Smelts breed in the spring. They have to swim up their native river to spawn. They need clear water and a gravel river bottom to deposit their eggs.
Spawning (laying and fertilization of the eggs) takes place at night. Males and females gather in close rows, each female (which are bigger) surrounded by two males or more. Sperm and eggs are then released. Fertilized eggs become adhesive and will stick to the gravel or vegetation on the bottom.
The female can lays between 8,000 and 69,000 eggs. These hatch after seven to ten days, and then the larvae travel down the current as far as the mouth of the river, where they begin to feed.
The males mate at the age of two or three years, and don’t always survive the spawning. The females, meanwhile, grow faster than their male counterparts, but don’t reach sexual maturity until the age of three years.
After the spawn, smelts that have survived return to the sea, along the coasts, to spend the summer.