We used historical sources from the end of the nineteenth to the mid-twentieth century and current data to retrace fish assemblage trajectories for the past 150 years on 29 river stretches distributed throughout the Seine River basin. We based our analyses on species presence/absence, occurrence of amphidromous and non-native species, and species ecological traits related to habitat preference, oxygen and temperature requirements, and diet. In accordance with general trends observed for Western Europe, we detected a general decline of amphidromous species and an increase in non-native species, even if, at some sites, several non-native species were extirpated. These changes affecting amphidromous and non-native species led to a weak increase in beta-diversity in fish assemblages. Independently of amphidromous and non-native species, for a reduced set of sites, we noted that changes in the balance of ecological traits over time, trace, in a consistent way, the major steps that affected watercourses like waterway development, increasing pollution, dam construction, or, locally, the recent wastewater treatment improvement. Despite local variations, we found contrasted trends between, on one hand, large rivers and/or catchments which had experienced strong expansion in human population, where fish assemblages had deteriorated, and, on the other hand, upstream catchments, with a declining human population, where fish assemblages showed signs of improvement. Because our results suggested that long-term changes affecting fish assemblages cannot be summarized as an unequivocal gradual degradation, we questioned the use of historical data to define ecological reference conditions for river assessment and management purposes.

Using both historical and current data, we retrace the long-term evolution of fish assemblages in the Seine River basin since 1880, from headwaters to upstream of the Seine River estuary. Successive phases are observed, related to anthropogenic impacts on habitat conditions and river water quality. Temporal trajectories were thus reconstructed on several reaches based on the change of the proportion of species’ ecological traits, in order to detect the main drivers of alteration. Contrasted trends occur between large rivers and small streams of the basin. In this context, migratory fish declined, whereas the proportion of non-native species increased in the fish community of the Seine River.

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The spread of non-native species is nowadays recognized as a major threat to the biodiversity of freshwater ecosystems. However, for a very long time the introduction and acclimatization of new species has been perceived mainly as a source of wealth for human societies. Here, we examined the establishment of non-native fish species in the Seine River basin from a historical perspective by adopting a twofold approach. In a first step, at the whole basin scale, considering various written and archeological sources, we traced the chronology, over the last millennium, of the establishments of non-native species. In a second step, by analyzing fish monitoring from several hundred sites covering the diversity of rivers and streams, we examined the changes in numbers and abundance of non-native species in local fish communities over the last three decades. The first documented species introduction dates back to the 13th century but it is from the middle of the 19th century that the introduction attempts accelerated. Today, these introductions have reached an unprecedented level and 46% of the species recorded in the basin are non-native. During the last three decades, non-native species have continued to increase within fish communities both in terms of number of species and abundance of individuals. The most pronounced increases are noted on large rivers and sites where anthropic pressures are strong. Waterways connecting European basins, globalization of trade, and ongoing climate change provide a general background suggesting that the increase in the proportion of non-native species in the fish communities of the Seine River basin is likely to continue for several decades.