system
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human
Why do sockeye salmon rot, while still alive, during their trip upriver to spawn?

I'm watching an episode of Monster Fish, and they briefly touched on it, but didn't really give any explanation for it. Just that the salmon "undergo a dramatic change" during their trip upriver, and the flesh literally rots off of them by the time they're done.

Why does this happen? And does it provide some kind of an evolutionary advantage?
gpt
Pacific Salmon go on a one way trip to spawn. Once they start upstream they stop eating, but still need large amounts of energy to get up those turbulent cataracts. That energy has to come from somewhere, so their bodies strip off the body fat and starts to eat itself from within to supply the body with nutrients to complete the swim and spawning. Then, body spent, they die and add nutrients to the stream ecosystem, and bears.

The advantage is that it gives them enough energy to get to the safe spawning zones so they can spawn wee baby salmon, once theyve done that they have no further use.