Fish kills

DEFINITION: Significant, sudden mass die-offs of fish

The cause of a fish kill is rarely obvious, and its determination often requires complex analyses of water quality, fish tissues, and environmental and human events that have occurred in the watershed.

Notification of a fish kill usually occurs after, rather than during, the event, in which case causative agents may have dissipated or become diluted, and the tissues of the affected fish may have decomposed. Consequently, formal field investigation involves trained personnel, fishery biologists, fish pathologists, and conservation officers. A relevant field investigation includes the collection of factual and precise data. These are obtained through careful observation, interviews with witnesses, and meticulous sampling of water and dead fish. Because fish kills caused by human activity may result in litigation, samples and other evidence must be collected according to prescribed methods and quickly transported to an analytical laboratory.

About one-half of reported fish kills are the result of such natural causes as oxygen depletion; blooms of toxic algae; infectious diseases caused by bacteria, viruses, protozoans, or parasites; stranding of fish schools; overabundant runoff of silt or ash from forest fires; toxic runoff water; and life-cycle events. In most cases, combinations of causative factors are present. For example, during the summer, fish can become physiologically stressed when water temperatures exceed the fish’s optimal limits. If all other environmental factors remain optimal, the fish may tolerate the increase in temperature. However, oxygen solubility is lowered as temperature increases; thus, the concentration of dissolved oxygen is lower at a time when stress requires the fish to increase their oxygen uptake. Also, the increased metabolism of all other biological organisms puts additional demands on the limited supply of oxygen. The killer in this case is not temperature but oxygen depletion, the primary culprit in fish kills. For example, many marine species, including fish, crabs, and lobsters, died and were found off the New Jersey coast in 2023. Researchers found both low levels of dissolved oxygen, less than 3 parts per million (ppm), in shallower, more inshore areas. Seawater usually includes dissolved oxygen greater than 7 ppm.

Oxygen depletion can also be caused by input of high organic loads, algae blooms and their degradation, low water flow, and ice cover on a lake or pond during the winter. Snow cover reduces light penetration, which inhibits photosynthesis, the only source of dissolved oxygen in ice-covered lakes. This results in winter fish kills. Natural fish kills in coastal waters are often caused by blooms of microorganisms that generate red tides and other epidemiological effects.

A variety of human activities can cause significant fish kills. The dumping of treated and untreated sewage waste into natural waters artificially increases the nutrient load of the system. This practice in itself does not cause a fish kill, but it stimulates increased production and biodegradation processes, which reduce dissolved oxygen below critical levels. A more obvious cause is the introduction into a water system of toxic pollutants such as pesticides, heavy metals, and industrial wastes.

Alterations of the landscape of a watershed can increase water temperature and siltation; silt reduces the efficiency of oxygen uptake by fish’s gills. Dams impede fish migration and, during low water levels, create pockets of deoxygenated water. The turbines of hydroelectric power plants and other water-intake facilities are another detriment to fish. The disposal of undesirable fish by commercial fishing operations often generates fish kill reports.

Into the mid-2020s, fish kills were increasingly linked to climate extremes, such as heat waves and droughts, which exacerbate oxygen depletion and trigger harmful algal blooms. High-profile cases included the 2023 Menindee fish kill in Australia’s Darling River and the 2022 Oder River disaster in Europe, both caused by a mix of extreme heat, pollution, and algal toxins. In the United States, large die-offs along the east coast in 2023 were associated with hypoxia in warming coastal waters. Research emerged showing that larger-bodied fish were particularly vulnerable to hypoxia. While natural causes remained significant, the combination of warming temperatures, nutrient enrichment, and human alterations of waterways made fish kills more frequent and severe worldwide.

Bibliography

Davies, Anne. “Menindee Fish Kill May Have Been Partly Caused by Release of 'Black' and Clean Water by Authorities, Researchers Claim.” The Guardian, 10 Apr. 2023, www.theguardian.com/australia-news/2023/apr/11/menindee-mass-fish-kill-satellite-images-researchers-blackwater-release-darling-baaka-river-australia. Accessed 24 Aug. 2025.

"Fish Kills." U.S. National Office for Harmful Algal Blooms, hab.whoi.edu/impacts/impacts-wildlife/fish-kills/. Accessed 24 Aug. 2025.

Helfman, Gene S. Fish Conservation: A Guide to Understanding and Restoring Global Aquatic Biodiversity and Fishery Resources. Island, 2007.

Müller, Johannes, et al. "On Being the Wrong Size, or the Role of Body Mass in Fish Kills and Hypoxia Exposure." Environmental Biology of Fishes, vol. 106, 2023, pp. 1651–1667. Springer Nature Link, doi.org/10.1007/s10641-023-01442-w. Accessed 24 Aug. 2025.

"Rutgers Scientists Saw Low Oxygen, Fish Mortality in 2023." National Fisherman, 19 Jan. 2024, www.nationalfisherman.com/mid-atlantic/rutgers-scientists-saw-low-oxygen-fish-mortality-in-2023. Accessed 24 Aug. 2025.

Walters, Carl J., and Steven J. D. Martell. Fisheries Ecology and Management. Princeton UP, 2004.