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    -Eric Tabacchi \ CNRS
    "The projects to analyse and model fish populations presented during the symposium were facilitated by the existence of comprehensive and standardised databases in France..."

    -Gilles Cherrier \ Seine- Normandie water agency
    "The great benefit of this type of symposium for a Water agency, which finds itself in a daily struggle to inform and negotiate with its partners, lies in the solid information and the tools made available to reinforce our arguments..."

Trends in fish populations over time

How have fish populations, confronted with modified habitats, climate change, alien species, etc., changed over the past 20 years in continental France? During the symposium on Aquatic biodiversity organised by Onema in November 2012, a number of studies used the long-term data series collected by Onema to provide some answers.

The national symposium organised by Onema delivered an array of new knowledge on aquatic biodiversity in France, spanning microbial processes and wetlands, DNA used for inventories and the cost of plant invasions. A number of projects looked at current dynamics in freshwater fish populations based on nation-wide monitoring programmes since 1978, carried out first by the High council on fisheries, then by Onema. An unprecedented study (N. Poulet, S. Dembski, L. Beaulaton, Onema), based on 7 748 fishing operations from 1990 to 2009, produced information on the temporal trends in population for 47 species. The data show results that are mixed for the various species, but nonetheless positive on the whole. In 58% of the monitoring points, more species are present now than 20 years ago. For 42% of the species, their occurrence (presence at a monitoring point) increased significantly since 1990 and it dropped for only 11%. Similarly, the average density increased for a large majority of the species.

Expansion of introduced species

The species undergoing strong growth are often introduced species, e.g. Wels catfish, asp, topmouth gudgeon, etc. That is particularly the case for tubenose goby, giant goby and black-spotted goby. They were detected for the first time in the Rhine basin in 2007, 2010 and 2011 respectively and the last two in particular have since undergone rapid expansion (S. Manné, N. Poulet et S. Dembski, Onema). Conversely, a number of native species would appear to be in decline. That is the case for common bream, tench, eel and brown trout. A number of hypotheses have been proposed, including eutrophication of rivers for bream and the drying up of side channels for tench. To make progress in the research, the various trends must now be studied on different spatial scales (river, river basin, etc.) and filled out where possible with older data, similar to the project (S. Beslagique, Irstea) to trace the evolution of fish trends in the Seine since the end of the 1800s.

Are shifting ranges a response to climate change?

Again using the monitoring data stored by Onema, a study (L. Comte, University of Toulouse III) looked into recent changes in the ranges of fish in conjunction with climate change over the past 30 years. A comparison (presence or absence of species) of over 9000 sites during two periods, a «cold» (1980-1992) and a «hot» (2003-2009) period, revealed significant differences depending on the species. For example, brown trout are disappearing from the edges of their initial distribution zone, whereas barbel are increasingly found along the edges of their zone. For most species, the study revealed shifts to higher elevations, as if the species were following their ecological niche, but at speeds far slower than the progression of climate change. These results raise a number of questions. Are the shifts accompanied by changes in strategies concerning reproduction, feeding and size? How will species deal with future changes in the climate? Some initial answers were provided for diadromous fish by a number of modelling projects presented during the symposium. This extremely productive research highlights the great value of the long data series produced by Onema for efforts to understand and restore aquatic biodiversity. National monitoring programmes can be filled out with in-depth, local projects. Examples are the systematic observations since 1984 on migrating salmonids in the Bresle river (G. Euzenat, F. Fournel, Onema), showing a drop in marine survival rates of salmon, and the exceptional project since 1974 on Lake Geneva, studying the modifications in the dynamics of phytoplankton communities caused by climate change and the corresponding explosion in whitefish populations (D. Gerdeaux, INRA). Clearly, long-term observations form the basis for our understanding of aquatic biodiversity.

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