Some are still eggs in the redd (the term for a nest of salmon eggs), but many have hatched into the alevin, or newly hatched baby salmon, you see above.
Female salmon dig their redds in river beds and deposit their eggs in the fall; the eggs hatch during the winter. What if humans were like salmon? What if, instead of the protection of our mother’s womb, we got the nurturing abilities of a rocky river-bottom? It’s a wonder that any of these salmon survive, especially when we consider that a river is much more dynamic than the hatchery trough at the Aquarium.
Only the strong survive
In ideal conditions (hardly ever present in a natural stream setting), 80% of the eggs will make it out of the gravel to a free-swimming life. Storms, disease and predation take some of them out of the running. Also, female salmon may dig their redds on top of existing redds—this is called superimposition. The original eggs may not survive the digging activity.
Life at the bottom
At what point will the alevins pictured above leave the gravel and start swimming? After they’ve used up all the nutrition from their “lunch boxes,” the yolk sacs attached to their undersides. At that point, they will become free-swimming juvenile salmon, or fry. How long it takes for a salmon egg to hatch and emerge from the gravel are largely dependent on temperature. This is because salmon are ectothermic animals (remember this blog post?), so their growth and metabolism increase as the surrounding temperature increases. Chinook salmon eggs take roughly 47 days to hatch in 52°F (11°C) water. In those same conditions, it will take those chinook alevin 84 days to absorb their yolk sacs and become fry.
Chinook on top
Chinook salmon are roughly three times more likely than pink, chum, or sockeye to make it from egg to the fry stage. Why? They spawn at lower densities, so there are fewer of the problems that come with competition for space. As the largest salmon species, they can dig the deepest (and therefore safest) redds. They also spawn in large rivers, which may be able to buffer the effects of storms better than small streams.
Come see the alevins at the Seattle Aquarium—and learn more about salmon!