Salmon swim upriver to spawn as part of their natural reproductive cycle. This behavior is known as anadromy. Here are the reasons why salmon undertake this challenging journey:
- Reproduction: The primary purpose of salmon swimming upriver is to reproduce and lay their eggs. Adult salmon return to the same rivers or streams where they were born, guided by their remarkable sense of smell and memory. They navigate through various obstacles and swim against strong currents to reach their specific spawning grounds.
- Suitable Environment: Salmon choose freshwater habitats for spawning because they provide a more favorable environment for their eggs and fry (young fish). Freshwater habitats generally have lower predation risk and more abundant food resources compared to the open ocean. The specific conditions of each river or stream, including water temperature, oxygen levels, and gravel substrate, play a crucial role in ensuring successful egg fertilization and fry development.
- Egg Development: Female salmon excavate nests, called redds, in the riverbed by using their tails to clear away debris and gravel. They deposit their eggs in these nests, and male salmon release sperm (milt) to fertilize the eggs. The eggs then develop and incubate in the gravel, protected from predators and fluctuations in ocean conditions.
- Parental Care: Once the eggs are fertilized, the female salmon covers them with more gravel to provide protection and create a suitable microenvironment. The female remains near the redd, fanning water over the eggs with her tail to supply oxygen and remove sediment that could smother them. This parental care enhances the survival rate of the developing embryos.
- Nutrient Transport: The journey upriver is physically demanding for salmon, and they often stop feeding once they enter freshwater. As they swim upstream, their bodies undergo physiological changes to adapt to the transition from saltwater to freshwater. They metabolize their stored fat reserves, converting them into energy for the challenging upstream swim. The nutrient-rich bodies of spawning salmon can also benefit the ecosystem as their carcasses provide valuable nutrients to the freshwater environment, supporting the growth of other organisms.
- Genetic Diversity and Population Dynamics: Salmon populations rely on genetic diversity for their long-term survival. By returning to their natal streams for spawning, salmon help maintain genetic variation within their population. This diversity is important for adaptability to changing environmental conditions and resistance to diseases.
It’s worth noting that not all salmon species exhibit anadromy. Some species, known as “non-anadromous” or “landlocked” salmon, spend their entire life cycle in freshwater lakes or rivers without migrating to the ocean. Examples include kokanee salmon and certain populations of Atlantic salmon.