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Impact of climate warming on fish

International consensus on the reality of climate change is now apparent: global warming is ascribable in large part to human activities. It is causing rapid deterioration of the environment and is increasing the threat to biodiversity.

However, the mechanisms of its impact are still poorly known, particularly in the aquatic environment. At Cemagref, two researchers, who have been analysing the freshwater fish community over the two last decades, have observed profound changes that are more intense and long-lasting than predicted.

Global warming, whose impacts have been demonstrated since the 1980s and 1990s, influences the functioning of the world’s ecosystems as well as the structure and the diversity of animal and plant communities. The GIEC’s (International group of experts on global warming) latest report in September 2007 states that the global temperature should increase by 1.4–5.8°C between now and the year 2100. This phenomenon is an added pressure on the environment that will become increasingly intense over time and should be taken into account in planning sustainable resource and ecosystem management. However, its impact and the possible influence of the effects of other non-climate factors still remain difficult to determine.

- Towards reduced stream biodiversity?

Two Cemagref researchers, Martin Daufresne and Philippe Boet, have studied the effects of this phenomenon on aquatic environments and particularly on the fish communities residing in French streams. Changes in the number, size and representativeness of fished species have been observed for approximately 20 years. Fishermen believe that the fish caught today are no longer the same as before. New species living in warm or Southern waters, such as spirlin and the common bitterling, are gradually replacing the traditionally fished species.

With a large-scale analysis, combining data collected on several streams and several sites for 15–25 years, these researchers have shown a substantial impact of global warming on the structure of fish communities. It appears that the proportion of South-living and warm-water fish in French rivers has increased from 20% and 40%, respectively, to 50% between 1979 and 2004. Whereas large numbers of these small fish tend to become predominant within the communities, large fish, more sensitive to the temperature increase, tend to disappear little by little. However, this change is accompanied by a global drop in biodiversity. The large number of fish observed is distributed within an increasingly limited number of species.

- Distinguishing the impacts of global warming from those of other factors

In large rivers, fish can also be subjected to non-climatic factors such as water development programmes, dams or nuclear power stations, which may influence the vast changes identified within aquatic communities. According to the data collected at this scale, the impact from these sources appears to be quite low on the trends observed. On the other hand, since dams act as natural barriers, they hinder the Southern species in their migration towards the North. In a context of increasing climate warming, the lack of a fish flow from the South, as has been observed on these sites, could be harmful to the current process of species renewal and subsequently intensify the threat weighing on the biodiversity of large rivers. Current research on a finer, individual scale, continues in a partnership with EDF (French electricity company) in order to better assess the real impacts of these developments on fish and most particularly their physiology. In the long run, this research has shown itself to be a requirement for a better understanding of these slowly progressing phenomena.

Marie Signoret | alfa
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