Sea trout are known for their remarkable journey between freshwater and marine environments. They exemplify a unique migratory lifestyle, spending different stages of their lives in distinct aquatic habitats. Their ability to thrive in both fresh and saltwater sets them apart, inviting a closer look into their biological makeup and behaviors.
Characteristics and Classification
Sea trout are not a distinct species but rather a migratory form of the brown trout, scientifically known as Salmo trutta. They belong to the Salmonidae family, which also includes salmon and other trout species. Their differing lifestyles are a result of environmental factors and genetic predispositions. The term “anadromous” describes their life cycle, where they hatch and spend their juvenile years in freshwater before migrating to the sea to mature and grow, eventually returning to freshwater to spawn.
The Salmo trutta species is highly adaptable, exhibiting various ecotypes, including river-resident, lake-resident, and the anadromous sea trout. Sea trout are formally referred to as Salmo trutta morpha trutta, distinguishing them as the sea-run form. Like all trout, they possess an adipose fin, a small fleshy fin located between the dorsal and caudal fins. Their body is typically torpedo-shaped and elongated.
Migratory Life and Habitats
The life cycle of a sea trout begins in freshwater rivers, where eggs are laid in gravel nests called redds, typically between November and February. After hatching, the young fish, known as alevins, remain in the gravel, feeding on their yolk sacs before emerging as fry. They then develop into parr, spending one to five years in freshwater, establishing territories in clear, oxygenated river systems.
As they prepare for their journey to the sea, these juvenile trout undergo a physiological transformation called smoltification, becoming “smolts.” This process involves developing a silvery coloration and adapting their internal organs to cope with the transition from freshwater to saltwater. Smolts then migrate downstream to coastal waters and estuaries, usually around late March or April, often moving under the cover of darkness.
In the marine environment, sea trout mature and grow due to abundant food sources. They generally remain in coastal areas and estuaries, typically not venturing more than 80 kilometers from their natal river. When ready to spawn, adult sea trout migrate back to their birth rivers, often arriving from summer to early autumn and waiting in deep pools until winter. Unlike some salmon species, sea trout do not die after spawning; many return to the sea to feed before spawning again in subsequent years.
Distinguishing Features
Identifying a sea trout, especially when it is in freshwater, can be challenging due to its close resemblance to the brown trout. When in saltwater or freshly returned to freshwater, sea trout exhibit a silvery coloration on their sides, with a grey-green back and a white belly. This silvery appearance is a result of guanine crystals on their scales, providing camouflage in the open marine environment.
Upon returning to freshwater, their silvery sheen fades, and they adopt the brownish or olive coloration of resident brown trout, often with brown and black spots. While brown trout have prominent red spots, sea trout possess dark or black spots, and any red spots present may turn black upon migration to the sea. Adult males preparing for spawning may also develop a hook-like, upward-facing protrusion on their lower jaw called a kype.
Feeding Habits and Behavior
The diet of sea trout varies depending on their life stage and environment. In their freshwater juvenile phase, young trout primarily feed on insects and small invertebrates. As they transition to the marine environment, their diet expands significantly due to richer feeding opportunities.
In the sea, sea trout become opportunistic predators, consuming a range of prey including small fish like herring and sand eels, as well as crustaceans such as shrimp and crabs. Their feeding habits can scale with their size, with smaller sea trout consuming crustaceans, medium-sized ones preferring shrimp and smaller fish, and larger individuals targeting other fish. When sea trout return to freshwater for spawning, they cease feeding, relying on energy reserves built up during their time in the sea. While in the sea, they may form schools, but during their return to freshwater for spawning, they tend to be more solitary or gather in deep pools.