Le black bass (M. salmoides), originaire d’Amérique du nord, est une espèce de plus en plus présente en eau douce.

© Laurent Madelon

Biodiversity of freshwater fish in turmoil


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Updated 24.06.2020

In the last few centuries, 15% of new fish species have been introduced by Man into rivers across the world. Researchers have found that these introductions alter the ecosystem to a much larger extent than originally anticipated.

A team of researchers has just finished measuring the morphological characteristics associated with the locomotion and nutrition of 9,500 species of freshwater fish, out of the 13,000 currently known worldwide. Their objective is to assess the environmental impact of the introduction of new species, intentional or otherwise, by Man 1 .

We used a database developed within our laboratory, which identifies fish species in various water courses around the world”, states Thierry Oberdorff, IRD’s research director and ecologist. “We took into account the physical characteristics of each species, such as the size of the eyes, mouth, fins, body, etc. We examined a total of ten morphological features”, explains Aurèle Toussaint, researcher in macro-ecology who conducted this thesis project within the EDB and Marbec laboratories (Marine biodiversity and its uses).


Functional diversity: + 150%        

Why such painstaking work? Because all these features help define the ecological characteristics of a given species via two basic functions: locomotion (ability of fish to swim at a certain speed and withstand the current) and nutrition (ability of fish to capture and eat prey). On the scale of a river, all the characteristics of the species involved are used to measure functional diversity. The researchers were therefore able to compare this parameter before and after introduction.

As a result, the average 15% increase in the number of species per river led to a 150% rise in functional diversity in the communities in question.

This is huge, we did not expect such a high figure, says Aurèle Toussaint. This is proof that the characteristics of non-native and native species are very different. This changes the functioning of ecosystems, the effects of which on native species are as yet unknown”.

Les grands barrages – responsables de la surreprésentation des espèces aplaties latéralement sont de plus en plus nombreux. Il en existe aujourd’hui plus de 59 000 à travers le monde, comme le barrage Théodore Roosevelt, près de Phoenix en Arizona.

© CC Bureau of Reclamation

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On a global scale, the biggest changes occur in the most industrialised regions, where the largest part of trade is concentrated, a factor which largely promotes the deliberate or accidental introduction of species. “In North America, we identified more than 27% of new species in water courses, which resulted in an average increase in functional diversity of more than 500%, while we only recorded 5% of new species in Asia and Africa, associated with an average increase in functional diversity of 35% to 50%”, notes the researcher.


Are dams the cause?

The researchers also observed, at worldwide level, an increase in the average size of the body of fish and an over-representation of laterally flattened species, such as carp (Cyprinus carpio), native to Asia, and largemouth bass (Micropterus salmoides), native to North America.
These species are able to survive in a stagnant environment, states Aurèle Toussaint. This change is probably attributable to the effects of dams, which are increasing in number throughout the world and alter the structure of water courses by creating artificial lakes, thereby reducing water circulation”. According to scientists, these results show that studying changes in the number of species is not sufficient: other characteristics must be taken into account, such as morphology, to assess the impact of these changes.


Notes :

1. Toussaint A., Charpin N., Beauchard O., Grenouillet G., Oberdorff T., Tedesco P., Brosse S. & Villéger S., Non-native species led to marked shifts in functional diversity of the world freshwater fish faunas . Ecology Letters , 5 septembre 2018 ;

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