Systems
Sanitary sewer systems contain a constantly flowing mixture of household and industrial wastewater, creating an environment immediately lethal to almost all fish. The greatest barrier to aquatic life is the rapid depletion of dissolved oxygen (DO). As organic material like human waste and food particles decomposes, vast populations of bacteria consume virtually all available oxygen, quickly rendering the water hypoxic (low oxygen) or anoxic (no oxygen).
Fish require a certain level of DO to survive, often struggling or dying when concentrations drop below 3 to 4 milligrams per liter (mg/L). Raw sewage, however, often has a DO concentration near zero, making breathing impossible. Furthermore, the decomposition process releases high levels of ammonia and hydrogen sulfide, both highly toxic to fish and other water-breathing organisms.
Beyond the lack of oxygen, the water chemistry is fundamentally hostile due to heavy metals, pharmaceuticals, and concentrated cleaning chemicals. These substances disrupt fish physiology, affecting their gills, growth, and reproductive systems. Physical conditions are also unsuitable, characterized by fluctuating temperatures and rapid, turbulent flow rates that offer no stable habitat for fish to rest, feed, or spawn.
The True Aquatic Life of the Sewers
Since fish cannot survive in the low-oxygen, high-nutrient environment, the actual aquatic life in the sanitary sewer consists of organisms adapted to these extreme conditions. The ecosystem is built upon anaerobic and facultative bacteria that break down organic matter without needing free oxygen. These microorganisms form the base of a unique food web thriving on human waste.
Among the most common inhabitants are Tubifex worms, often called sludge worms or sewage worms. These segmented annelids are highly tolerant of low dissolved oxygen and organic pollutants, using hemoglobin to maximize oxygen absorption. Tubifex worms anchor their heads in the sediment, ingesting sludge and bacteria, while waving their posterior ends in the water to breathe.
Another creature adapted to survive in stagnant, oxygen-deprived sewage is the rat-tailed maggot, the larva of the drone fly (Eristalis tenax). This larva is named for its long, telescoping breathing siphon, which functions like a snorkel. The maggot remains submerged, feeding on organic matter, while extending its siphon up to the air surface to breathe.
How Fish Sometimes Enter Drainage Systems
The rare instances of fish being found in underground systems are almost always linked to storm drains or combined sewer systems, not the sanitary sewer line carrying household waste. Storm drains carry rainwater runoff directly to local waterways like rivers and lakes without treatment. During heavy rainfall, fish living in these natural bodies of water can be washed into the storm drain network as the water level rises.
These fish may temporarily survive because the water in a storm drain is rainwater, not raw sewage, and still contains some dissolved oxygen. A more complex scenario occurs with Combined Sewer Overflows (CSOs), which are older systems mixing sewage and stormwater in the same pipes. Extreme weather can overwhelm the system, causing the mixture to overflow into natural waterways or creating temporary backflows that pull fish from rivers into the drainage network.
The fate of pet fish, such as goldfish, flushed down a toilet is swift and fatal in the sanitary sewer environment. Even if they survive the initial journey through the home’s plumbing, they quickly succumb to the lack of oxygen and the toxic chemical cocktail present in the wastewater. Any large aquatic animal, like the occasional eel or turtle found in a pipe, is either an anomaly that swam in from a connected natural water source or was lost in a storm drain. Their presence is not indicative of a thriving sewer fish population.