Overfishing, water pollution, excessive riverside development, etc. have contributed to the regression of amphihaline water migratory fish populations in Europe. Today, most species are in danger. Restoration programs at different spatial and temporal scales have been initiated with, notably, support to populations and rehabilitation of habitats.
However, in the current context of climate change, the distribution of species and the characteristics of their migration need to change. In Bordeaux, Cemagref researchers have been using biogeographic models to predict the distribution of European amphihaline migratory species on the 2100 horizon.
Amphihaline species are those that pass periodically, at well defined stages of their life cycle, from salt to fresh water and vice versa.
The scientific community is beginning to apprehend the impacts of global warming on the distribution of forest essences and the progression of nesting birds in Europe. To date, few studies have been conducted on the effects of the rise in river and ocean temperatures on the distribution and the future of migratory amphihaline fish. Yet most European species are now in danger.
As indicated by the label “amphihaline migrators,” these fish migrate great distances to complete their life cycle between ocean and river. A simulation of the future geographic distribution of migratory fish integrating climate change has just been completed for a doctoral dissertation at the Bordeaux Cemagref. On the 2100 horizon, the results predict consequential losses in climatically favorable watersheds, which raises the question of the future of the majority of European species.
A historical model of species distribution
The researchers began by inventorying migratory amphihaline fish species throughout Europe, the Middle East, and North Africa. This large geographical scale covered the near totality of the geographical area occupied by each of the 28 European species included in the census. How will temperature limit the distribution zone of these species?
To answer this question, 200 watersheds were studied to describe the distribution of each species in terms of presence-absence and abundance. The study established a distribution model for each species at a time when humans exerted little pressure on the milieux. The 1900s were chosen as the reference period. More than 400 bibliographical references were analyzed and lists were made by partner laboratories of the European Diadfish network. In addition to air temperature, four other factors known to influence freshwater fish distribution were studied: longitude at the mouth of the watershed, the surface area of the watershed, altitude at the river’s source, and precipitations.
What future is in store for amphihaline migratory fish in 2100?
The next step applied these distribution models to a context of climate change, using the four reference climate scenarios developed by the Intergovernmental Expert Group on Climate Change (Groupement d’Experts Intergouvernementaux sur l’Evolution du Climat, GIEC, 2000). The study’s term was set at 2100 so as to have sufficient distance to measure significant evolution in the fish populations.
In addition, this date corresponds to the order of magnitude for most restoration plans successfully carried out for migratory amphihaline fish. Based on a temperature evaluation between 1 and 7°C, species’ responses can be classified into three categories: one shrinking their distribution area, one extending their distribution area, and one showing little or no change in distribution. This study shows that for most species the situation will deteriorate. For example, smelt and arctic char should lose approximately 90% of the favorable watersheds for zero or reduced gains. Only two species, the thick-lipped grey mullet and the twaite shad, will be able to expand toward the north, beyond their initial distribution area.
Finally, as predicted, the southern watersheds risk losing most of their species. Could this be an opportunity for more exotic migratory amphihaline fish? Researchers are very reserved on the subject, even pessimistic, because the migratory amphihaline species swim along the coast of West Africa since they have no permanent rivers in which to live.
It is therefore urgent to restore milieux and populations. The predictive models established within these studies are relevant tools to be used to set up conservation programs over the long term at different scales of action.
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