Sea trout smolt migrations

April and May are the peak time for sea trout smolt migration, when some young brown trout decide it is time to leave the river and go to sea. But what is a smolt? Denise Ashton explains.

The phrase ​‘it depends’ is so often applied to trout behaviour that it can be hard to get a straight answer to a simple question. Our native trout are amazingly adaptable and variable (‘plastic’ in the science terminology). What is written below about sea trout smolts is more of a general indication than any sort of rule. It is one of the things we love about trout, this variability. Salmon, cuckoos and swallows are more consistent and predictable! 

Brown (resident) trout and sea trout are the same species, Salmo trutta. Why do some trout go to sea? It is a mixture of genetics and environmental factors (for more detail see this article by Andy Ferguson on the complexities of trout migration and this summary). Sea trout will generally migrate to sea when they are between 1 and 3 years old (although they can be up to 9 years old) and around 13 to 18cm long, going through a process of ​‘smoltification’ to ready them for life in sea water. For example, special cells in the gills adapt to excrete salts and the fish’s gut and kidney changes to produce more urine, essential to maintain life at sea.
The timing of these changes is partly determined by increasing day length (photoperiod), hence most smolt migrations take place in March, April and early May, though river flows (discharge) and temperature also have an effect. Smolt migrations take place in the autumn on some rivers and while the numbers migrating at this time might (probably) be lower, it’s likely an important life cycle strategy to spread risk. 

In the river, most young trout find comfortable niches around rocks or weed; this is where they can find small areas of slower flow, their food is easily found both on the river-bed and drifting past and there are nearby bolt holes to avoid predators. The dark backs and spotted sides typical of trout in rivers is good camouflage, especially when viewed from above. At sea, however, countershading with a dark back and silvery flanks and belly makes trout much less visible when swimming mid-water, so smoltification effects such a colour change. Sea trout also become more streamlined in shape. 

Left: a trout parr avoiding predators and other trout. Right: Salmon (top) and sea trout (bottom) smolts. Sea trout smolts (and adults) have a thicker ​‘wrist’ above the tail than salmon. 

Juvenile trout (fry and parr) are essentially solitary and they will go to considerable lengths to find a niche that avoids seeing any other trout. However, smolts form small shoals and will drift tail first downstream in a group, with the group ​’leader’ changing in much the same was as the lead goose in V formation will swap around to share the burden of trail blazing. Travelling in a group is perhaps an adaptation to avoid predators, in much the same way as small fish will form a ​‘bait ball’. Another strategy to avoid predation is to travel mainly at night. 

Given the right conditions, smolts can make fairly rapid progress to the sea (a median speed of 62 – 69km per days has been recorded on the River Frome in Dorset) but they are often held up at weirs. Smolts seem to either hesitate or rest before going over a weir, and they become very vulnerable to predation a these points (see diagrams below)
Low flows can delay migration – the very low flows in April 2021 seem to be delaying smolt runs in many rivers. A combination of low flows and weirs can be fatal to many smolts — Research on the River Tweed showed that up to 80% of smolts fail to migrate past weirs in low flow years. 

The fairly predictable timing of smolt migrations and the way they travel in shoals is an ideal opportunity for research, and especially salmon research. Smolt traps are placed in many rivers by fisheries researchers to monitor populations and much has been revealed about trout and salmon behaviour by tagging smolts and tracking their movements using telemetry or acoustic tags.
For example, the Game Conservancy and Wildlife Trust have been running a research smolt trap on the River Frome for many years, and the ​‘Missing Salmon Project’ is tagging and tracking salmon smolts as they travel to the Moray Firth in Scotland. In the Irish Sea, the COMPASS project is unearthing some intriguing detail about sea trout and salmon smolt migration, with much, much more to be learned. 

To find out more about trout, including sea trout, go to our Trout Resource Hub.