1. Pacific Salmon (Oncorhynchus spp.)
Pacific salmon live in the Pacific Ocean and include several species, such as Chinook, Sockeye, Coho, Pink, and Chum salmon. These species display a unique life cycle pattern characterized by semelparity. After hatching in freshwater rivers or streams, juvenile Pacific salmon spend a period of time in their natal waters, feeding and growing before migrating to the ocean. Once in the ocean, they undergo extensive feeding and growth, maturing into adults. When the time comes to reproduce, adult Pacific salmon undertake an arduous journey back to their natal freshwater streams and rivers. This migration is often referred to as the "salmon run." During the upstream migration, the salmon face various challenges, such as waterfalls, rapids, and predators.
Upon reaching their natal waters, Pacific salmon engage in the spawning process. Male and female salmon create nests, called redds, in the gravel beds of rivers or streams. The females deposit eggs into the redds, and the males fertilize them. The fertilized eggs develop in the redds, protected by the gravel substrate, until they hatch into alevins. The adult salmon, having completed their reproductive journey, deteriorate rapidly and eventually die after spawning.
2. Atlantic Salmon (Salmo salar)
Atlantic salmon, on the other hand, exhibit iteroparity. After hatching in freshwater rivers or streams, juvenile Atlantic salmon spend several years in their natal waters, growing and maturing. When they reach a certain size and maturity level, they migrate to the ocean, where they undergo feeding and growth. Unlike Pacific salmon, Atlantic salmon can endure multiple migrations between freshwater and saltwater environments, returning to their natal rivers or streams to spawn multiple times throughout their lives.
During the spawning season, adult Atlantic salmon undertake similar upstream migrations to reach their natal waters. They establish redds and engage in the spawning process, with females depositing eggs and males fertilizing them. The fertilized eggs develop in the redds, and the adult salmon return to the ocean, where they continue to feed and grow.
In conclusion, the difference between Pacific and Atlantic salmon lies in their reproductive strategies. Pacific salmon exhibit semelparity, meaning they die after reproducing, while Atlantic salmon display iteroparity, enabling them to reproduce multiple times over their lifespans. This fundamental distinction shapes their respective life cycles and migratory behaviors.