External Fertilization In many aquatic animals, such as salmon, the female releases her eggs into the water and the male then releases his sperm over the eggs. Since the sperm and the egg unite outside the body, this method is called external fertilization. Hundreds or even thousands of egg and sperm are produced to ensure that enough with unite and grow to be adults. Both the gametes and the developing young are vulnerable to the environment and predators.

Hatcheries Fish hatcheries use technology to ensure a higher rate of survival in wild fish populations. For example, wild salmon use external fertilization, which means that relatively few eggs are actually fertilized. Just before they are ready to reproduce, male and female fish are caught. The eggs and sperm are collected and mixed together in a container. The fertilized eggs are incubated in special trays that have a constant supply of cold running water. Once the eggs hatch, the young are fed at the hatchery before they are released into the wild. This process produces a much greater number of young salmon than would occur naturally.

1: Gamete Combine A redd, or a female's nest of eggs, is fertilized by sperm on the bottom of the river. These eggs remain in the gravel throughout the winter, and the embryos develop.

2: ALEVINS In the spring, the eggs hatch and alevins emerge. These are tiny fish with the yolk sac of the egg attached to their bellies. Alevins stay close to the redd for a few months. 

3: FRY  When they have consumed all of the yolk sac and grown in size, these fish emerge from the gravel, and are then considered fry. Fry swim to the surface and begin to feed and grow. When they feel the urge, they begin their migration toward the ocean.

4: SMOLT To manage the transition between freshwater and saltwater, salmon fry must go through a physical change known as smolting. Smolting begins in freshwater and ends when they enter into the ocean when it is time. Smolts have a silvery coating over their scales to camouflage them from predators and shield their bodies from fresh to saltwater.

5: Adults Salmon enter the ocean as young adults and leave it as mature adults who have grown large and sexually mature, ready to spawn.

6: Spawning When they are sexually mature, salmon obey their homing instinct and travel back to the streams where they were born to spawn. It is a challenging journey, and only the toughest and luckiest salmon complete it to reproduce at the end.