The suffocating event may be brought on by factors such as algae blooms, droughts, high temperatures[4] and thermal pollution.

Fish kills may also occur due to the presence of disease, agricultural runoff, sewage discharges, oil or hazardous waste spills, hydraulic fracturing wastewater, sea-quakes, inappropriate re-stocking of fish, poaching with chemicals, underwater explosions, and other catastrophic events that upset a normally stable aquatic population.

Fish are subject to various viruses, bacteria and fungi in addition to parasites such as protozoans, flukes and worms, or crustaceans.

Signs of disease include sores, missing scales or lack of slime, strange growths or visible parasites, and abnormal behavior–lazy, erratic, gasping at the water surface or floating head, tail or belly up.

For example, since 2004 fish kills have been observed in the Shenandoah River basin in the spring, from the time water temperatures are in the 50s (°F) until they reach the mid-70s.

In addition to altered behavior, affected fish have swollen gills that are mottled and have the appearance of ground hamburger meat.

For example, hundreds of thousands of fish died after an accidental spill of bourbon whiskey into the Kentucky River near Lawrenceburg.

However, officials could not determine whether the fish kill was due to the bourbon directly or to oxygen depletion that resulted when aquatic microbes rapidly began to consume and digest the liquor.

Chlorine introduced as alkaline hypochlorite solution is also extremely toxic,[14] leaving pale mucilaginous gills and an over-production of mucilage across the whole body.

[16] One of the more bizarre and recently discovered diseases produces huge fish kills in shallow marine waters.

[19][20] The term "red tide" is also commonly used to describe harmful algal blooms on the northern east coast of the United States, particularly in the Gulf of Maine.

[21] These blooms are natural phenomena, but the exact combination of factors that result in red tide outbreaks is not fully understood.

[23] Excessive anthropogenic nutrient enrichment of phosphorus and nitrogen allow for rapid growth and multiplication of phytoplankton in the Mississippi River.

In the water, higher concentrations of organic matter are present because of the high reproductive rate of the phytoplankton over a short period of time.

Furthermore, a significant detrimental outcome caused by eutrophication in the Mississippi River is the increased uptake of dissolved oxygen by bacteria, in response to higher concentrations of organic matter.

[24] As the dead phytoplankton accumulate, detritus, or organic matter waste, forms at the surface along with other bacteria and algae.

Consequently, as more bacteria, phytoplankton, and algae exponentially grow and multiply, the more submerged aquatic vegetation die, because they do not have access to sunlight due to eutrophication.

As a result of the excess nutrient enrichment in the Mississippi River, dead zones appear in the Gulf of Mexico, created from the process of eutrophication.

The dead zones in the gulf are mainly created by the nitrogen and phosphorus enrichment of the Lower Mississippi River.

An August 2010 fish kill in Delaware Bay was attributed to low oxygen as a result of high temperatures.

[25] A massive (hundreds of thousands) fish kill at the mouth of the Mississippi River in Louisiana, September, 2010, was attributed to a combination of high temperatures and low tide.

Native to Africa's Nile River, the tilapia stop feeding when water temperatures drop below 60 °F (16 °C) and die when it reaches 45 °F (7 °C).

In many places, an assessment of potential effects of underwater explosions on marine life must be completed and preventive measures taken before blasting.

Low river flows also reduce the available dilution for permitted discharges of treated sewage or industrial waste.

Recommended stocking densities are available from many sources for bodies of water ranging from a home aquarium or backyard pond to commercial aquaculture facilities.

The discharge of red aluminium sludge from a reservoir in Hungary into the Marcai River is acknowledged as causing environmental devastation,[31] The loss of adult fish also can have long-term impacts on the success of the fishery as the following year's spawning stock may have been lost and recovery of the pre-kill population may take years.

In small ponds, mechanical aeration and/or removal of decaying matter (such as fallen leaves or dead algae) may be reasonable and effective preventive measures.