Here's why:
* Oxygen levels: Cold water holds more dissolved oxygen, which is crucial for salmon to survive and grow.
* Metabolic rate: Cold water slows down salmon's metabolism, allowing them to conserve energy and grow more efficiently.
* Spawning: Many salmon species migrate to cold, freshwater rivers to spawn, as it provides a suitable environment for eggs to hatch.
While some salmon species can tolerate slightly warmer water, they generally prefer temperatures below 20°C (68°F) for optimal growth.