Exotic fish are a threat to river ecosystems, but what happens when invasive species are native to an area and have been introduced into waters that are not their native habitat? A new study conducted by the UB and published in the journal science of the whole environmenthas analyzed the impacts that the native fish receive from these species translocated speciescompared to the impact of exotic invasive species, ie those not native to any basin in the territory.
The study’s conclusions show that habitat quality is the most important factor in the well-being of native fish, but the study also points out that translocated species can become just as problematic as the exotic ones.
According to the researchers, these findings may have implications for river management, particularly in the context of climate change, since species relocation is a common consequence of interregional water transfers undertaken by some countries to mitigate the effects of global warming.
“Our data suggest that invasions by translocated native species should be taken at least as seriously as those by exotic species in the systems we study, ie the typical medium-sized Mediterranean streams and rivers,” notes Alberto Maceda, researcher at the Biodiversity Research Institute (IRBio ) of the University of Barcelona and first author of the article. The study involved Adolfo de Sostoa, researcher at IRBio, and experts Ralph Mac Nally and Jian DL Yen from the University of Melbourne (Australia).
A landmark study of the impact of translocated species
Researchers studied the characteristics of different fish species at fifteen sites in the basins of the northeastern Iberian Peninsula, but particularly focused on cyprinid species (Cyprinidae), one of the most biodiverse species in the world and the most common in Europe.
In particular, researchers analyzed indicators of major ecological relevance, such as native biodiversity, abundance and size distribution of native fish when subjected to invasion by exotic or translocated native species. “Prior to 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 our study is the first to analyze the problem from a broader perspective and combined different indicators,” says Alberto Maceda, lecturer at the Faculty of Biology at the UB.
Negative impact on native fish
The results, consistent with previous studies, indicate that habitat quality is critical for the conservation of native species. That is, environmental traits such as temperature, water depth or velocity, pH, or nutrient content are the variables that best explain the variability of traits such as abundance or weight of the native species analyzed. However, the major novelty of the study is that after accounting for these environmental variables, the results suggest that the translocated species had potentially greater impacts on native fish than the exotic species, although the latter contained some widely recognized exotic species, such as carp (Cyprinus carpio) or bleak (Alburnus Alburnus).
As stated by the authors in the publication, the presence of translocated fish was associated with lower abundance and abundance of native fish and smaller native individuals, while the presence of exotic fish was associated with higher abundance and abundance of native fish, and generally larger individuals .
There are still many unanswered questions about translocated species
In light of the results, the researchers emphasize the need to examine more closely the ecological impacts of translocated native species. “It is not good to assume that exotic species’ impacts are worse because they come from outside our borders, as we do not yet have enough information to make such claims. In fact, we have a great lack of knowledge about the diseases, hybridization problems, trophic competition, etc. that translocated species can bring,” the researcher points out.
Legislation and flow management challenge
The conclusions of this study pose a significant challenge to current legislation and river management, such as being in the situation of protecting and eradicating the same species depending on which hydrological catchment it occurs in. Alberto Maceda notes that “Species are usually declared problem species in a political area, but we can determine that a species is native and has invasive populations in the same political area. To make matters worse, we can even find that a species is declining in its original tank, but is spreading into tanks where it was previously introduced.”
In this context, the researcher points to habitat conservation as the aspect on which managers should focus most in order to conserve native fish. “Generally, we think that habitat conservation measures should be taken, as the benefits have multiple dimensions that can even make native species better competitors against exotic species,” he says.
Despite this general recommendation, Alberto Maceda adds that sometimes intervention against introduced species can also be a solution, particularly when dealing with translocated natives, as they may have similar habitat requirements to natives. “Rivers with poorly preserved habitats also experience the most biological invasions, and it is often difficult to distinguish between the impacts of exotic species and habitats. However, in some cases the main adverse effect is that of the translocated native or exotic species and therefore acting on them, if total eradication is feasible, the river will certainly benefit,” he concludes.