What is the main reason for fish kills in streams caused by pollution?

Depletion of oxygen is the main reason for fish kills in streams caused by pollution because aquatic organisms, including fish, rely on dissolved oxygen in the water for survival. Pollution can lead to a variety of issues that decrease the oxygen levels available to fish. For example, the introduction of organic matter into the water can foster the growth of bacteria that consume oxygen during decomposition. This process can severely reduce the levels of dissolved oxygen in the water, making it difficult for fish and other aquatic life to breathe.

When the oxygen levels drop significantly, it can lead to hypoxia, a condition where the oxygen is insufficient to support marine life, ultimately resulting in fish kills. While increased algae growth can contribute to oxygen depletion through processes like algal blooms, it is the depletion of oxygen itself that directly threatens fish populations.

In contrast, heavy metals, while harmful, typically affect fish through chronic exposure rather than causing immediate kills associated with acute oxygen depletion. Similarly, high temperatures can exacerbate oxygen depletion but are not the primary cause. Therefore, the primary mechanism leading to fish kills due to pollution is the depletion of oxygen in the water.