Aquatic ecosystems threatened by the size of non-native fish

This modification in the size structure of fish assemblages poses a serious risk in terms of the alteration of aquatic ecosystems. The results of the project are published in the April 2010 issue of Ecology Letters.

Since the Neolithic era, human beings have been transporting and introducing new species. This practice has accelerated over the past 150 years along with the development of transportation methods and international trade. The phenomenon extends to freshwater fish, several hundred species of which have been introduced throughout the world, either unintentionally or for food or recreation purposes.

By cross-referencing data on the fish present in 1,050 river basins around the world, researchers at the University of Toulouse, CNRS, IRD and MNHN, as well as the universities of Antwerp (Belgium) and Utrecht (the Netherlands), have determined that the fish species introduced by human intervention are an average of 12 cm larger than the species naturally present in these rivers, which increases the average body size of the fish assemblages in a given river by about 2 cm. This modification affects, to a moderate but significant degree, Bergmann's empirical rule. A general rule that applies to most living beings, it expresses the fact that the farther an organism lives from the equator, the greater its body mass. This principle is the result of the joint evolution of species and their environment over millions of years, and, as this research reveals, humankind seems to be in the process of altering its profile.

Beyond these historical considerations, the introduction of species whose ecological characteristics differ from those of the native species can also affect the functioning of the ecosystem. Some of the larger species widely introduced throughout the world are predators (trout, black bass, catfish, etc.) whereas others are detritus feeders or herbivorous (carp, tilapias, etc.). These ecological characteristics are likely to modify the food chain or the recycling of organic matter. Modifications in the average body size of assemblages observed in river basins on a global scale could thus go hand-in-hand with modifications in the functioning of aquatic ecosystems.