The native range of the brown trout (Salmo trutta) extends from Iceland and the White Sea in the north, to the Atlas mountains of Morocco in the south, making them one of the most widespread freshwater fish in Europe. They have been widely introduced across the globe for angling purposes and can now be found as far afield as Chile and New Zealand. However, they remain, along with salmon, one of the fish most readily associated with Scotland.
In Scotland, the brown trout is one of the most recognisable of our native fish and it can be found in a variety of freshwater habitats, including rivers, large lochs and upland lochans. This species has two alternative life-cycle patterns – the first is the freshwater resident form referred to as ‘brown trout’ and the second is the anadromous (sea going) form referred to as ‘sea trout’.
Brown trout are polymorphic and a large variety of morphotypes exist throughout Scotland. The most important of these, are the original colonisers – those fish which occupied isolated loch and streams immediately after the retreat of the ice ~10,000 years ago. Populations of brown trout which have remained isolated since that period, including ‘ferox’ (or piscivorous) forms, are of considerable conservation value.
‘Resident’ brown trout, as this term suggests, complete the whole of their life cycle in fresh water. Most populations do, however, undertake significant migrations within fresh water. The most common life-cycle pattern in Scottish brown trout populations is the migration of juvenile fish from nursery areas, where they begin to feed, into lochs where they may remain until becoming adults. Some other migrations may occur between feeding areas in the summer months.
The sea trout is not a species in its own right, but a migratory form of the brown trout (Salmo trutta L.). Sea trout are like Atlantic salmon in that they migrate to the sea to feed and grow before returning to fresh water to spawn. They do not, unlike Atlantic salmon, migrate to far off feeding grounds, but instead utilise coastal areas. These areas provide sea trout with increased opportunities for feeding and growth before they return to freshwater to spawn.
Sea trout are native to Scotland and are found widely in Scandinavia, Iceland, the Baltic. They are not restricted to northern Europe and can be found in many parts of the European Atlantic seaboard as far south as Portugal.
In Scotland, some rivers may have brown trout populations which are entirely resident in freshwater, whereas some may have populations which are comprised of both life history types i.e. both brown trout and sea trout. The reasons for this are poorly understood but it is accepted that both environmental and genetic factors play a part.
The life history of sea trout is similar to, but not entirely the same as that of Atlantic salmon – that is they spend a variable time in freshwater as juveniles before undergoing the physiological changes that allow them to migrate to sea as smolts. Many populations reach the ‘smolt’ stage after two years, although in some locations, it may take longer. Interestingly, female trout are more likely to become smolts and migrate to sea than males. Like salmon, migration downstream to the sea usually takes place between the months of April-early June.
Sea trout differ from Atlantic salmon in that they do not venture off to distant feeding grounds in the sea, but instead, remain in coastal areas. Relatively little is known about the ecology of sea trout in these coastal environments, although it is known that the length of time spent at sea varies considerably between individuals and some ‘populations’. The time spent at sea can be quite short, with some fish returning to the river after just a few weeks or months between July and September. These small fish are often referred to as ‘finnock’. Many adults return as larger ‘maiden’ fish after 12 or more months at sea and these fish can be seen in the river between May-October.
Spawning takes place in their natal river and normally begins in mid-October and continues through to early January. The timing and duration of spawning is may be determined by environmental and genetic factors. Once on the spawning grounds, sea trout lay their eggs in gravel pockets or ‘redds’ that have been excavated by the female fish. Once the eggs have been fertilised the female then covers the fertilised eggs with gravel. The young trout will emerge from the gravel between mid-March and early May. Many of the spent adults, known as ‘kelts’, die, but a significant proportion of them survive and make their way back to the sea to recover and grow. Sea trout can spawn up to thirteen times in their lifetime.
Neither forms of trout, freshwater resident or sea trout, receive extensive protection within conservation legislation. Some protection exits in terms of exploitation controls exist within fisheries legislation and sea trout are further protected within fisheries acts relating to the protection of ‘salmon’. In 2007, however, both ancestral brown trout forms and sea trout were added to the UK Biodiversity Action Plan Priority Species List.
Image credits – First image of brown trout and River Helmsdale (c) Linda Pitkin/2020Vision, Ferox trout, Loch Ericht trout and Ferox trout eggs (c) Martin Hughes; Loch Awe (c) Lorne Gill.