El total emptying of the Alcollarín reservoir, in the province of Caceres, was carried out with the aim of eradicating the invasive fish pseudorasbora parvaWhat began as a biological control intervention has led to a fierce controversy over its effects on the ecosystem and the territory.
After the maneuver, the landscape is desolate: dead fish on the bed, bad smells that reach the urban area and the disappearance of waterfowl. The debate has ignited between residents, fishermen and the administration. on the suitability of the method and its execution in this case.
What happened and why the emptying was decided
Until a few months ago, the reservoir stored its maximum level, with 50 cubic hectometres (50.000 billion litres), regulating the flow of the Alcollarín River. The Ministry for Ecological Transition, through the Guadiana Hydrographic Confederation, ordered the complete emptying to stop the spread of the pseudorasbora parva, already present in the Alcollarín and with colonization detected towards the Ruecas and the Guadiana. The management of invasive species de peces as this is explained in our section dedicated to invasive species de peces ..
The device contemplated passive and active fishing arts, and barriers to prevent passage downstream of the wallThe technical documentation also warned of the risk of spread to infrastructure such as the Orellana Channel and demanded avoiding any mass release of individuals during the exploitation of the dam.
In practice, during critical phases of water descent, the fishing nets were overwhelmedEyewitnesses shared videos showing numerous specimens escaping downstream, which, according to local residents and fishermen, may have facilitated their dispersal throughout the basin.
Cost, public contract and execution
The intervention was tendered by MITECO as Actions to control Pseudorasbora parva y awarded for 953.313,01 euros (VAT included), with financing from Recovery, Transformation and Resilience PlanThe resolution, published in the State official newsletter, details the obligation to take extreme measures to prevent the spread of the species.
In the region, numerous testimonies consider that the planning and deployment were insufficient at key momentsWell-known figures in the Extremadura naturalist field have denounced that the operation, far from containing the problem, allowed the escape of thousands de peces towards the lower reaches of the Alcollarín, reaching the Ruecas and Guadiana rivers. The management of these situations is also explained on our pages about causes and consequences of mortality de peces.
The administrations involved must now provide detailed technical evaluations on the actual effectiveness of the operation and on the side effects, in order to determine responsibilities and establish evidence-based corrective measures.
Environmental impact and reaction in the region
The reservoir has been left completely dry and the bottom is covered with dead fish, both invasive and native species, in the process of decomposition. On days with easterly or northeasterly winds, the stench reaches the urban center from Alcollarín, causing discomfort and numerous complaints.
The sudden disappearance of the sheet of water has expelled acuatic birds that used the reservoir as a habitat. This has brought the recreational fishing and the ornithological tourism, activities that provided economic movement to localities such as Alcollarín y ZoriteThe conservation of these habitats and species is also essential for biodiversity in rivers and reservoirs, as discussed in our article on the impact of fires on aquatic fauna.
The social climate reflects indignation. Neighbors remember the loss of land with the construction of the dam and now consider that the social and environmental use which had been consolidated. It is requested to the Extremadura joint Programs of restocking of native species and they emphasize that before doing so, it is essential to completely contain the invader.
Among the citizen proposals, the following are included: intensive fishing of exotic species as a method of control and the tightening of sanctions by transporting live specimens. More precise planning is also called for, with retention devices that do not become saturated during critical phases of the reservoir's decline.
Legal framework and the problem of Pseudorasbora parva
Conservation regulations prohibit massive and non-selective methods of wildlife elimination, which generates debates regarding the legal framework of actions such as a total emptying if selectivity is not guaranteed. Experts and residents request a independent evaluation of the operation and the possible damage to native species, in addition to a habitat recovery plan.
La pseudorasbora parva is a invasive alien species of Asian origin, present in Europe since the sixties and extended to more than 30 countries. This species preys on eggs and fry, consume zooplankton and promotes eutrophication processes, in addition to acting as a vector of the pathogen Sphaerothecum destruens, which affects native fish and their reproduction.
In Spain it was detected in 2002 with inflows mainly into the basin of the ebro, and has expanded into different areas: Internal Basins of Catalonia, Delta of the Ebro, Manzanares (Tajo), Hozgarganta y Guadiaro in Andalusia, as well as in the middle basin of the Guadiana. It is located in the Spanish Catalog of Invasive Alien Species and in European regulations regarding priority species.
Experts agree that effective management requires sustained control, effective barriers, targeted captures and continuous surveillance, avoiding decisions indiscriminate that may harm native fauna and habitats.
Priority is given to safe removal of debris, the monitoring of the water quality and a plan for the restoration of the water level that guarantees the containment of the invasive species.
In addition, it is proposed to intensify the downstream monitoring in Alcollarín, Ruecas and Guadiana, as well as shielding critical points such as Orellana Channel, with retention devices that prevent saturation during rapid descents.
From the institutional level, it is required transparency in management, coordination with independent scientific teams and, if necessary, advances in regulations to act with agility, maintaining the selectivity and the protection of native species.