Sea trout are fish known for their ability to inhabit both freshwater and saltwater environments throughout their lives. This migratory pattern allows them to thrive in diverse ecosystems, moving between rivers and the open sea.
Defining Sea Trout
Sea trout are the anadromous form of the brown trout, scientifically classified as Salmo trutta. They share the same genetic makeup as resident river brown trout, but their life history patterns differ due to their migratory behavior. In some regions, they are known by various common names such as sewin in Wales, finnock or herling in Scotland, and white trout in Ireland.
They belong to the family Salmonidae. Despite their distinct migratory lifestyle, sea trout are not a separate species but rather a morphotype of Salmo trutta. Environmental factors, particularly the availability of food, can influence whether an individual brown trout adopts an anadromous lifestyle and migrates to sea.
The Anadromous Life Cycle
Sea trout begin their lives in freshwater rivers, hatching from eggs laid in gravel nests called redds. The hatching time can range from one to three months, influenced by water temperature and food availability.
Newly hatched trout are called alevins. Nourished by a yolk sac, they become fry after about a month and begin foraging for small insects. They then develop into parr, spending one to five years in freshwater rivers, feeding on invertebrates like nymphs and caddisfly larvae. As they prepare for their sea migration, parr undergo a physiological transformation known as smoltification, which enables them to tolerate saltwater.
During smoltification, their gills adapt to process salt water, and their bodies become silvery. These smolts migrate to the sea. In the marine environment, they feed on fish, small crabs, and shrimp, growing substantially larger than their freshwater counterparts. After several months to a few years at sea, sea trout return to their natal rivers to spawn. Unlike most salmon, a significant portion of sea trout, around 75%, do not die after spawning and can return to the sea to feed before migrating back to the river to spawn again in subsequent years.
Physical Traits and Habitats
Sea trout display distinct physical characteristics that vary depending on their life stage and environment. When in the sea, they exhibit a silvery coloration with faint black spots, providing camouflage in the open ocean. Upon returning to freshwater for spawning, their coloration quickly changes, adopting the more typical olive-brown hues with brown and black spots seen in resident brown trout.
Their body is generally elongated and torpedo-shaped, with silver-grey sides and grey-green backs, and a white belly. Like all trout species, they possess an adipose fin, a small fleshy fin located between the dorsal and caudal fins. Adult sea trout typically range from 35 to 60 centimeters (14 to 24 inches) in length and can weigh between 0.5 to 2.4 kilograms (1.1 to 5.3 pounds), although larger specimens up to 10 kg have been recorded. Breeding males may develop a hook-like protrusion on their lower jaw called a kype.
Sea trout inhabit diverse environments, utilizing both freshwater and coastal marine habitats. For spawning and early development, they rely on gravelly riverbeds in freshwater streams and rivers. After smoltification, they migrate to coastal marine environments, where they feed and grow. Their distribution is wide, found along the Atlantic and Baltic coasts of Europe, the United Kingdom, and the coasts of Iceland, though they are not present in the Mediterranean Sea.