Description: Fish of the family Salmonidae have a small fleshy fin (adipose fin) on the back just in front of the tail fin, a characteristic which is absent in other European fishes. Colouration is very variable, both within and among waters. Typically the back is brown or bluish-grey in colour grading to grey, silver or yellow on the sides and white on the belly. The back and sides are covered in spots that are black, and sometimes red, in colour and vary in size, shape and distribution. Sea trout (brown trout that have been feeding in the sea) are silvery in appearance with black spots. Brown trout typically live for about 5-6 years and reach several kilograms in weight. However, the ferox ecotype/subspecies can live for more than 20 years and fish of up to 9 kg have been taken in Lower Lough Erne and Lough Melvin.

Habitat: Brown trout can be seen in most clean rivers at any time where the water is slow flowing and clear although specific types/species are restricted to certain water systems.

Distribution: Brown trout, in the generic sense, is widespread and found in all non-polluted rivers and lakes in Northern Ireland. Presence of brown trout is a good indicator of a healthy aquatic environment. Genetically unique populations are often referred to by local names such as dollaghan, ferox, gillaroo, salmon-trout, and sonaghen. These are present in the Melvin, Erne and Neagh systems, and in some coastal rivers, especially in the Mournes (for example, Whitewater, Spence’s, Shimna) and Glens of Antrim (for example, Glynn, Glenariff).

Similar Species: Atlantic salmon can be confused with brown trout (Salmo trutta) in the freshwater juvenile stage and adults returning from sea can be confused with sea trout — the marine migratory form of brown trout. The Atlantic Salmon Trust website is a good resource, and shows the main characteristics used to differentiate the two species. Hybrids with brown trout also occur at about one per cent in some rivers and are difficult to identify definitively, except by molecular genetic analysis. There is also some confusion about whether there are distinct species within Salmo trutta or whether these are subspecies or ecotypes.

Key Identification Features:

• Adipose fin just in front of tail.
• Variable colouration but typically they have a dark back grading to a lighter colour on sides with a white belly.
• Back and sides covered in black or red spots.
• Various distinct types/populations with distinctive charaterisics.

Distribution Map from NBN: Salmo trutta at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Salmo trutta at iNaturalist World Species Observations database.

GBIF data for Salmo trutta

WoRMS: Salmo trutta at World Register of Marine Species. Accepted name: Salmo trutta Linnaeus, 1758. AphiaID: 127187.

Classification: Biota; Animalia; Chordata; Vertebrata; Gnathostomata; Osteichthyes; Actinopterygii; Actinopteri; Teleostei; Salmoniformes; Salmonidae; Salmoninae; Salmo