Why did many Northwest Coast peoples return salmon bones or remains to the water?

Northwest Coast peoples returned salmon bones and remains to the water because they viewed salmon as supernatural beings who voluntarily sacrificed themselves for the benefit of humanity.1

According to their traditional beliefs, these spirit-beings would return to their homes beneath the sea after being caught, where they would be reincarnated if their bones or offal were returned to the water. It was believed that if the salmon were offended by the improper treatment of their remains, they would refuse to return to the river in future seasons. Consequently, these rituals were essential to ensure the continued abundance of the salmon runs upon which the people relied.1