Exotic fish are a threat to the river ecosystems they invade. In a new study from the University of Barcelona, experts set out to understand what happens when invasive species have been introduced into waters that are not their original territory. The study analyzed how native fish are impacted by these species, called translocated species, compared to the effects of exotic or invasive groups.
The results show that the quality of the habitat is the most important factor for the well-being of native fish. However, translocated species were found to be as problematic as the exotic ones.
These results have implications for river management, especially in the context of climate change – as species translocation is one approach to mitigate the consequences of global warming.
“What our data suggest is that invasions by translocated native species should be taken at least as seriously as those by exotic species in the systems we studied, i.e. the typical medium-sized Mediterranean streams and rivers,” explained study first author Alberto Maceda.
The characteristics of fish species were studied in 15 sites in the basins of the northeastern Iberian Peninsula. The focus was on the cyprinid family (Cyprinidae), which has the highest species richness in the world and is the most common in Europe.
Indicators of great ecological relevance were used, such as diversity, abundance and size distribution of native fish when exposed to invasion by exotic or translocated native species.
“Before our study, there were studies that highlighted the problem of mixing populations of Mediterranean and Atlantic trout, and some examples of competition between native and translocated native species, but ours is the first study that has analysed the problem from a broader point of view and has combined different indicators,” said Maceda.
The results indicate that habitat quality is essential for conservation of native species. This includes temperature, water depth or velocity, pH or nutrient levels – which all influence characteristics such as the abundance or weight of native species.
The results suggest that translocated species have potentially greater impacts on native fish than the exotic species. The presence of translocated fish was associated with a lower abundance and richness of native fish and smaller native individuals. Meanwhile, exotic fish led to impacts such as higher abundance and richness of native fish and generally larger individuals.
There are still many unanswered questions about translocated species. Further research is needed to fully understand the ecological impact of translocated species.
“It is no good assuming that the impacts of exotic species are worse because they come from outside our borders, as we do not yet have enough information to make such assertions. In fact, we have a great lack of knowledge about the diseases, hybridisation problems, trophic competition, etc. that translocated species can bring,” said Maceda.
The findings present a challenge for current river legislation and management. According to Maceda, species are usually declared a problem in a political territory, but we can find that a species is native and has invasive populations in the same political area. “To make things worse, we may even find that a species is in decline in its native basin, but it is expanding in basins where it has been previously introduced.”
The researchers emphasize that habitat conservation should be a priority, as the benefits can even make native species better competitors against exotic species. Furthermore, in some cases, intervention against introduced species can also be a solution, as translocated natives can have similar habitat requirements to natives.
“Rivers with poorly conserved habitats also experience the most biological invasions, and it is often difficult to distinguish between the effects of exotic species and habitat,” said Maceda. “However, in some cases the main detrimental effect is that of translocated native or exotic species, and therefore acting on them, if a complete eradication is feasible, will certainly be beneficial to the river.”
The study is published in the journal Science of The Total Environment.