Spain is home to between 900 and 1,100 species of fish, many of them key to ecological balance. However, close to a dozen have been classified as invasive species, with the standout being the brook trout (Salvelinus fontinalis), a trout native to North America that has raised concerns throughout the territory.
With a streamlined body, it rarely exceeds 30 centimeters and 500 grams, although its vivid colors —green back, white spots, reddish flanks— may deceive: it is a true ecological enemy.
Introduced in the 19th century to boost sport fishing in rivers and mountain lakes, the brook trout ended up spreading uncontrollably. It is currently established in the basins of the Tagus, Douro, and Ebro rivers, as well as in Pyrenean ibones, Gredos lagoons, and areas of the Aigüestortes National Park.
Ecological impact: predation, competition, and trophic imbalance
The brook trout represents a multifaceted threat:
- Preys on insects, crustaceans, fry, and amphibians, altering the food base.
- Competes with native species such as the brown trout (Salmo trutta).
- Causes an increase in phosphorus and alters the nutrient cycle in high mountain lakes, extremely fragile ecosystems.
- Can hybridize with brown trout, posing genetic risks to native populations.
Furthermore, its presence affects traditional salmonid fishing, with economic consequences for several autonomous communities.
Ongoing control and eradication measures against this invasive fish species
Despite the difficulties, various strategies are already being implemented to control its spread:
- Awareness campaigns to prevent illegal translocations.
- Inspection of fishing gear that could transport eggs or fry.
- Specific eradication plans, such as the one implemented in Peñalara lagoon, where after five years of intensive net fishing, species of zooplankton and benthos that had disappeared were recovered.
- Regional regulations and inclusion in the Spanish Catalog of Exotic Invasive Species (Royal Decree 630/2013).
Restoring ecological balance in aquatic ecosystems
The brook trout, initially introduced for recreational purposes, has become one of the most problematic invasive species in Spanish waters.
Its control is vital to preserve aquatic biodiversity, prevent the genetic loss of endemic species, and restore the balance of mountain lakes and rivers.
