Why?

The goals of the Salmon change when they reach mating age. Now they don't need to survive in the ocean, they need to make sperm/eggs and survive the long trip to their birthplace. This trip is through fresh water lakes and rivers. That is a very different environment from the ocean. They will need to jump/swim up waterfalls. They will need to avoid predators, but very different ones than they faced in the ocean.At the end of their journey they will mate and die.

Because the goals and environmental pressures change so much the Salmon must also change to meet those new challenges. If they didn't they wouldn't be as successful in reaching their final destination and making more Salmon.

How?

Just like humans, hormones trigger biological changes. The change to fresh water also triggers changes and a countdown clock. The changes use a lot of energy, as does their trip and mating. Their body will literally consume itself to make the changes and meet the challenges it needs to so that the salmon make it home to mate.

In case anyone was curious

His humps, his humps his humps his humps

I've seen this happen to human men who got too horny, tbh.

Is it all salmon species? I stumble upon dead salmon after spawning all the time during field work and I've never seen one that looks like that. They usually look like the top photo.

This is a good way to illustrate what "gooning" is.

Do they taste different?

Same

Everything I know about salmons is thanks to The Chemical Brothers

https://youtu.be/dDj7DuHVV9E?si=QapEpbKgj3PkiSlW

are they still salmon colored?

Happens to the best of us.

Because they do not need to survive like that for long lol they die like immediately after.

Do da humpty hump

That thing looks like a fish skeksis.

splatoon moment