The brown trout can reach sizes of more than 80 cm, but generally the trout that live in our rivers do not exceed the 50 cm. In Catalonia, the indigenous trout populations belong to the Mediterranean lineage, except in the Garonne basin which are of Atlantic lineage. The coloration of the body is very variable according to the rivers. On the Mediterranean side, the native brown trout has a large number of small black dots. In the Garonne basin, trouts have fewer black spots and are larger in size. Trout of both lineages also tend to have more or less numerous red dots.

The brown trout is native to Europe, North Africa and West Asia. In Catalonia it inhabits the headwaters of most river basins.

Typical trout habitat is cold-water mountain rivers. It breeds in autumn or early winter. The eggs are laid in stream areas with gravel substrates, where the females excavate a redd.

The trout feed mainly on invertebrates, but older individuals also feed on fish and crayfish.

The stocking of rivers with hatchery trout has led to genetic pollution on native populations in many rivers. For this reason, genetic reserves have been created to preserve the indigenous populations of each basin.