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 Diadfish1 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.