- Ecology and Evolution
In this activity, the students will see how the environment in which salmon eggs are placed affects how well they will be protected from predators. This activity is adapted from the Stream to Sea program, which contains many great salmon education resources.
Streams contain two different and distinct types of regions, riffles and pools. A riffle is an area of a stream or river where the water is shallow and fast moving and where gravel or rocks lay on the bottom. A pool is an area of a stream or river where the water is deep, slow moving, and silt or clay lay on the bottom. Salmon are born in gravel beds in the riffles of streams 16 to 1,200 km away the sea.
The female spawner will search around the same area as she was born for a good place to lay her eggs. She will look for a riffle, where the fast-flowing water will provide plenty of oxygen for the eggs, the gravel will give the eggs protection from predators, and the force of the water will remove any sediment that builds up, preventing suffocation. The female spawner digs a nest, known as a redd, with her tail in the stream gravel bed to remove any small sediments. She lays an average of 2,500 eggs but can lay up to 8,000 eggs in the gravel. If she were to create a redd where there is a lot of soil, sand, clay, or silt, the eggs would suffocate from loss of oxygen.
The male fertilizes the eggs by covering them with milt, a milky substance containing the sperm and seminal fluid. The female then covers the eggs in the redd with gravel using her tail. Gravel hides the eggs from birds and other predators, and prevents them from being washed away and protects them from direct sunlight.
This environment within the riffle of the stream is where the eggs will stay until they hatch into alevin. Even as swimming alevin, the salmon will stay in this familiar safe area of the stream, and won’t venture far until they start searching for food as fry.