Why do fish kills occur when herbicides are used?

Fish kills can occur when herbicides are used primarily due to reduced dissolved oxygen levels in the water. When herbicides are applied to control aquatic weeds, the subsequent die-off of these plants can lead to significant changes in the aquatic ecosystem. As the plants decompose, the process consumes oxygen from the water. This increased biological oxygen demand can cause the dissolved oxygen levels in the water to drop sharply.

Fish and other aquatic organisms rely on sufficient dissolved oxygen to survive; low levels can lead to stress or even suffocation. The rapid decomposition of plant material can dramatically alter the balance of oxygen, especially in warmer water, which further exacerbates the problem, as warmer water holds less dissolved oxygen than cooler water.

While increased water temperature, high levels of sunlight, and excessive nutrient loading can affect aquatic ecosystems, they do not directly result from the application of herbicides in the same way that oxygen depletion does. Therefore, reduced dissolved oxygen is a direct consequence of herbicide use leading to fish kills in aquatic environments.