I would not consider the salmon to be a zombie. It is still alive, its body is just changing with age. That happens to humans too. Our bodies grow old and wrinkly, our muscles become smaller, and just overall, our bodies deteriorate. The same thing is happening here. The only difference is, the salmon's physical appearance is changing a bit more and it looks a little more dead. It is not dead however, so it is not a zombie. I personally don't think there can ever be zombies. The only thing i could maybe see happening is that a virus could infect us, and cause us to look like dead people, but we still wouldn't be zombies.