Once salmon return to their birthplace to spawn, they choose a spot in the cold current to nest. Some Pacific salmon females even "dance" for their mates before turning onto their sides to excavate gravel with their tails. Meanwhile, the male salmon guard the females against predation. Once the nests, which are also called, "redds," are formed, the females proceed to lay their eggs in them, after which the males deposit clouds of sperm, called "milt," over the eggs, externally fertilizing them. Then the females bury the eggs under the gravel. Exhausted, the females then die.
Upon fertilization, each egg absorbs water, which serves to harden its outer layer. Once this occurs, cleavage ensues, followed by the blastoderm formation. Next to the blastoderm sits a relatively large yolk sac. As the process continues, the blastoderm envelopes the yolk sac, forming a fish-like appearance. These fertilized eggs incubate for at least 50 days before hatching. Water temperature is directly related to the hatching time, so the colder the water, the longer it takes for the eggs to hatch.
Once hatched, these young, translucent larvae, called, "alevins," remain in the gravel for three to six weeks. Conveniently, they each have an attached yolk sac as their food source, while their mouths and digestive tracts develop. Once the yolk sacs are absorbed, the larvae enter what is termed the "fry" stage of development. They are now one inch long and emerge from their nests to begin their quest for food.
Shortly thereafter, the young fry develop into the parr or fingerling stage. Reaching two inches in length, they begin to develop a more definitive shape.
Before leaving their native streams, salmon grow into their next stage of life, in which they are now called "smolts." In order to successfully migrate to the ocean and live there for the next few years of their lives, these juveniles undergo anatomical, physiological, metabolic and endocrinological changes. At this time their fins and color patterns distinctly denote their species.
Salmon growth is temperature dependent. Living in warmer regions, they are allotted much more food than a colder climate can provide, so they grow more quickly when temperatures are higher. In contrast to birds and mammals, once salmon reach reproductive maturity, they continue to grow for the rest of their lives. This is one instance when size does matter. The larger the salmon, the more offspring it can produce.