Brown trout Salmo trutta; Linnaeus, 1758. Family: Salmonidae
The European Brown trout (Salmo trutta) is found throughout Scotland in both running and still waters. Our damp, equable, oceanic climate and varied geology with an abundance of gravels provide very favourable conditions for spawning, growth and survival. Brown trout are typically streamlined, highly-spotted fish, with brown background colouration varying to yellow, and with black, orange, or red spots.
Brown trout occur in a wide variety of forms, differing substantially in appearance and in life history. Small, growth-stunted, but highly-coloured, resident trout are common in headwater streams. Others, which may be more silvery as juveniles, migrate downstream to better feeding opportunities in rivers, lochs, estuaries, or the sea ( see sea trout), and eventually return to their natal areas to spawn in early winter (Oct-Dec). The eggs (4 to 6 mm diameter) are buried in gravel in moderately flowing riffles of small streams, usually from 1 to 5 metres wide. Spawning can also occur on clean gravel areas in lochs, particularly where there is up-welling of groundwater.
The brown trout is an extremely variable species in both habit and appearance. Large migratory sea trout, for example, differ markedly in most respects from the smaller non-migratory trout that choose to remain in rivers - despite the fact that, in many cases, single families give rise to both migratory types. River and loch trout are well-known for their wide range of colourations and spotting patterns. Formerly, this led to their being classed as many separate species, or sub-species. Now, however, all trout are regarded as belonging to a single species, Salmo trutta, although this is not the whole story. Modern techniques show that trout from different locations differ genetically and that the effect is very marked. The patterns of variation appear to reflect the routes taken by invading trout at the end of the last glaciation, as they re-colonised Scotland's emerging freshwaters from rivers or lakes on the edge of the ice.
Diet and Behaviour
Adult Brown trout can range from 10-100 cm in length, but typically are between 20-50 cm. Smaller trout feed almost entirely on crustaceans, worms and molluscs and on insects and their larvae. In summer, much of the diet in highland lochs is wind-blown insects of terrestrial origin. As they grow, larger trout feed increasingly on smaller fishes, including minnow (Phoxinus phoxinus L.), stickleback (Gasterosteus aculeatus L.), and on other small trout and salmon parr. A large form (known as ferox trout) found in many highland lochs feeds largely on Arctic charr. Trout in lowland waters tend to grow faster than those at higher altitudes. Trout in warmer waters grow more quickly, but tend not to live as long as those in the colder, less productive highland waters.
The original genetic patterns have been modified considerably by later events involving exchange of fish, and genes, among populations - and this probably continues today. On the otherhand, genetic exchange among populations is limited by other factors such as fidelity to particular spawning sites, differences in the timing of reproduction and the choice of particular types of mates. The most notable example of such segregation comes from Lough Melvin in Ireland, which contains three forms of trout - sonaghen, gillaroo and ferox. Sonaghen are silvered plankton feeders, gillaroo are highly coloured bottom feeders and ferox are large fish that prey on smaller ones. All three forms spawn separately and, in spite of their proximity during most of their lives, each form remains genetically distinct.
Brown trout is widespread and ubiquitous in Scotland, but favours cold, clean, well-oxygenated rivers and lochs.
Brown Trout and Angling
In Scotland, there has been little commercial exploitation of brown trout as a food fish, either by netting or through aquaculture. However, the species is held in very high regard by anglers. The value of the national fishery for brown trout angling is clearly substantial but has probably been under-rated in the past. For example, case studies now show that brown trout lochs bring several million pounds annually to the economies of the Outer Hebrides and Orkney through visiting anglers.
In more populated areas of the country where the demand for angling is particularly high, some Brown trout populations are supported by stocking from fish farms. However, although the restocking trade in Brown trout has largely been overtaken by production of the alien Rainbow trout (Oncorhynchus mykiss), the growth in angling interest in rainbow trout, often for 'put-and-take' fisheries, and the rapidly growing acceptance of 'catch and release' are helping to reduce pressure on natural populations of Brown trout.