Salmon engage in sexual reproduction, a fundamental process in the animal kingdom. This requires the genetic contribution from two distinct parents, a male and a female. Unlike asexual reproduction, this method ensures significant genetic diversity within salmon populations, which is important for their adaptation and survival.
Salmon’s Spawning Migration
Adult salmon migrate from the ocean to freshwater rivers and streams to reach their natal spawning grounds. This migration can span thousands of kilometers, with some species traveling over 3,000 kilometers upstream, equivalent to running a marathon daily. During this trip, their bodies undergo significant physiological transformations. Salmon cease feeding upon entering freshwater, redirecting all energy reserves towards the development of reproductive organs. Their physical appearance also changes, with males often developing a hooked snout, known as a “kype,” and sometimes a humped back.
The upstream journey presents numerous challenges. Salmon must overcome natural obstacles such as rapids and waterfalls, capable of jumping up to two meters to clear barriers. They also face threats from predators like bears and eagles. The instinctual drive to return to their birthplace, known as homing, guides them. Scientists believe salmon navigate using the Earth’s magnetic field in the ocean and then rely on their keen sense of smell, recognizing unique chemical signatures of their home stream once they enter freshwater, ensuring the salmon are physiologically ready for reproduction.
The Spawning Process
Upon reaching their freshwater spawning sites, female salmon construct nests, known as “redds.” A redd is a shallow depression excavated in gravelly riverbeds, which provides oxygen to the developing eggs. The female uses her tail fin to dislodge gravel. She may dig multiple pockets within a single redd, or construct several redds, each containing thousands of eggs, which are typically orange to red in color and about the size of a pea.
Once eggs are deposited into the redd, male salmon release “milt,” their seminal fluid containing sperm, over the eggs. This external fertilization is a hallmark of salmon reproduction, where the genetic material from both parents combines in the water. The male and female exhibit synchronized behaviors during this moment. Following fertilization, the female covers the eggs by disturbing gravel from the upstream edge of the depression, burying them several centimeters deep to protect them from predators and the current. She may then guard the redd for several days before succumbing to exhaustion, completing her life cycle.
Early Life and Development
Salmon eggs remain buried within the gravel of the redds throughout the winter months. The rate of embryonic development is influenced by water temperature, with colder temperatures leading to a longer incubation period. During this period, the eyes and other developing organs become visible through the eggshell.
In early spring, tiny fish called “alevins” hatch from the eggs. Alevins possess a yolk sac attached to their bellies, which serves as their primary food source. They remain hidden within the gravel, feeding on this yolk sac until it is absorbed. Once the yolk sac is consumed, the young salmon, now called “fry,” emerge from the gravel. Fry swim to the surface to gulp air, and this freshwater environment provides shelter and food sources before they transition to larger bodies of water.