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  • Freshwater fish and climate change. Current situation and adaptation strategies


    Aquatic environments constitute ecological treasure houses, represent a unique cultural heritage and provide society with important economic resources. What will be the consequences of climate change on the hydrology of aquatic environments? How will aquatic ecosystems and organisms react? What can be done to limit their vulnerability? Fish are emblematic life forms in rivers and an important resource for humans. They are one of the lifeforms that will potentially be impacted by climate change.
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High risk of local extinction of fish species

Fish are an excellent indicator of water quality, but are also among the first to signal and suffer from climate change. Evolutionary changes in certain fish species have already been noted.

Over the past three decades, rapid and major changes in the climate have been observed. These disruptions affect ecosystems, but also the species living in those ecosystems and particularly fish which are found in virtually all surface waters. Increases in temperature and evapotranspiration, combined with a reduction in precipitation, will impact fish populations that are also subject to significant anthropogenic pressures. Thanks to a number of scientific studies on fish, in which Onema participated, the initial impacts have been identified and models have run projections for France, however a degree of uncertainty remains concerning climate change.

Changes in the ranges of species

Statistical models using mid to long-term data series have revealed a number of changes concerning notably species physiology, biological rhythms and the distribution of fish communities. Depending on the species, contractions, expansions or shifts in ranges have been observed. "On the whole, the results tend to show a reduction in favourable habitats for cold-water fish, e.g. trout and Atlantic salmon, whereas fish inhabiting warmer waters, e.g. chub, have seen their habitats expand", indicates Nicolas Poulet, Onema scientific officer. All species of fish now tend to move upstream to higher altitudes. However, climate change (57 m/decade gain in altitude for isotherms) is moving much faster than fish (13 metres per decade), with as a result a high risk of local extinction for the species subjected to unfavourable conditions.

Colonisation faster than extinction

Caution is advised in interpreting certain results, e.g. the observed increase in the number of species a mid altitudes. It has since been shown that extinctions take place more slowly than colonisation by new species. "On a given site, colonising species and others headed for extinction can co-exist, thus giving the impression of greater species richness, whereas in fact, certain species will disappear at some point in the future", says Nicolas Poulet. Finally, concerning their physiology, a reduction in the average size of fish has been noted over the past 20 years. The cause may be metabolic, given that smaller animals draw greater profit from a rise in temperatures than larger animals, thus making them more competitive. The impact of climate change on fish in continental France is already visible, but efforts on the local level should be undertaken to assist ecosystems in maintaining their resilience, a vital factor in buffering the effects of climate change. "Above all, we must reduce anthropogenic pressures given that we cannot really do much to limit climate change on the local level", concludes Nicolas Poulet.

Contact : nicolas.poulet@onema.fr

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