Impact of Gambusia in Iberian Rivers and Beyond
A small fish, not exceeding 6 centimeters, has caused significant disruptions in Spanish rivers. Introduced a century ago to combat diseases carried by mosquitoes, gambusia, or mosquitofish, has spread broadly across much of the country. It threatens native river species, including a locally important fish unique to the Iberian Peninsula, and faces the risk of extinction in parts of its range.
Gambusia holbrooki, commonly known as the mosquitofish, originates from the eastern United States and can inhabit fresh, brackish, and salty waters. Today, it is present in about 30 countries across five continents, used for ornamental purposes or as a biological control agent against disease-carrying mosquitoes.
In Spain, the species was introduced in 1921 to address malaria. The World Health Organization recognizes malaria as a potentially fatal disease transmitted to humans by infected mosquitoes, though it is preventable and treatable. Spain was declared malaria-free in 1964, and most cases since then have been imported; nevertheless, mosquitofish remain abundant in slow, warm waters across much of the Iberian Peninsula, notably along the southwest, the Mediterranean coast, and the Ebro basin.
This persistence is due to gambusia’s preference for slow-moving, shallow waters rich in vegetation. The species shows remarkable resilience in degraded and polluted environments, including areas with sewage discharge or low dissolved oxygen. In some locations, native species struggle to adapt to these conditions.
As efficient predators, mosquitofish frequently feed on dipteran larvae (flies and mosquitoes), copepods, and small herbivorous insects that inhabit surface waters. By consuming larvae, gambusia can reduce pest populations, but they also disrupt the food web by depriving native species of prey and habitat.
Gambusia’s reproductive capacity is high, producing 15 to 30 offspring per birth. This fecundity can be adjusted by environmental conditions, with spawning increasing when resources are scarce and decreasing when resources are abundant, further challenging native ecosystems.
Alterations to river courses caused by dams, reservoirs, and other infrastructure support the species’ persistence by creating hydric zones with shallow flow that favor its survival.
biological effects
The established presence of this hardy species outside its natural range is a result of early 20th-century efforts to control a parasitic disease. In some cases, public health measures overshadowed the protection of rivers and wetlands and the status of native species living there.
These historical decisions in the malaria fight continue to shape hydrographic ecosystems today, with native species displaced by predation and the adaptability of exotic species contributing to ecological imbalance.
This is a reminder of how exotic introductions can ripple through ecosystems. Among the native and endangered species affected are Aphanius iberus, an Iberian Peninsula endemic fish, and other regional fishes such as Aphanius baeticus in the Guadalquivir basin. There are also reports of native amphibians harmed by mosquitofish through predation on eggs, larvae, and tadpoles.
According to the Spanish Invasive Alien Species Catalogue, Gambusia contributes to environmental changes that promote eutrophication in rivers and wetlands, leading to nutrient enrichment in freshwater systems and excessive algal growth. This, in turn, reduces oxygen levels and threatens essential members of the aquatic community, posing a serious challenge to maintaining biodiversity. Gambusia is listed among the 100 most harmful invasive species by the International Union for Conservation of Nature (IUCN).
For further context, it is noted that the Guadalquivir basin exhibits a higher level of invasiveness than some native fish populations, underscoring the complex dynamics of non-native species in Iberian waters.