Brown trout
Salmo trutta fario
The brown trout is a stationary predatory fish inhabiting cold, oxygen-rich flowing waters from lowlands to high alpine regions. It is characterized by a spindle-shaped, slightly laterally compressed body and distinctive red spots with white margins along its flanks. As an ecologically demanding species, it often serves as a bioindicator for high water quality and intact aquatic structures. It feeds opportunistically on insect larvae, small crustaceans, and, as it matures, increasingly on other fish.
Details
Identification
Adipose fin present, light margins on pelvic and anal fins, red and black spots on flanks, usually olive-brown base coloration.
Social behavior
Strong territorial behavior; occupies and defends fixed holding positions (shelters) against conspecifics.
Diet
Broad spectrum of aquatic insect larvae, terrestrial insects (drift), freshwater amphipods, and small fish (e.g., bullheads or minnows).
Hunting strategy
Ambush predator; uses current shadows behind stones to grab drifting food or actively hunts prey fish.
Spawning substrate
Loose gravel and pebble substrate in fast-flowing sections (lithophilic spawner).
Overwintering
Activity reduction in deeper pools; reproduction (spawning season) takes place during the winter months.
Ecology
Ecological role
Apex predator in small streams; regulates the density of invertebrate and small fish populations.
Natural predators
Otter, grey heron, common merganser, kingfisher (for juveniles), and larger predatory fish such as the Danube salmon.
Competitor species
Rainbow trout (competition for food/space), grayling, bullhead (competition for shelter among juveniles).
Ecosystem service
Important food fish, significant target for recreational fishing (tourism factor), and central element of biodiversity in mountain streams.
Threats
River engineering, dams, pesticide runoff, climate change (rising water temperatures), and predation pressure from cormorants.
Scientific profile
Morphology & ID
Coloration
Back olive-brown to blackish, flanks lighter with characteristic red spots surrounded by white halos; belly whitish-yellow.
Distinguishing features
Presence of an adipose fin (often red-edged), red spots on the flanks, vomerine teeth present in the anterior part of the mouth.
Confusion species
Rainbow trout (Oncorhynchus mykiss - pink lateral band, black spots on adipose fin), Atlantic salmon (Salmo salar - no red spots as adult, slimmer caudal peduncle).
Sexual dimorphism
Males develop a pronounced kype (hooked lower jaw) during the spawning season and often show more intense, darker coloration.
Habitat
Fish region
Trout region (epirhithral), characterized by cool, oxygen-rich, and fast-flowing water.
Preferred zone
Benthic and pelagic; prefers cover such as roots, stones, or overhanging banks.
Flow preference
Rheophilic; prefers turbulent flow areas but utilizes resting zones to conserve energy.
Substrate preference
Lithophilic; requires coarse gravel and stones for hunting and reproduction.
Oxygen requirement
Very high; critical limit is approx. 5 mg/l, while values above 8 mg/l are optimal.
Migration
Migration behaviour
Potamodromous; performs short upstream migrations in autumn to spawning grounds in oxygen-rich upper reaches.