2. Trout and Its Life Roles in Ecosystem

Life cycle of the brown trout

The life cycle of the brown trout begins in autumn when the adults spawn on the gravelly beds of rivers and streams. The females lay their eggs in excavated hollows, where they are fertilized by the males and then covered with fine gravel for protection. In the spring, the larvae hatch and initially feed on the nutrients from their yolk sacs before becoming juvenile trout. These young fish seek shelter in shallow waters with plenty of hiding places. At the age of two to four years, the trout are sexually mature and migrate upstream to the spawning grounds to begin the reproductive cycle again in autumn.

• young individual (hidden in shelters)
• adult (2^nd–4^th year)
• spawning of adults in autumn
• the spawn (⌀ 4–6 mm) covered with fine gravel
• hatching of fry in spring

Life cycle of the freshwater pearl mussel

In summer, male freshwater pearl mussels release sperm into the water, which are then inhaled by the females, leading to fertilization of the eggs. The eggs hatch into microscopic parasitic larvae called glochidia. In August, the females release these glochidia, some of which attach themselves to the gills of the trout. In spring, the tiny mussels leave their hosts and burrow into the riverbed. They grow slowly and reach maturity after 5 to 10 years. The adult mussels establish a stable position in the riverbed, grow up to 13 cm in size and can live for up to 150 years, making them the longest-living organisms in the region.

• attachment to the gills (August)
• late summer to spring
• young individuals (0.4–0.7 mm, 5–10 years)
• adult (13 cm, 150 years)
• glochidia (0.04–0.07 mm, 6–8 days)

Brown trout as an apex predator

In the food pyramid of mountain streams, the adult brown trout occupies the position of apex predator. It primarily feeds on the larvae of aquatic insects such as caddisflies, mayflies and midges, as well as small crustaceans, such as amphipods, and terrestrial insects. Larger specimens also prey on smaller fish, amphibians and small mammals. The presence of trout as apex predators has a significant influence on the structure and composition of populations of aquatic organisms and thus contributes to maintaining the biological balance.

• European bullhead
• Eurasian minnow
• mayfly larva
• caddisfly larva
• amphipod (scud)
• organic matter

Importance of the brown trout

The brown trout is a well-known and popular fish but faces challenges such as habitat degradation and loss, pollution, and overfishing. Natural predators such as otters also pose a threat to its populations. Trout hatcheries contribute to breeding and releasing trout into water bodies, helping to restore populations and mitigate the negative impacts of human activity. Protecting trout is crucial for preserving this iconic species.

• trout hatchery
• fly angler
• Eurasian otter
• trout on the menu

The project "Living jewels below the water surface of Šumava" is supported by EU funds from the ERDF fund, INTERREG Bavaria-Czech Republic Program 2021–2027.

Illustration and graphic design: Lenka Pužmanová