Global Warming Science -


Shrinking Fish in Europe


[last update: 2009/10/01]



A widely reported news story in July 2009 – European Shrinking Fish due to Global Warming



The Discovery article states: “Daufresne and his colleagues examined long-term surveys of fish populations in rivers, streams and the Baltic and North Seas and also performed experiments on bacteria and plankton. They found the individual species lost an average of 50 percent of their body mass over the past 20 to 30 years while the average size of the overall fishing stock had shrunk by 60 percent.” []


The study conducted a meta-analysis of freshwater fish communities in large rivers in France (Seine and Rhone rivers) found that the mean body size of fish declined in the fish populations. Also, a meta-analysis of herring and sprat in the Baltic Sea also showed decrease in mean size in fish populations. []




French Rivers


A UNESCO report on the Seine River basin states: “Human development also harms the biology of the rivers: migratory fish cannot get past 60 percent of the hydroelectric power plants and less than 20 percent of the dams are equipped with fishpasses. Modifications to the basin’s major rivers, particularly for navigation purposes (1,427 km of navigable waterways, 550 of which have large or medium clearance), are the principal cause of decline in the population of migratory fish species. … Out of a total of thirty-three fish species that have been identified as belonging to the local ecosystem, twenty-six are still commonly found today, a considerable improvement over the 1960s when the diversity and number of fish had declined due to heavy water pollution. … While conditions near the edges of the basin are generally favourable to fish life, this is not the case in its centre (in the Seine River, in particular). In small rivers, non-point source pollution and the silting up of riverbeds are the main causes of the decrease in fish populations. In large rivers, the causes are mainly physical barriers and discharge from urban areas. Along with the negative impact of anthropogenic pressure (seven species are no longer present), about twenty new species have been introduced by humans.” []


A 2007 French study (Harby et al: “A mesohabitat method used to assess minimum flow changes and impacts on the invertebrate and fish fauna in the rhone river, France” []) states: “The change in mesohabitats gives impacts on the composition and abundance of fish and invertebrates … Other studies show that population densities of grayling, brown trout, nase and dace have decreased greatly and become endangered since hydropower development, while densities of gudgeon, minnow and stone loach have increased.


A 2006 French study (Lamouroux and Cattaneo: “Fish assemblages and stream hydraulics: consistent relations across spatial scales and regions” []) found “These reach-scale relationships were consistent across six large French basins, and consistent with the analyses made at the microhabitat scale. Therefore, microhabitat preferences for hydraulics are strong enough to generate consistent reach-scale community responses to hydraulics across regions, despite the influence of other filters such as temperature, nutrient levels or history. The distribution of basic geomorphic features (pools, riffles) in streams and their modification (by dams, weirs and dikes) can modify the proportion of fish guilds by up to 80%, probably contributing to the long-term decline of riffle-dwelling species in Europe.


A 2005 French study (Dufour et al: “Does global warming impact on migration patterns and recruitment of Allis shad ( Alosa alosa L.) young of the year in the Loire River, France?” []) stated: “The hydrological and thermal changes in the Loire River were investigated to test the influence of climatic changes on a short freshwater stage anadromous fish species … The year-on-year levels of young-of-the-year abundance showed wide variations, which were not explained by environmental parameters


The following figure shows the annual average water temperature in the Rhine River (dark green line) 1909 – 2006. [] The rivers have been warming since before CO2 could have caused warming, with an unchanged linear rate, and thus the warming cannot be attributed to anthropogenic CO2.




The Rhone River is “heavily perturbed by anthropogenic pressure from 19 hydroelectric plants (the first of which was erected in 1925), five nuclear plants and from industrial pollution” []


The Rhone River has also warmed throughout the 20th century. “the temperature of the French river course of the Rhône was affected by the impact of nuclear power plants. … The CNR had estimated the yearly average warming up impact of the chain of hydroelectric schemes at 0.14°C due to the slower velocity of flow in the 16 reservoirs (Cottereau, 1989). A far more important part of the warming up must related to the impacts of nuclear power plants. Indeed, the influence of these plants on the thermal regime has been demonstrated by Electricité de France (Desaint, 2004). 90% of time, the theoretical impact is less than 3°C just below the plants, while the average warming up is 1.72°C (Bugey plant), 1.03 (Saint-Alban plant), and 1.34°C (Tricastin plant).” []


The following figure shows the average annual temperature anomalies for land-based climate stations in the area of France encompassing the Rhone River from the NOAA Global Historical Climate Network for 1950 - 2008 (plotted at [

]). All of the warming occurred in a couple of years in 1988 – 1990. Prior to 1988 there was no warming trend and after 1990 there has been no warming trend. The warming jump of the late 1980s left residual warming of about 0.9 degree. Anthropogenic CO2?




Conclusion: Most studies attribute fish changes in the French rivers to the effect of dams and power plants, not due to global warming. The warming that has occurred in the rivers is not due to CO2.




Baltic Sea Herring / Sprat


A 2003 Danish study (Moolman et al, “Feeding ecology of central Baltic Sea herring and sprat”) studied herring and sprat in the Baltic Sea with data from 1977 to 1999 and found “The results of this study support the hypothesis that growth reductions observed in Baltic herring and sprat are due to combination of a change in food availability and an increase in densitydependent competition.” []


The European Commission Fisheries report for the Baltic Sea for 2010 states: “The Western herring stock has declined further, following a 2009 TAC [Total Allowable Catch] which was set well above scientific advice. The incoming year class is only a quarter of the long term average. This stock intermingles with the North Sea herring stock, which is also suffering from poor recruitment. It is therefore important to adopt a common approach for the management of these two stocks.” []


The ICES WGBFAS Report 2005 [] examined Baltic herring and sprat fisheries. The marked decrease in mean weights that started in the early 1980's stopped and was reversed after 1998, but dropped again in 2004”. This is shown in the figures below from this report. The study also predicts increasing catch sizes and spawning stock biomass in the future as a result of changes in total allowable catch.




A 2006 study “Variability of Physical Factors Relevant to Fisheries Production in the Mediterranean Sea, North Sea and Baltic Sea” (Lloret, Wegner, Lleonart, Stergiou, Fromentin, Mackenzie, Damm and Drakopoulos - Boletin Instituto Espanol de Oceanografia, 2006) examined sea surface temperatures []. The following figure is from that study, showing sea temperatures for two sea basins – very little warming has occurred.




Conclusion: Most studies attribute fish changes in the Baltic Sea to the effect of fishing, not CO2. Very little warming has occurred.



See also: for more info on the Baltic Sea area.