Freshwater fish that choose to go to sea
Sea trout are native to Scotland and are also found in Scandinavia, Iceland, around the Baltic and as far south as Portugal.
Scotland’s brown trout have two possible life-cycles: some spend all their lives in freshwater, while ‘sea trout’ migrate to the sea.
Some Scottish rivers are home only to resident brown trout populations while others have brown trout and sea trout. These are the same species but a combination of genetics and environmental factors, principally food shortage, mean that some trout will go to sea to feed before returning to spawn.
There is more food in the sea, so sea trout will often be bigger than brown trout in the same river.
Juvenile sea trout spend two or more years in freshwater before migrating to sea as smolts. Smolts shoal together to migrate, usually around late March/April and usually at night, but while Atlantic salmon travel to far-off feeding grounds, sea trout stay in coastal areas.
Female trout are more likely to migrate . Like salmon, this mostly happens in the spring. Some small fish (‘finnock’) return between July and September, after just a few weeks or months at sea but many adults come back as larger ‘maiden’ fish, after spending more than a year at sea.
Sea trout spawn in the river where they were born, from mid-October to early January. Many of the spent adults will die but some return to the sea. A sea trout can spawn up to 13 times in its lifetime. The young trout emerge between March and May.
Like salmon, sea trout do not usually feed in fresh water, even though they may enter rivers months before spawning. At first they are silver in colour but they gradually turn brown. The only sure way to tell if a trout has been to sea is to look for growth rings on its scales that show it has grown quickly on the rich food in the sea.
Salmon have a more streamlined shape, with a concave tail and slimmer tail wrist, an upper jaw that reaches no further than rear of the eye, few if any black spots below lateral line and 10-15 (usually 11-13) scales counted obliquely forward from adipose fin to the lateral line; trout have 13-16 of these scales.
Both brown trout and sea trout are UK Biodiversity Action Plan priority fish species and sea trout are a Scottish Government Priority Marine Feature species.
History
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. An even larger sea trout, weighing 13 kg (28.6lbs), was illegally netted in the River Tweed in 1987.
Sea trout once provided a valuable fishery, for instance on the River Add, in the Hope Spot and angling for them provided many jobs on the west coast of Scotland, but the populations in many rivers have collapsed, for instance in the River Ewe.
Many observers put this down to establishment of fish farms near the mouths of spawning rivers, and in the coastal areas used all year by adult sea trout.
Research
Scottish Government (Marine Scotland) research has shown that sea lice can have an impact on sea trout up to 30km from fish farms. More research is needed.
Threats
Sea lice multiply in fish farm cages. Migrating sea trout smolts and adult fish that are resident all year can be harmed by these parasites. Diseases can also spread from farmed fish to wild sea trout. Climate change may be affecting them too.