Redd

Lore & Background

In salmonid biology the redd is far more than a hole in the riverbed. The female selects a site by sight and smell, favouring reaches with moderate current, clear water, and a gravel bed that will allow oxygenated water to flow over the eggs once they are buried. She excavates with rapid tail-flicks, pushing gravel downstream in a characteristic 'redd pit' that can be 20–60 cm deep and up to a metre or more in length, depending on species. As she digs, she releases a stream of eggs into the deepest part of the depression; the male, hovering above, discharges milt to fertilise them externally. The female then sweeps gravel back over the eggs, leaving them sealed in a pocket of oxygenated water.

For Pacific salmon (Oncorhynchus spp.) the act is semelparous: the female will die within days to weeks of completing the redd, her body decomposing and delivering a pulse of nitrogen, phosphorus, and micronutrients to the stream ecosystem. This nutrient transfer is a well-documented trophic link between marine and freshwater food webs. Atlantic salmon (Salmo salar), by contrast, are iteroparous and may survive to return to the ocean after spawning, though mortality is still high.

The redd's integrity is extraordinarily fragile. Fine sediment from upstream disturbance, a single degree of sustained temperature rise, or a burst of invertebrate predation (caddisfly larvae, crayfish, or birds probing the gravel) can wipe out an entire clutch. Fisheries managers therefore treat intact redd density as a primary metric of spawning-habitat condition and a trigger for habitat-restoration interventions such as gravel augmentation and riparian shading.

Reader's Guide

Spawning & Redd Construction (≈ 3–14 days, species-dependent): The female selects a gravel site, excavates a pit with tail-flicks, and deposits eggs in one or more batches. The male fertilises externally. Key risk: predation of eggs during the brief exposed window; siltation from storm runoff.

Incubation (≈ 4–14 weeks, strongly temperature-dependent): Fertilised eggs develop in the gravel pocket. The chorion protects them; oxygen diffuses through the interstitial water. Mortality risks: siltation sealing the pocket, temperature spikes, invertebrate predation (caddisfly larvae, crayfish), and avian probing.

Alevin (≈ 2–6 weeks): The embryo hatches into a yolk-sac larva that remains buried in the gravel, absorbing its yolk. It is immobile and highly vulnerable to anoxia if the gravel clogs.

Fry (≈ 1–3 months): The yolk sac is fully absorbed; the fry emerges from the gravel, begins exogenous feeding on zooplankton and invertebrates, and develops a lateral line and functional fins. Predation by birds, larger fish, and invertebrates is the dominant mortality.

Parr (≈ 6 months–2 years, varies by species): The fish grows to a few centimetres, develops parr marks (dark vertical bars), and feeds actively in the stream. It remains in freshwater, building muscle and condition.

Smoltification (≈ 4–12 weeks): A profound physiological shift—gills adapt for osmoregulation in seawater, the body silvers, parr marks fade, and the fish becomes seawater-ready. Behaviour shifts from territorial to migratory.

Ocean / Adult phase (≈ 1–5+ years, species-dependent): The fish feeds and grows in marine or estuarine waters. For Pacific salmon this is the sole ocean phase before a homing return.

Return & Re-spawning (≈ 2–8 weeks): The adult navigates back to its natal stream, feeds little or not at all, and spawns. In semelparous species the adult dies; in iteroparous species it may survive and return to the ocean.

Did You Know?

• The decomposition of a dead Pacific salmon after spawning can deliver up to 80 % of the nitrogen in a stream's invertebrate biomass, linking marine productivity to freshwater food webs.
• A single female Chinook salmon (Oncorhynchus tshawytscha) may deposit roughly 2,000–7,000 eggs in one redd, though fewer than 1 % typically survive to adulthood.
• Salmonid smolts undergo a complete gill-epithelium remodelling during smoltification, switching from freshwater ion-uptake cells to seawater ion-extrusion cells—a change that takes only a few weeks.
• The characteristic downstream 'redd pit' left by a spawning female is used by fisheries biologists as a field survey unit; counting intact redds per stream reach is a standard spawning-habitat index.