Why does the Pacific Salmon die after spawning but the Atlantic Salmon do not and return yearly?

Pacific salmon, such as sockeye and chinook, typically die after spawning due to a life history strategy that prioritizes reproduction over longevity, allowing them to allocate all their energy to one reproductive event. This semelparity is thought to have evolved in response to the harsh conditions of their upstream migration. In contrast, Atlantic salmon are iteroparous, allowing them to spawn multiple times throughout their lives, which provides them with the opportunity to return to the ocean to recover and gain strength between spawning events. This difference in reproductive strategy reflects adaptations to their respective environments and life cycles.