The reproductive effort of salmon begins when they return to freshwater streams to spawn. A female excavates a depression, known as a redd, in the gravel streambed where she deposits her eggs. These eggs, or roe, are a highly concentrated, energy-rich package of marine-derived nutrients, making them a prize food source for nearly every river organism. This vulnerable stage contributes to the naturally low survival rate, as a large percentage of eggs are consumed in the wild.
Primary Fish Species Consuming Eggs
The most consistent predators of salmon eggs are resident stream-dwelling fish, particularly those in the trout and char family. Dolly Varden, a type of char, synchronize with the salmon run and move into spawning areas specifically to feed on this nutrient pulse. They act as opportunistic scavengers, positioning themselves downstream of active redds to intercept eggs swept out by the current. Char species are strongly attracted to the chemical cues released by the eggs, signaling a readily available meal.
Other salmonids, including Rainbow Trout, Cutthroat Trout, and Brown Trout, also consume large quantities of eggs during the spawning season. These resident trout often gorge on the eggs, which represent a significant caloric boost that sustains them through the winter months. In some systems, even juvenile salmon (parr) will feed on the eggs of other salmon species. This predation behavior is generally passive, focusing on collecting loose eggs rather than actively digging into the protected gravel nests.
Small, bottom-dwelling fish called sculpins are also effective egg predators. Species like the Slimy Sculpin are well-adapted to life on the streambed, using their flattened bodies to hold position in the current just below a spawning site. They intercept eggs that roll or drift out of the redd, utilizing their large mouths to consume them whole. While small individually, the abundance of sculpins means their collective impact on exposed egg survival can be considerable.
Environmental Factors Increasing Egg Vulnerability
While predators are abundant, eggs must become accessible, a process often triggered by environmental conditions or the salmon themselves. A major factor is superimposition, where a later-spawning female inadvertently digs up a previously constructed redd while preparing her own nest. This action dislodges and exposes thousands of eggs, sending them into the water column to be consumed immediately by waiting fish. This phenomenon is common in areas with limited high-quality spawning habitat.
Hydrological events, such as flash floods or severe spates, can compromise the integrity of the redd. High-velocity water scours the streambed, washing eggs out of the protective gravel and making them available to downstream predators. Conversely, poor water quality increases vulnerability by killing developing embryos in place. Low levels of dissolved oxygen, often caused by fine sediment filling the spaces between the gravel (siltation), can asphyxiate the eggs.
Eggs that die from low oxygen or disease become an easy target, as predators act as scavengers removing non-viable eggs from the ecosystem. Furthermore, if a redd is constructed too shallowly, the eggs may be exposed to light or physical disturbance, increasing the likelihood of fungal infection and mortality. Any egg not securely buried in the clean, oxygenated gravel is a likely target for fish predation, whether exposed by flood or by a subsequent spawner.
The Ecological Impact of Egg Predation
Egg consumption by fish acts as a pathway for transferring energy from the marine environment back into the freshwater food web. The rich fats and proteins in salmon eggs provide a massive seasonal subsidy to the resident fish community. This concentrated food source allows species like trout and char to build up fat reserves necessary to survive the lean winter months. This phenomenon is a natural part of the annual ecosystem function.
From a population perspective, the high rate of egg predation is considered a form of compensatory mortality. Many eggs consumed are those that would have died anyway (unfertilized, diseased, or poorly buried). This natural thinning helps prevent overcrowding and ensures that the few eggs remaining in the redd have adequate resources and oxygen flow for development. Survival to the alevin stage is typically only around 10 to 15 percent.