The various stages of development of salmonid fish have individual descriptive names, the most important of which are alevins, parr and smolts. Adult fish that have spent time in the sea are known as Grilse (one sea winter) or Salmon (multi-sea winter).
In the UK, Atlantic salmon spawn in October and November, burying their eggs in gravelly-cobble areas in streams where the prepared depression that the female fish create create are called redds. Females lay a total of 7,000 to 8,000 eggs in two or more redds (NB that this number of eggs is in the mid-range of fecundity across all species of salmonids). A steady supply of clean, well oxygenated water is critical to sustain these eggs whilst they are buried. The eggs remain in the gravel throughout the winter before hatching in the spring. Newly hatched salmon, called sac fry or alevins, obtain food from their attached yolk sac and may remain buried for some time. The free-swimming young salmon (known as fry) eventually emerge from the redd, primarily from April to June, when the yolk sac has been completely absorbed. Feeding activities by the fry begin at this time.
Salmon fry, approximately one and one quarter inches long at emergence, quickly set up feeding territories which they defend from other fish. Growing salmon prefer stream habitat lined with cobble-sized stone and clean, cool (15° C or less) water that is free of sediment. Fish are found in riffles and along the interface of fast moving water, under overhanging cover and generally toward the bottom of the water column.
Fry that have spent their first summer in the stream where they hatched are 7 to 10 cms long by autumn and are called parr. After one full year in freshwater, the parr will have grown to a length of 10 to 15 cm. Parr remain in freshwater for a period of one to three years. The freshwater residence period is largely dependent on growth rate. The fastest growing parr, usually from warmer, more productive tributaries, spend only one year in freshwater. Slower growing parr, often from colder, less fertile tributaries, spend three, or rarely, four years in freshwater. Most parr in the UK spend two years in freshwater. During their first autumn, parr may disperse widely from their first summer location to seek new habitat.
Parr destined to leave the freshwater environment the following spring begin a process called smoltification during the preceding winter. Pronounced physical changes occur during the spring after salmon reach a size suitable for migration to the sea, 15 to 20 cms or more. These changes allow juvenile salmon to adapt to life in marine waters. Throughout the smoltification process a series of behavioral, physiological, and morphological changes occur that transform young salmon from territorial, bottom-dwelling, freshwater fish to schooling, saltwater fish. Juvenile salmon leaving for the ocean are called smolts. Smolts lose the dark vertical stripes, parr marks, on their sides and become bright silver in color. Smolts migrate to sea from April through June. Some smolts may commence pre-smolt movement in the fall to start their long migration.
UK smolts begin a long migration westward and northward first along the coast and then further out to sea. The salmon eventually arrive at waters off of the west coast of Greenland where they share feeding grounds with other Atlantic salmon from North America and Europe. In recent years, most UK salmon, return after spending only one winter at sea (1SW) and are called grilse. Other salmon return to spawn after residing in the ocean through two sea winters (2SW). and others wait until after their third sea winter to return (3SW). Grilse (1SW) are around 2 kgs in weight. The 2SW salmon grows to about 75 cms and 4.5 kgs as a returning mature salmon. and 3SW salmon often weigh more than 7 kgs.
Adult salmon return to their home rivers at various times of year depending on various factors. In the freshwater environment, the color of the adult salmon slowly changes from silver to a very dark color. Salmon attempt to reach their natal streams as conditions allow. If they are early, they spend the summer holding in deep, cold pools before spawning in the autumn. From the time they enter the freshwater until spawning, often six months later, the salmon do not feed; feeding begins again after they return to saltwater in the autumn or spring if they survive the experience. Unlike Pacific salmon, not all Atlantic salmon, die after spawning, though many die as a consequence of the rigors of the upriver migration, the spawning effort itself, and not feeding for up to one year while in freshwater. Adults that survive the rigors of migration and spawning are called kelts. Kelts return to the ocean in late autumn or early spring, at which time they regain their silver color. A small percentage of salmon survive several spawning runs, alternating between freshwater and marine environments. Repeat spawners and grilse are valuable to the salmon population for maintaining genetic variability and providing a buffer against all sources of mortality affecting the population whether in fresh or salt water.
Click here to view a diagram of the salmon life cycle in a separate window.
Page Author: Dr Rick T Leah, Univ of L'pool - Contents last reviewed 05/01/2007