Salmon lay eggs, a fundamental part of their life cycle, culminating in spawning. They return to freshwater environments to ensure species continuation. This complex process involves biological and environmental interactions, from egg deposition to the emergence of new life.
The Spawning Journey
Salmon begin their spawning journey with an arduous migration from the ocean back to their natal freshwater rivers and streams. This upstream migration typically occurs during autumn (September-November), though timing varies by species and location. Adult salmon cease feeding during this journey, relying on stored energy reserves. Their bodies undergo physical transformations, including color changes and, for males, the development of a hooked snout.
Upon reaching suitable freshwater spawning grounds, the female salmon prepares a nest, called a redd. She uses her tail to dig a depression in the gravel riverbed, clearing sediment. This creates a clean, oxygenated environment for the eggs, often resulting in a lighter gravel patch. Males compete for the opportunity to spawn with the female. Once the redd is ready, the female deposits her eggs, which the male simultaneously fertilizes by releasing milt. A single female can lay 1,000 to 17,000 eggs, depending on species and size. After depositing and fertilizing eggs, the female moves slightly upstream, digs again, and covers the laid eggs with gravel, creating a protective mound. This process repeats over several days until all eggs are laid.
From Egg to Alevin
After being laid and fertilized, salmon eggs (roe) begin developing within the gravel redd. These pea-sized, orange, translucent eggs allow the developing embryo to be seen. Incubation varies from 6 to 20 weeks, depending on water temperature and species. Colder water prolongs incubation, while warmer temperatures accelerate development.
As the embryo develops, features like eyes and an early nervous system become apparent through the eggshell, a stage called “eyed eggs.” Hatching occurs when the developing salmon, now an alevin, breaks free from the eggshell. Alevins are small (about an inch long) and have a prominent yolk sac attached to their bellies. This yolk sac provides primary nourishment (proteins, fats, vitamins), sustaining them for initial weeks. For weeks to months, alevins remain hidden within the protective gravel of the redd, absorbing the yolk and developing before emerging as free-swimming fry.
Factors Influencing Spawning Success
The success of salmon spawning and the survival of their eggs are influenced by a combination of environmental and biological factors. Water temperature is a primary concern, with an ideal range for egg development typically between 5°C and 9°C (41°F-48°F). Temperatures exceeding 20°C (68°F) can be lethal to the eggs, and overly warm conditions can lead to accelerated development and deformities. The quality of the gravel in the redd is also important; it must be clean and free of fine sediments like mud and silt, which can smother eggs by reducing water flow and oxygen availability.
Sufficient water flow is necessary to provide a continuous supply of oxygen to the buried eggs and to remove waste products. Fluctuations in water levels can expose redds, leaving eggs vulnerable to drying or freezing. Predation poses a significant threat to eggs and newly hatched alevins, with various fish, birds, and other animals feeding on them. Human activities also impact spawning success. Habitat degradation through logging, pollution, and the presence of dams can reduce suitable spawning areas, alter water quality, and impede migration routes, collectively affecting salmon populations.