The presence of exotic fish in rivers does not alter the native fish response to the environmental pollution, according to an article published in the journal Science of the Total Environment and signed by the researchers Alberto Maceda Veiga, from the Biodiversity Research Institute (IRBio); Adolfo de Sostoa, from the Faculty of Biology and IRBio, and Ralph Mac Nally, from the University of Canberra (Australia).

Exotic fish are able to adapt extreme environmental conditions (droughts, environmental pollution, etc) and show territorial behaviour. They are active predators, alter the food web and can make local fauna disappear. However, knowing whether the presence of exotic fish is a factor that hardens native fish response to the environmental stress -poured wastewater for instance- is still an unsolved doubt among the scientific community.

Exotic fish and fluvial ecosystems: the forgotten factor

This is the first scientific study analysing whether exotic fish affect the response of native populations towards environmental stress factors. Therefore, the authors study the interaction between exotic species and fluvial pollution through the analysis of indicators -abundance, pathologies, size, etc.- on the ecological state of native fish populations.

"The most common indicators of water quality are based on protocols that do not take the effect of exotic species, and this element could add a new stress factor to the native fauna. In this study, we wanted to see whether there is proof of this additional stress and whether it could alter the reference values we have to assess the quality of water for native animals" says Alberto Maceda Veiga, member of IRBio and expert at the Doñana Biological Station (EBD-CSIC).

All factors that alter the flow, morphological conditions and fluvial connectivity -that is, hydromorphological alterations- are the main threat to the preservation and protection of rivers, according to the authors, even more than quality of water or the presence of non-native species in the natural environment.

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New strategies to fight fluvial pollution

"In the study, we stated a certain proof that only by improving the environmental flow regimes in rivers could notably improve the environmental situation in our rivers" says Alberto Maceda. "Therefore, fighting environmental degradation in river basins is more efficient than using resources to extract exotic fish from the water environment."

"However, this does not mean there is not a serious environmental problem related to pollution or non-native species. But if we solve hydromorphological problems we could get many benefits for the rivers: improve the quality of water -more water, higher dilution of pollutants- and the natural control of exotic fish, which usually live in still waters (reservoirs for instance)."

Tackling factors that alter the environmental equilibrium independently is the most effective strategy to preserve the ecological quality of rivers. "Environmental management or continental waters is less efficient if treated globally. Therefore, it is best to prioritize actions and act independently on each stress factor" notes Adolfo de Sostoa, lecturer from the Department of Evolutionary Biology, Ecology and Environmental Sciences of the University of Barcelona and member of IRBio.

Rivers and pollution: future forecast in the Mediterranean area

In the Mediterranean area, the river regime is quite influenced by the effects of climate, a situation that could worsen due the effects of the climate change. Environmental pollution, moreover, could depress the fish immune system and favour the appearance of diseases, in particular those brought by the exotic animals -the case of the parasite Lernaea cyprinacea- which put native fish populations at risk.

In a situation of global change, preserving the good ecological state of fluvial ecosystems is a priority that affects administrations, managers and society as a whole. "Water is a limited resource, especially in the Mediterranean climate, therefore overexploitation of water resources can have severe consequences for our rivers" warns Maceda.

The lecturer Adolfo de Sostoa believes that "we should prioritize the improvement of hydromorphological conditions in water basins, which is the main factor of degradation for continental water environments, and then act on other factors such as the presence of non-native exotic species."