Climate change: a dark future for migratory fish

Although restoration programmes have been set up, the future distribution of these species may be modified because of climate change. At the Bordeaux Cemagref, scientists have developed biogeographical models to predict their distribution on the 2100 horizon.

Migratory fish are noteworthy in that they use both the sea and freshwater environments to complete their life cycle. Since the last glacial period 18,000 years ago, this allowed them to progressively colonize all parts of Europe. However, over-fishing, river development, pollution, etc. have contributed to these migratory fish populations regressing and today most of these species are endangered.

Moreover, they must adapt to global warming, already implicated in the reduced numbers of individuals of certain species, such as the reduction in smelt numbers observed over the past few years in some of the southernmost parts of their distribution area. To identify sensitive species that may be the most severely affected by this climate change, their future geographical distribution, integrating rises in temperature and changes in precipitation, was simulated as part of a doctoral thesis at the Bordeaux Cemagref.

A historical model of species distribution
First, Cemagref researchers inventoried the migratory fish species throughout Europe, the Middle East and North Africa. This large geographical scale covered nearly the entire geographical area of each of the 28 European species counted in the census. How does temperature limit the distribution area of these species? To answer this question, 200 catchment areas were studied to determine 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 put little pressure on the environment. The first decade of the 20th century was chosen as the reference period. More than 400 bibliographic references were analysed and the lists made were completed by the partner laboratories in the European Diadfish network . In addition to air temperature, four other factors known to influence the distribution of freshwater fish were retained: longitude at the mouth of the watershed, the watershed’s surface area, the altitude at the source and precipitations.
What does the future hold for 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 timeline covered the period to 2100 so that significant changes could be measured in the fish populations with a sufficiently long-term perspective. Moreover, this duration corresponds to most of the restoration plans successfully carried out for migratory fish. Based on a temperature rise between 1 and 7°C, the response of the species can be classed into three categories: shrinkage of the distribution area, extension of the distribution area and no change in the distribution area.

This study has shown that for most species the situation will deteriorate. For example, the smelt and the Arctic char will lose approximately 90% of the watersheds that are favourable for reduced or null gains. Only two species, the thinlipped mullet and the twaite shad, will be able to expand their territory towards the north, beyond their initial distribution area. Finally, in accordance with the predictions, the southern watersheds risk losing most of their species. Could this be an opportunity for more exotic migratory fish? Researchers remain very reserved, even pessimistic, on this point, because few of these species are found along the coast of West Africa because of a lack of permanent rivers to accommodate them.

The priority is therefore restoring the fish environments and populations. The prediction models within these studies are good tools that can be used to set up conservation programmes over the long term at different scales.