The early life of salmon unfolds entirely within freshwater environments before their migration to the vast ocean. This freshwater phase involves distinct stages, each with specific physiological changes and behaviors that prepare them for the sea.
The Early Life Stages of Salmon
Salmon begin their freshwater journey as eggs, typically laid in gravel nests known as redds. These eggs remain buried within the gravel for several weeks to months, depending on water temperature, undergoing incubation. During this time, the developing embryo is protected from predators and strong currents.
Upon hatching, young salmon enter the alevin stage, characterized by a large, visible yolk sac attached to their underside. They remain hidden within the gravel, absorbing nutrients from this yolk sac. Alevins remain largely immobile, dependent on these stored energy reserves for development and growth.
As the yolk sac is fully absorbed, alevins transform into fry, emerging from the gravel to begin actively swimming and feeding in the water column—a process often called “swim-up.” They are very small and begin to hunt for microscopic invertebrates and insect larvae.
Fry then develop into parr, a stage lasting one to five years. Parr are recognized by their vertical parr marks, which provide camouflage against the riverbed. They become increasingly territorial, defending small feeding areas and consuming a wider variety of aquatic insects and small organisms.
Life in the Freshwater Nursery
Young salmon, particularly during their fry and parr stages, inhabit freshwater streams and rivers. Cover, such as submerged rocks, fallen logs, and overhanging vegetation, offers protection from predators and strong currents. Varied water flows, from riffles to deeper pools, support diverse invertebrate populations.
Their diet primarily consists of aquatic invertebrates, including the larvae of caddisflies, mayflies, and stoneflies. Young salmon also consume terrestrial insects that fall into the water, such as ants, beetles, and small spiders. They actively search for prey in the water column and along the riverbed, consuming a wide range of small organisms to fuel their rapid growth.
Preparing for the Ocean Journey
The transition from freshwater to saltwater is marked by a physiological and behavioral transformation known as smoltification. This process prepares young salmon, now called smolts, for survival in the marine environment. Internally, their gills develop specialized cells to excrete excess salt. Their kidneys also adapt, shifting from producing large volumes of dilute urine to conserve water.
External changes also occur during smoltification, with the parr marks fading and their skin developing a silvery, iridescent sheen. This new coloration provides camouflage in open ocean waters. Behaviorally, smolts lose their aggressive, territorial instincts and begin to form schools, which offers increased protection during their downstream migration. They cease active feeding as they begin their journey, relying on stored energy reserves as they move towards estuaries and the ocean.
Navigating a World of Dangers
Young salmon face numerous threats throughout their freshwater existence. Predation is a threat, with larger fish such as trout and sculpins preying on smaller fry and parr. Avian predators like kingfishers and herons also hunt young salmon, while mammals such as otters and raccoons can capture them.
Human activities and environmental changes also pose dangers. Dams and culverts can block migration routes, preventing movement or ocean access. Water pollution from agricultural runoff, industrial discharges, and urban areas can degrade water quality, introducing harmful contaminants. Habitat degradation, often caused by logging, urbanization, and altered land use, reduces the availability of cool, clean water and protective cover.