Down to the Sea

By Bill Hauser

Salmon smolt. What do you know about salmon smolts?

Let’s begin with a brief overview of a generalized salmon life cycle starting with the egg.

The egg is deposited during summer and incubated until mid-winter when the fish hatch and in spring, become free swimming fry. After some development and growth, they become juveniles called parr or fry until they become smolts that migrate to the ocean. In the ocean, they grow and mature until they migrate back to freshwater to spawn. This sounds easy enough but it is really not so simple.

What, exactly, is a smolt? The smolt lifestage is brief but it is crucial in the life of a salmon. It is brief because it lasts only long enough for the young fish to migrate from freshwater to saltwater. It is critical because at this time, the little salmon undergoes a severe physiological transformation from an organism that is designed for life in freshwater to another that can survive only in the salty seawater. In addition, it is during this time that smolts imprint – they memorize the sequence of odor changes as they migrate downstream. They will need this memory when they return as adults to the stream of their birth. (Some imprinting occurs during the fry stage as well.) With the parr-smolt transition comes a dramatic change in coloration. The distinct narrowly oval dark parr marks disappear and are replaced with silvery sides and a dark back and they resemble miniature adults.

Smolt migrations begin in early spring and usually peak in late May or early June. This transformation is triggered by increasing day length and increasing water temperatures and begins earlier in the more southerly parts of their ranges. Most of the migration is during nighttime to minimize predation. Although a smolt migration may last 4 to 6 weeks, most of the migration occurs within a just a few weeks.

Most smolts average about four inches in length but sockeye salmon smolts may be a bit smaller and Chinook salmon smolts may be a bit bigger. The ages of smolts are more variable. Many sockeye salmon smolts are one year of age but may be up to three years. Chinook salmon smolts most commonly rear only one year in freshwater and coho salmon typically become smolts after one year but may be up to four years. A colleague reports that he once aged a large coho smolt at eight or nine years. He opined that it had become marooned above a particularly robust beaver dam. (Actually, most beaver dams are much more porous than most observers would believe.)

But here is an important question. Where do smolts end up after their migration from freshwater? Estuaries and other brackish water. What is their job? To eat and avoid being eaten (not necessarily in that order) and continue out into the ocean.

Within a particular year, the largest an oldest migrate earlier in the migration period and the migration is not a passive ride downstream but, rater, an active downstream movement.

Of course, here is the usual disclaimer. These descriptions are only the most common actions but each population is unique. There are often small differences.

Finally, recall that pink and chum salmon are outliers in this descriptions of smolts and smolt migrations because they migrate to the ocean as fry soon after emerging from the redd.