Redd house for safe spawn
A salmon redd with the female in front of the cleaned and sorted gravel. And a male competing behind her.
Photo courtesy of the Idaho Department of Fish and Game
Redd — that’s right, with two Ds — is another funny word that fish biologists use: It's the nest where salmon, trout and steelhead lay their eggs in the gravel of a stream.
Every August and September in central Idaho’s high country, returning salmon start cleaning their spawning streams. The word “redd” comes from the Scottish dialect, meaning “to put in order, to tidy, or to clear,” which female salmon do to prepare a good place to put their eggs.
When it comes time to spawn, female salmon will look for a spot with good gravel. “Good” gravel is free of mud and silt that can smother eggs and has the right sizes and shapes to incubate eggs and keep them safe, plus the right water speed to supply oxygen and keep eggs cool.
Salmon will first dig a test pit with a few strokes of their tails, then slightly back over it. The theory is they are feeling the flow of water through the gravel with their fins because eggs need a supply of clean, aerated water to develop properly. If the female likes the spot, she will dig further, excavating a deeper pit.
Male salmon don’t do any digging, but compete with each other, with the largest trying to drive off the other males. The salmon then spawn as females expel eggs into the pit. A properly dug pit holds eggs and milt together, ensuring good fertilization.
Females move just upstream and dig again, covering the eggs and starting the next egg pocket. As they dig, current flushes dirt, debris and smaller gravels downstream. After all the digging and sweeping, the redd is a clean spot in the stream with neatly sorted gravel. That keeps the egg pocket clean and creates a hump in the water, which flows through the gravel and into the egg pocket.
Females will keep digging and cleaning the gravel until they have released all eggs. They then move to the sides, digging trenches to focus stream flow toward the redd, hence pushing more gravel on top of the egg pockets to protect their precious content.
Female salmon stay with their redd to apply finishing touches and protect it from other females who might want to use the same spot. Eventually, the soon-to-be-mother fish drift downstream and die, while eggs develop and hatch during fall and winter, snug in their redd.
In spring, young emerge through gaps in gravel and start the entire life cycle once again.
Timothy Copeland is a fisheries biologist with the Idaho Department of Fish and Game.