In an earlier post of the freshwater life cycle of steelhead, I described the fry stage and touched on its potential to have an outsized influence on the overall productivity of steelhead. The value of fry receives less focus in science and conservation. This raises the question: why is the fry stage often ignored, and why can it have such a profound influence on how we estimate the productive potential of a river for steelhead?
The whole challenge with early life occurs because of the sheer fecundity of salmonids and the vulnerability of their tiny offspring.
For example, a single female steelhead may lay 3,000–6,000 eggs, and because they have just emerged, fry are very abundant — albeit also weak and poor swimmers. Imagine wandering around as a toddler alone in the forest, everything would feel dangerous. It is no different for fry.
The fry are naïve to predators and particularly vulnerable because of their small size. They must also find food rapidly and consistently. No food, no survival.
Not surprisingly, survival is brutal. In some cases fewer than 1% of fry survive, and even in good years more than half often perish. The cumulative level of mortality is greater than at any other stage of life.
A newly emerged sac fry. This fish still carries the yolk sac its mother provided — its first and only food source for the days between emergence and the moment it must start feeding on its own. At this stage, fry are tiny, naïve, and poor swimmers.
A Critical Period
Most of the mortality occurs during what is referred to as the "critical period," which begins at emergence and lasts for several weeks to a few months. A short burst of dispersal happens when fry first emerge from the gravel; however, most fry remain close to the redd and the vast majority do not move more than 200–300 meters during the early period.
The critical period is a pivotal stage when survival hinges on balancing the need to eat and grow against the risk of predation. This stage marks the period of highest overall mortality in a steelhead's life.
Fish such as juvenile bull trout, sculpin, and even older steelhead may gorge themselves on fry during the early critical period.
Crowding, Competition, and Consequences
The average angler probably believes predation presents the greatest risk to young fry. This isn't necessarily true.
Survival strongly depends on the abundance of neighboring fry. When fry are scarce, the search for space and food is relatively easy. But when fry are abundant, especially when several redds cluster together, competition among fry intensifies. Those with a competitive edge hold prime spots close to the redd, while weaker and less dominant fry are forced to the margins, or in worse situations, longer distance migrations. Because fry are highly reluctant to undertake long-distance movements, most fish cram into the limited habitat within the immediate vicinity of the redd, which in turn dramatically increases potential effects on survival due to increased competition.
Regulation Starts (and Stops?) Here
Ecologists describe this phenomenon, where the number of animals influences the rate at which they survive, as population regulation. In steelhead, regulation occurs most strongly during their first year of life but can be especially strong during this early critical period. For those fry that survive, life progressively gets easier as they grow and become stronger swimmers, better foragers, and more skilled at evading predators.
Regulation occurs because resources are limited and as fry abundance increases, survival rates fall, creating a natural equilibrium with the amount of available food and space.
Hence, the only possibility to increase the number of juveniles a river can support is to increase the amount of food and space. This is the basic concept that drives the emphasis on habitat restoration in recovery and management plans. The assumption is simple: more high-quality habitat should increase juvenile survival and the number of smolts that migrate to the ocean.
The Untold Story
Despite decades of well-intended efforts and untold millions of dollars, there is thin evidence that habitat restoration directly leads to measurable increases in the number of returning adult steelhead. This may seem at odds with the general assumption that habitat restoration is improving steelhead runs.
When a population is in equilibrium with its environment, adding habitat through restoration tips that balance. The catch is that fry don't disperse very far during their first months and restoration only covers a small portion of any watershed. Since steelhead abundance is largely regulated by the fry stage, adding more habitat will have little benefit unless fry can access it.
Spawning steelhead — like many animals — tend to cluster during spawning, leaving some areas crowded while others sit underutilized. This patchiness can create intense competition early in life and trick us into thinking this is the limiting factor. In reality we may need to increase spawner escapements to spread them out and fill up the habitat with fry.
In short: restoring habitat only helps if fry can get there.
Lasting Effects
These nuances of the ecology and behavior of fry mean they leave an outsized fingerprint on subsequent life stages. For example, the number of fry that survive to the end of their first summer is a good predictor of the number of smolts that will migrate to the ocean. The growth of fry during the first summer even plays a large role in determining the age of smolts — poor early growth leads to older smolts, while strong growth early in life leads to younger smolts. Surprisingly, this remains true even when these fish spend 2–3 years or more in freshwater before smolting. Ultimately, this short window during the first summer of life often sets trajectories that persist through later life stages.
Unfortunately, the fry stage is understudied in steelhead and is rarely, if ever, accounted for in management plans or models. If fry are so important, how do we account for them? Fry themselves can be time consuming and difficult to track and count. Luckily, measuring the distribution of spawners can be an effective surrogate for directly observing fry. Are redds tightly clustered? Do some streams account for most of the spawning, while others only receive a few? Answering these questions can help us determine whether populations are at equilibrium with their environment, or if unused potential might exist. And the first step is as simple as GPS-locating redds, something that is becoming more common but is still not widely done.
Stay tuned as we continue to examine the life of juvenile steelhead.
Support the Work
Better Science. Better Decisions. More Wild Fish.
Every donation helps us put more anglers in the field as scientists — gathering the data that protects wild fish across the Pacific Rim.
Make a Donation