What typically causes fish kills in summer?

The primary cause of fish kills during the summer months is oxygen depletion, often resulting from algae blooms. In warm weather, nutrient-rich waters can promote the rapid growth of algae, leading to what is known as algal blooms. When these blooms become too dense, they can block sunlight from reaching aquatic plants, reducing photosynthesis and subsequently decreasing oxygen production in the water. Additionally, when the algae die off, their decomposition by bacteria consumes even more oxygen, further depleting the supply available for fish and other aquatic organisms. This lack of dissolved oxygen can lead to hypoxia, a condition where oxygen levels drop too low for fish to survive, ultimately resulting in significant fish kills.

Cold water temperatures, high ice cover, and excessive light exposure do not typically contribute to the mass mortality of fish during the summer. Instead, the conditions created by algal blooms—combining high nutrient levels, warm temperatures, and reduced light penetration—directly contribute to the oxygen depletion that endangers aquatic life. Understanding these dynamics is crucial for effective aquatic pest management and ecosystem health.