What primarily causes fish kills in aquatic environments?

Fish kills in aquatic environments are most commonly associated with the removal of too much oxygen from the water. This situation can arise from various factors, leading to hypoxia, a state where the dissolved oxygen levels in the water become critically low. Oxygen is essential for the survival of aquatic organisms, including fish, and when levels drop significantly, fish are unable to breathe, resulting in extensive mortality.

This phenomenon can occur due to factors such as algal blooms that consume oxygen as they decompose, excessive organic matter from decaying plants and animals, and increased biological activity in the water. In these scenarios, the balance within the aquatic ecosystem is disrupted, directly impacting fish populations.

While excessive nutrient runoff, increased water temperature, and overcrowding of fish can also contribute to fish kills, their impacts are often indirect or situational compared to the immediate and acute effect of low oxygen levels. Therefore, the critical role that dissolved oxygen plays in maintaining fish health makes the lack of oxygen the primary cause of fish kills in aquatic environments.