Tuesday, July 14, 2020
Check Out the Redds
Fish and Game Officer Terry Thompson captured this picture of a leaping rainbow trout trying to jump over a small diversion dam on the Big Wood River a couple weeks ago.
Tuesday, May 26, 2020



This spring’s record low water in the Big Wood River has a silver lining.

It’s making it easier to witness the annual spring spawning ritual of rainbow trout in the Big Wood River and its tributaries.

Terry Thompson shot this picture of a large rainbow trout swimming near a newly built redd in the East Fork of the Big Wood River during May 2020.

The rainbow trout normally become sexually mature at two or three years of age. Their spawning is triggered by the length of day and the temperature of the water, said Terry Thompson, regional communications manager for Idaho Department of Fish and Game.

As the days grow longer and the water warms to the upper 40s Fahrenheit, they begin traveling upstream to spawning habitat, picking areas where the water is well oxygenated and there is a gravel stream bottom that is clean and not filled with silt or sediment. Often, they will even jump over waterfalls or small diversion dams to get to these spawning grounds.

Once they’ve arrived at a suitable place, female rainbow trout build a nest, or redd, where they deposit their eggs. Using their tails, they fan the gravels causing the gravel to move and creating a depression in the gravel. It may take up to a week to build a redd.

Once the redd is made, males compete for the opportunity to mate with the female, timing the release of sperm into the red as the female deposits her eggs.

A typical two-pound female lays between 2,000 to 3,000 eggs into small spaces between the gravels where they can receive oxygen from the passing water. (She thinks of everything!) The female then fans her tail across the gravel like a magic wand and covers up the eggs, securing them in place while providing protection from predators.

When the water is in the upper 40s, the eggs take about seven weeks to hatch. The small sac fry will remain hidden in the gravels dining on their nutritious yolk sac for food until they can swim up out of the gravels.

That takes a few weeks. Once the fry emerge, they start to eat aquatic insects or zooplankton as they begin their journey to adulthood.

To spot a redd, look for bright clean gravel exposed on the river bottom that is lighter than the surrounding gravels.


Both look the same physically. But rainbow trout eggs produced in Idaho’s hatcheries are pressure shocked in the eyed-egg stage, making them sterile.

This is designed to reduce the risk of hatchery trout spawning with wild trout. Unlike hatchery steelhead, which are ocean-going rainbow trout, the majority of hatchery rainbow trout do not have their adipose fin clipped before release.


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