The Salmon Life Cycle
Salmon spend about half their lives in streams and estuaries and they spend the other half of their lives in the ocean. This diagram represents the life cycle of Pacific salmon. For simplicity sake, the life cycle is broken into five life stages: upstream migration, spawning, rearing/parr, downstream migration, and feeding/adult growth. The first four of these life stages occurs in streams and estuaries. The last stage, adult growth, occurs exclusively in the ocean.
At each stage of their life, salmon are dependent on certain environmental attributes and conditions to help them survive and mature into the next life stage. At each stage, if conditions are wrong, salmon populations begin to decline. Click on a life stage.
When ready to become adults, young salmon make their way downstream to the ocean.
Barriers to downstream migration are a threat to the survival of the salmon population. These include very low or discontinuous surface flow, impassable culverts, dams and unscreened diversions.
Once at sea, adult salmon grow rapidly. If enough adults, originating from a particular watershed grow and survive, the cycle begins again, and the population is sustained.
Threats at this life stage include predation (including fishing pressure), ocean conditions and available food supply, and food competition with hatchery spawned salmon.
When salmon are preparing to spawn, they make their way from the ocean back to the mouth of stream where they were born. From there they swim upstream, through the estuary, up main stream and often up various tributaries, until reaching the stream gravels in which they will build their nests (or redds) and spawn.
Barriers to upstream migration are a threat to the survival of salmon populations. These barriers include sandbars across the mouth of the stream, very low or discontinuous surface flow, and natural and manmade physical barriers such as waterfalls, impassable culverts and dams and weirs without functional fish ladders.
Salmon build nests in course stream gravels. These nests are called redds. First, the salmon churn up the gravels, to wash out some of the fine sediments. Then the female lays her eggs into the voids in the gravel, while the male fertilizes them.
The fertilized eggs are oxygenated by the flow of cool water through the gravels of the redd. The eggs hatch and emerge from the gravel as tiny young salmon (fry). If during this period the flow of cool, oxygenated water through the redd is diminished by fine sediment, elevation of stream temperatures or reduction of flows, the fertilized eggs will suffocate and will not survive to become fry. Likewise, if scouring flood flows occur during this critical period, the redds along with their fertilized eggs may be swept away.
Key controllable sources of fine sediment in California’s north coast watersheds include unpaved roads adjacent to stream and stream crossings.
To survive and grow juvenile salmon need:
- Pools for refuge from the effects of summer heat and cool well-oxygenated water at depth.
- Backwater areas and pools in which to hide and avoid becoming swept downstream during swift storm flows.
- Abundant food in the form of macro invertebrates.
- The stream has enough scour producing objects, such as large woody debris (LWD), boulders or exposed bedrock, and
- The input of bulk (coarse and fine sediment) into the stream’s bedload is in balance with the output.