What is a redd? Salmon keep cleaning out a 'nest' for spawning

That's right- redd with two Ds. Another funny word that fish biologists use. In this blog post, I am talking about 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, the 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." Female salmon do exactly that 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 good supply of clean, aerated water to develop properly. If the female likes the spot, they will dig further, excavating a deeper pit.

Male salmon don't do any digging, but compete amongst each other, with the largest trying to drive off the other males. The salmon then spawn as the female expels her eggs into the pit. A properly dug pit holds the eggs and milt together, ensuring good fertilization. The female salmon moves just upstream and digs again, covering the eggs and starting the next egg pocket. As she digs, the current flushes dirt, debris, and smaller gravels downstream. After all the digging and sweeping, the redd looks like a clean spot in the stream with neatly sorted gravel. This keeps the egg pocket clean and creates a hump in the water, which flows through the gravel and into the egg pocket.

The female salmon will keep digging and cleaning the gravel until she has spawned all her eggs. Then she moves to the sides, digging trenches to focus the stream flow toward the redd and pushing more gravel on top of the egg pockets to protect their precious content. A female salmon will stay with her redd to put on the finishing touches and protect it from other females who might want to use the same spot. Eventually, she will drift downstream and die, while her eggs develop and hatch during the fall and winter, snug within their redd. In the spring, the young emerge through gaps in the gravel to start their life cycle again.

Check out this closeup underwater view of salmon spawning in Red River, Idaho!

Watch this video to see how Idaho Fish and Game trains people to count redds - an important way biologists keep track of salmon populations.