The most common causes of fish kills are oxygen depletion, algal blooms (could deplete oxygen or be toxic), pesticide toxicity and disease. By far, the most common cause of fish kills is oxygen depletion in lakes or ponds.

A large number of dead fish in a lake, known as a "fish kill," is most commonly caused by hypoxia (oxygen depletion) rather than direct poisoning. In 2026, climate-driven factors like sustained heatwaves can raise water temperatures, which naturally hold less dissolved oxygen. This is often exacerbated by Harmful Algal Blooms (HABs); these blooms thrive on agricultural runoff (nitrogen and phosphorus) and, while they produce oxygen during the day via photosynthesis, they consume massive amounts of it at night and during their decomposition phase. Other common causes include thermal shock from rapid temperature changes, the spread of infectious diseases/parasites during spawning, or "turnover" events where deep, oxygen-poor water suddenly mixes with the surface layer after a storm. If you see fish "gasping" at the surface before a mass die-off, it is a definitive sign of an oxygen crisis. Reporting these events to local environmental agencies is crucial for identifying if localized pollution or sewage leaks are a contributing factor.