What Causes Fish Kills?

Dead rainbow and brown trout.
Dead rainbow and brown trout. Michael Melford / Getty Images

Fish kills are events when a massive fish die-off occurs. Fish populations are largely invisible to us, but a fish kills will be evidenced by a large number of fish floating and washing up on shores.

Usually an Oxygen Problem

Ultimately, it is the lack of oxygen which commonly is the cause of fish kills. In the summer as surface waters warm, algae proliferate and provide a boost to microscopic life. In itself that is fine, until the algae switches from photosynthesis (producing oxygen) to respiration (using oxygen) during low light periods such as at night or during prolonged cloudy periods. That process leaves less oxygen for fish, which will start dying if they are already under stress from overcrowding, low water levels, or high water temperature. To complicate matters, oxygen gets further depleted when algae starts dying off in large quantity. Bacteria-driven decomposition then uses up a lot of oxygen, driving down the oxygen concentration in the water.

Humans can play a facilitating role in depriving oxygen from water when they release some types of pollutants in river or lake waters. Nutrient pollution is generally to blame, in the form of manure from farm runoff, fertilizer, or ineffective wastewater treatment plants. The phosphorus and nitrogen in these effluents boost algae production, increasing oxygen consumption.

Thermocline and Oxygen Profiles

To understand fish kills in lakes, we need to understand a few key physical characteristics of these bodies of water. An important feature of lakes experiencing seasons is the thermocline. As the surface waters of a lake warm up in the summer, a temperature gradient gets established, with denser, colder water near the bottom and warmer water near the top. That is not at all surprising, except that the temperature change as you go deeper is not gradual. Instead, there is a sharp discontinuity a few meters down, with warmer waters above, and cold water locked in below. The dividing line is the thermocline. This cutting off of the two large masses of water is very significant for fish.

While winds can usually whip around water enough to thoroughly mix it and bring up cold, oxygen-rich water from the depths, the thermocline blocks that process. Mixing only occurs above the thermocline, keeping the warm waters oxygen-poor and facilitating fish kills.

Winter Fish Kills

In snowy regions, fish kills can happen in winter too and there again it is a question of oxygen. During particularly severe winters, snow can lay thick over lake ice, blocking sunlight from reaching the water. As a result, algae die off and decompose, consuming oxygen, and leaving little of it available for fish. The author of these lines has observed a peculiar manifestation resulting from fish kill conditions. On a small lake in the Midwest in late winter, dozens of catfish congregated at a hole in the ice gulping air to survive the oxygen-less water. A red-tailed hawk had made the best of this unexpected bounty, picking off a few desperate fish from the edge of the hole.

Other Causes of Fish Kills

Not all fish kills are the result of fluctuations in dissolved oxygen. Many types of pollutants are toxic to aquatic life and can cause catastrophic events when released at high enough concentrations. Here are some examples of major fish kills:

  • In spring 2016, dead fish turned up on over 200 km of coastline in Vietnam. Conservative estimates put the number of dead fish recovered at over 100 tons, representing millions of fish. Heavily relying on the fishing industry, the people in that region were negatively affected by the fish kill which also impacted farm-raised fish and shellfish. The Vietnamese population responded with outrage, accusing the different levels of government of allowing industries to heavily pollute waters. Anger was long aimed at a large steel factory which discharges waste off the coastline, but it took several weeks for the government to formally identify the company, Formosa Ha Tinh Steel, as responsible. The discharged chemicals included phenols, iron hydroxide, and cyanide. According to Asia Times, the steel manufacturer reportedly agreed to pay $500 million in compensation.
  • The Ogeechee River in Georgia experienced a major fish kill in May 2011. Victim of a toxic release, 38,000 fish were found dead, in addition to alligators, turtles, and birds. The carcasses were spread out over 70 miles of river banks. The King America Finishing textile plant was found guilty of illegally releasing formaldehyde, ammonia, and hydrogen peroxide.
  • Accidents involving mining waste commonly lead to fish kills. The Mount Polly Mine Disaster involved a dam failure, releasing huge amounts of toxic mine tailings and affecting a sockeye salmon population. In many mining regions, acid mine drainage lowers stream pH enough to kill fish (for example in the Gold King Mine spill in Colorado and New Mexico).
  • In the summer of 2003, over a million fish died in the Narragansett Bay near Warwick, Rhode Island. The cause was low oxygen conditions brought about by a really intense rain event dumping tons of runoff containing polluting nutrient waste. In the late summer, warm waters algae proliferated and then sank and died off, their decomposition using up vast amounts of oxygen throughout the water column.​

Fish Kills…On Purpose?

Fisheries managers and aquatic ecologists have a rarely used but powerful tool at their disposal when trying to improve aquatic habitat quality. They sometimes purposefully cause fish kills as a last resort to get rid of invasive fish. Rotenone, a chemical extracted from the roots of a tropical plant, is often used as it kills everything with gills. Rotenone conveniently breaks down rapidly, leaving waters safe for fish after a few days. Once the lake or pond is rid of undesirable species, native fish can be reintroduced and now have a much better chance of establishing a resilient population. Recently non-native carp and goldfish were thus removed from Mountain Lake, an iconic body of water in the Presidio of San Francisco. Native three-spined sticklebacks, Western pond turtles, and chorus frogs will be reintroduced.