Background
In addition to the all-purpose threat of CO2-induced global warming, the authors write that "over the past 200 years, owing to the industrial and agricultural revolutions, various new and intense pressures have been applied to freshwater ecosystems resulting in the decline of many species both in range and number (Maitland, 1995)," and of these additional threats, they highlight five: overexploitation, water pollution, flow modification, destruction or degradation of habitat, and invasion by exotic species, citing Dudgeon et al. (2006).

What was done
Using the extensive electrofishing database of the French National Agency for Water and Aquatic Environment, Poulet et al. evaluated time trends of 48 freshwater fish taxa at 590 sites distributed across France over the period 1990-2009, in order to assess the recent and ongoing net impact of these several modern threats to the fish of France.

What was learned
In the words of the three French researchers, "the results demonstrated that species richness increased steadily from the beginning of the monitoring period," congruent with the finding that "the number of species displaying a significant increase in spatial distribution or abundance was greater than those showing a significant decrease." In addition, they say that "these results are in agreement with recent findings which describe an increase in the number of species and density of fish communities in large French rivers over the past 25 years," citing Daufresne and Boet (2007). And they add that "predictions about the future distribution of 30 common fish species in French streams in 2008, suggested increases in local species richness and in occurrences of the majority of species (Buisson et al., 2008)," making a point of noting that "both studies described population growth of warm and cool water species in response to global warming [italics added]."

What it means
Over the past two decades, when climate alarmists claim the earth warmed at a rate and to a degree that was unprecedented over the past millennium or two, the majority of French fish - plus some newcomers to the country's streams and rivers - actually found the supposedly deadly heat refreshing, as they increased in terms of both species numbers (diversity) and population size.

References
Buisson, L., Thuillier, W., Lek, S., Lim, P. and Grenouillet, G. 2008. Climate change hastens the turnover of stream fish assemblages. Global Change Biology 14: 1-17.

Daufresne, M. and Boet, P. 2007. Climate change impacts on structure and diversity of fish communities in rivers. Global Change Biology 13: 2467-2478.

Dudgeon, D., Arthington, A.H., Gessner, M.O., Kawabata, Z.I., Knowler, D.J., Leveque, C., Naiman, R.J., Prieur-Richard, A.H., Soto, D., Stiassny, M.L.J. and Sullivan, C.A. 2006. Freshwater biodiversity: importance, threats, status and conservation challenges. Biological Reviews 81: 163-182.

Maitland, P.S. 1995. The conservation of freshwater fish: past and present experience. Biological Conservation 72: 259-270.