Sea trout Salmo trutta; Linnaeus, 1758. Family: Salmonidae
Sea trout are the sea-running form of the Brown trout (Salmo trutta), a species that adopts many alternative life history strategies. In Scotland, where there is access to and from the sea, some of the juvenile trout (generally at about 150-200 mm in length and aged 2-3 winters) become highly silvered and migrate to feed in salt water. Their diet there is varied and gradually changes from small crustaceans to fish such as sandeels and sprats.
Many Sea trout return to fresh water as finnock (see below), although they may not spawn at this time. Such fish return to sea and return to spawn in subsequent years. Highly-silvered when recently back from the sea, the mature sea trout gradually darken with time in fresh water. The males, especially, become heavily pigmented. As spawning time approaches in late autumn, the mature sea trout are found together with Brown trout that have remained in fresh water and they occur on the same redds. Female Sea trout are more likely than males to migrate to sea.
After spawning, the surviving Sea trout kelts regain a silvery sheen by the following spring, when they return to the sea. Sea trout tend to live longer than salmon, and may make several annual spawning visits. The tendency to long life-span and multiple annual spawning is greater in highland than lowland rivers.
Like salmon, the seasonal timing of the return of Sea trout from the sea seems to depend on their place of origin - in simple terms, how far up a river system they hatched. In east coast rivers, Sea trout heading for the upper reaches come in from the sea from April to June. They may spend most of the summer in the main river stems, waiting for high water conditions in autumn before moving upstream to spawn.
Resting fish are found in groups, lying quietly during daylight and becoming more active at night, although seldom ingesting food. West highland Sea trout ascend clear rivers, often into lochs, where they remain for several weeks or months before spawning in tributary burns.
Immature Sea Trout (Regionally called Finnock, Herling or Whitling)
These small fish, much-loved by anglers, are common in most Scottish estuaries, feeding actively as they move in and out with the tide. As the season progesses, many gather in larger rivers and lochs, not necessarily in their natal systems, and over-winter in fresh water before returning to the sea in spring. In contrast to the rest of Scotland, immature sea trout are uncommon in the River Tweed (and the rivers of north-east England). There, the main migration strategy of the post-smolts is to head down, and then across the North Sea to feed off the coasts of East Anglia, Holland and Denmark. These more distant travellers often remain at sea feeding for two or more winters before they return. Not surprisingly, the Tweed sea trout are bigger on average than elsewhere in Scotland.
The current British rod-caught record for Sea trout, is a specimen weighing more than 10 kg (22lbs), which came from the River Leven (Loch Lomond) in 1989. A cast of an even larger sea trout, weighing 13 kg (28.6lbs), which was illegally netted in the River Tweed in 1987, can be seen at the Marine Scotland Science Freshwater Laboratory at Pitlochry.