The spread of the zebra mussel through Spanish rivers and reservoirs continues to grow, affecting the Júcar and Segura basins. This invasive species, recognized as one of the world’s most harmful due to its damage to native biota, infrastructure, and water management systems, is now present in eight of Spain’s fifteen river basin regions.
The Water Quality Area of the Júcar Hydrographic Confederation confirmed recent detections of zebra mussels (Dreissena polymorpha) inside the Alarcón reservoir (Cuenca), in the upstream section of the river, as part of the Shock Plan for the control and monitoring of invasive alien species.
The Ministry for Ecological Transition and Demographic Challenge announced this development this week, noting that the reservoir waters have been continually monitored since larvae of this species were first found in 2018.
The appearance of adult specimens in the Alarcón reservoir indicates that zebra mussels are congregating in the hydraulic infrastructure with the greatest capacity within the Júcar hydrographic limit. This discovery raises concerns for managers and users alike due to the ecological and economic damage this exotic invasive can cause.
Concha Durán, Head of Service for Water Quality, emphasized that the Alarcón reservoir plays a crucial role in the Júcar exploitation system and, in particular, in the Tajo-Segura transfer. The potential implications for irrigation and hydraulic infrastructure could be significant.
Thus, following the detection of several adult specimens, the CHJ initiated several prevention measures aimed at slowing the downstream expansion of the zebra mussel from Alarcón.
Browsing limitations
Regarding navigability, Durán explained that when the species is detected in a water body, navigation rules shift and a kind of reservoir quarantine is imposed. Ships registered to navigate Alarcón may be restricted from operating on other reservoirs, a constraint highlighted by officials.
CHJ administrators are analyzing how the colony behaves within the reservoir waters and identifying areas most prone to consolidation, noting that adult specimens of this inedible mollusk can reach about three centimeters in length and multiply rapidly, forming colonies that cling to hard substrates and block channels, pipes, and gates.
One major challenge in controlling this mollusk is the lack of specialized tools. Although water pressure and flow influence its spread, treatments available to destroy it also harm the broader ecosystem affected by the water.
Regarding monitoring efforts, the CHJ recently signed a new service outsourcing contract valued at approximately 250,000 euros for zebra mussel and other invasive species monitoring and expansion work. An innovative tool for reliable surveillance this year is environmental DNA, which detects traces of the organisms that once inhabited the water.
It also reached reservoirs in Albacete and Murcia
In the days ahead, the zebra mussel also appeared in populated adult forms at the Talave reservoir (Liétor, Albacete) and Camarillas (Hellín, Albacete) reservoirs, with initial indications of presence in the Cenajo reservoir (Albacete/Murcia). As approved by the Segura Hydrographic Confederation, this means it is now present along more than half of Spain’s hydrographic basins, with only the northwest corner of the peninsula currently spared.
Technicians have documented rising larval density in these reservoirs. In Camarillas, density rose from 0.003 larvae per liter in May to as high as 0.106 larvae per liter by August. In Talave, July sampling found 0.006 larvae per liter, increasing to 0.008 larvae per liter in August, while Cenajo recorded 0.005 larvae per liter in the latest sampling.
Adult presence has also been confirmed in Camarillas and Talave, indicating established populations in the Segura basin. Ongoing sampling will continue through mid-October to align with the species reproductive cycle. At the same time, CHS is placing information signs across the watershed and tightening controls on recreational boating to limiting movements that could spread larvae to additional water bodies.
Since the start of summer, navigation has been prohibited in the Camarillas reservoir due to adult populations, and Cenajo now offers a single access point with strict cleaning requirements for any water-contact items at an officially certified station.
One of the world’s most harmful invasive species
The zebra mussel (Dreissena polymorpha) is regarded by the International Union for Conservation of Nature as one of the most detrimental invasive terrestrial and aquatic species globally. Its main ecological and economic impacts arise from rapid growth, colony formation, and the tendency to clog pipes and channels.
As a major filter feeder, it clears large amounts of phytoplankton, altering water clarity and conditions while displacing native species. It can cover lake and river shores, affecting tourism and recreational activities such as sport fishing and boating. After a population takes root, the spread tends to accelerate in nearby basins, a pattern highlighted by researchers like Francisco Oliva, a University of Murcia professor and Life Invasaqua project coordinator who studies invasive aquatic species.
Given its high reproductive capacity, with a single specimen capable of releasing more than a million larvae annually, and the absence of significant predation, rapid colonization of affected reservoirs is anticipated. In a basin like the Segura, the socioeconomic impact could be severe for agriculture.
Some hydrographic confederations have already invested more than 30 million euros directly in managing the species. Indirect losses from zebra mussels were estimated at more than 1.6 billion by studies from other confederations. Experts stress the need to adjust water management and intensify monitoring to prevent further spread into the Talave reservoir and other water systems, including navigation and sport fishing restrictions to limit larval dispersal.
It is anticipated that this species will reach the Segura basin via the Tajo-Segura transfer. Years of warnings from scientists and technicians underscore the importance of persistent filtration of flows from nearby infected areas into the Talave reservoir. The risk remains high as the invasion continues to unfold in central and southern Spain, with ongoing surveillance and intervention plans in place to mitigate adverse effects on ecosystems and local economies.
Environment department contact address: [redacted]