Salmon are biologically programmed to die after spawning, a phenomenon known as semelparity. This is a reproductive strategy where an organism expends all its energy in a single, massive effort to reproduce. For Pacific salmon (Oncorhynchus), this means they invest everything into their journey upstream, battling rapids and leaping waterfalls, to ensure their genes are passed on.

Pacific salmon live most of their lives in the ocean, but they return to the freshwater streams where they were hatched to spawn. The journey to the spawning grounds is incredibly taxing. Some species swim more than 2,000 miles (3,200 km) to reach their spawning grounds. Salmon stop eating once they enter freshwater, relying solely on their stored energy reserves. This energy is used not only for the arduous journey but also for the physiological changes they undergo, such as developing secondary sex characteristics. For example, the males develop hooked jaws to attract females. By the time the salmon spawn, their energy reserves are depleted, leaving them too weak to survive.

Environmental stressors also play a role. The changes in water temperature and chemistry take a toll on their bodies. Additionally, the act of spawning itself is stressful, and the combination of these factors leads to a salmon’s eventual death. On the plus side, this is nature’s way of ensuring that the next generation has the best chance of survival, even if it means the current generation has to make the ultimate sacrifice.