What is a Smolt Anyway?

I often hear the term smolt thrown around by anglers. Like, “I caught a couple smolts,” or, “Did you see the smolts jumping?”

Basically, any juvenile steelhead anglers see or encounter is typically referred to as a “smolt.”  Technically, smolt refers to a highly specific life stage that exists during a narrow window of life when it is migrating to the ocean during the spring.

Outside of that window, which lasts weeks to a month, juvenile steelhead transition through a few stages of life. Below I define and describe those different stages of life leading up to the time when a juvenile steelhead actually begins smolting and migrates to the ocean.

When steelhead first hatch from their eggs they are defined as an alevin. An alevin has yolk sac protruding from its belly. That sac will provide the fish with food for up to several weeks as they continue to grow and develop. It’s like a sack lunch from mom. The yolk is critical because it allows the young fish to remain deeply buried in the substrate. Alevins may occasionally feed on small insects, but its not necessary for them to survive.

They begin to fully and consistently emerge from the gravel once the yolk sac is depleted, presumably because they need to find alternative sources of nutrition. 

A juvenile is referred to as a fry once the yolk sac is gone and they begin to spend more time outside of the gravel. During the first days after emergence, however, a small remnant of the yolk sac still protrudes from their belly. Because the small bit of yolk has not been buttoned up, they  are sometimes called swim up fry. In scientific research and journals, fry may also be defined as young-of-the-year, commonly abbreviated as YOY. Fry and YOY refer to the same life stage that briefly exists once they’ve burned through their yolk and before they transition to the next life stage.

Fry are very small and relatively poor swimmers and favor shallow channel margins, often spending several weeks in water that may only be a few inches deep. Not only does this provide them with slow water habitat with cover from predators, but it is also often warmer than the main flow in a river. Fry preferentially select water around 20°C, much warmer than the 16°C or so that older juveniles prefer. Warm, food rich channel margins help them grow quickly, and fast growth early in life is critical.

The fry life stage typically lasts for a few months, though slower growing fish may remain as fry for the first year. All told, a steelhead is a fry for the transitional period after they’ve burned through their yolk and before they begin to fully develop scales and spots. 

Once the fish has fully developed scales, spots, and prominent vertical oval bars on the lateral line (referred to as “parr” marks) the fish technically becomes as parr. The parr stage accounts for most of the time a juvenile steelhead spends in freshwater. Steelhead may spend 1 to several years at sizes ranging from 60mm up to 180mm.  Some parr may spend much of their time in freshwater within a few hundred meters of where they were born. However, because they are larger and stronger swimmers than fry, it’s not uncommon for them to undertake more extensive movements to find habitats with less competition. Further, larger-sized parr commonly begin moving out of creeks and headwater stream reaches during the fall before they smolt. They enter mainstem rivers as stream flow increases and temperatures decrease to seek out better feeding opportunities. They also gradually move downstream over the course of winter, offering those individuals a “running start” on smolting, and consequently, they often survive at higher rates than smolts that wait to initiate migration during spring.

After attaining a size of 180-220mm, or about 7 to 8.5 inches, and initiating a physiological transition that is stimulated by photoperiod – lengthening of the day – the parr marks disappear, spotting is reduced, and the fish become silvery. The outward change in appearance is driven by an internal physiological transition that prepares the fish for saltwater entry.  This brief window is referred to as smoltification and denotes the smolt life stage.  The transition from parr to smolt occurs during late winter or spring and lasts a few weeks at most.

Because of the costly physiological transition and the risks needed to enter the ocean and pass through the gauntlet of predators, smolt survival is often quite low and more similar to what fry experience than parr.

Smolts are quite sensitive because their protective scales loosen and can fall off much easier when handled.  And where parr are highly thermally tolerant, smolts must remain in cool water or risk shutting down the physiologic processes that allow them to enter saltwater.  For these reasons, its likely that a large portion of actual smolts perish when they are caught and handled by anglers, especially if they’ve been caught on larger gear meant for adult steelhead.

In summary, while parr and smolts are often used interchangeably among fisherman, they are very distinctive life stages. Much of the complexity in steelhead comes from how they use freshwater habitats as adults and juveniles, which is one reason they are so adaptable. In the coming weeks we will expand on these life stages and discuss why survival of fry is critical to understanding freshwater productivity, why parr have it relatively easy, and why we may be making incorrect assumptions about smolt survival, and how that could lead us to poor management decisions.