Why salmon die after giving birth?

Salmon are semelparous fish, which means that after they spawn (or give birth), they die. This is a natural part of the life cycle for salmon so it shouldn't really be thought of as a type of death. However, here is a more detailed explaination of why salmon die after releasing their milt or roe:

1. Exhaustion: The physical act of migrating upstream to their spawning grounds and then the actual process of spawning is incredibly strenuous for salmon. They expend a significant amount of energy during this time and their bodies become physically exhausted and weaken. By the time they have completed spawning, they are often too weak to survive.

2. Starvation: Salmon stop eating once they enter freshwater to spawn. They rely on their stored energy reserves to fuel their migration and spawning activities. Once these reserves are depleted, they can no longer sustain themselves.

3. Hormonal changes: Changes in hormone levels due to the reproductive process also contribute to the eventual demise of salmon. For example, the increase in cortisol levels, which is a stress hormone, may suppress their immune systems and make them more susceptible to diseases and infections.

4. Disease and parasites: Spawning salmon are also more vulnerable to diseases and parasites. The crowded conditions in spawning grounds can facilitate the spread of diseases, and the stress of spawning can further weaken their immune systems.

5. Predation: Salmon are also vulnerable to predation during the spawning process. Bears, eagles, and other predators target them during this time as they are relatively easy prey due to their weakened condition.

So while it's true that salmon die after reproducing, it's not necessarily a sad or unnatural event. It's a natural part of their life cycle and ensures the survival of their species.