Aquatic ecosystems threatened by the size of non-native fish

14 April 2010

Fish that were introduced into streams and rivers over the past 150 years have altered the average size of fish in many areas of the world. A study published in Ecology Letters shows that the species of introduced fish are larger than those found naturally in the water courses, which poses a heightened risk of modifying aquatic ecosystems.

Researchers from CNRS, University of Toulouse, IRD and the National Museum of Natural History in France, the University of Anvers in Belgium and the University of Utrecht in the Netherlands, cross-referenced data on fish in 1,050 rivers all over the world and found that non-native fish were on average 12 cm larger than native species. This raised the average size of fish in a river by about 2 cm across the board, contravening Bergmann’s empirical rule that, among living creatures, individuals in cooler environments tend to be larger.

Moreover, the introduction of non-native species whose ecological characteristics differ from those of native species can also affect how an ecosystem functions. Some of the large species that have been widely introduced (trout, black bass, catfish, etc.) are predators while others (carp, tilapia, etc.) feed on detritus or plants. These ecological traits are likely to alter the food chain and cycling of organic material. Changes in the average size of fish in communities in rivers at a global scale could entail changes in how ecosystems function.

“Non-native species disrupt the worldwide patterns of freshwater fish body size: implications for Bergmann's rule,” S. Blanchet, G. Grenouillet, O. Beauchard, P. A. Tedesco, F. Leprieur, H. H. Dürr, F. Busson, T. Oberdorff; S. Brosse, Ecology Letters, April 2010.