What causes large fish kills during thermal pollution events?

Large fish kills during thermal pollution events are primarily caused by insufficient oxygen levels in warm water. Warmer water holds less dissolved oxygen than cooler water, which is critical for the survival of fish and other aquatic organisms. As the temperature of a water body rises, the metabolic rates of fish increase, leading to a higher oxygen demand. However, the warming also reduces the solubility of oxygen in the water, creating a situation where oxygen availability can drop significantly. This can lead to hypoxia (low oxygen conditions), making it difficult for fish to breathe and survive, which can result in large-scale die-offs.

The other options, while related to aquatic ecosystems, do not directly address the specific mechanism behind fish kills due to thermal pollution. Excessive algae growth can occur due to nutrient influx, but this is often a result of eutrophication, which involves different dynamics. Natural predators and pollutants may affect fish populations or reproductive rates, but during thermal pollution events, the immediate and critical factor is the oxygen deficiency caused by elevated temperatures.