FI-I-2: Occurrence of thermophilic species in inland waters – case study

The picture shows an underwater shot of a large carp.Click to enlarge
Fish species such as carp are more competitive under warm breeding conditions.
Source: Photograph: © Vladimir Wrangel /

2019 Monitoring Report on the German Strategy for Adaptation to Climate Change

Table of Contents


FI-I-2: Occurrence of thermophilic species in inland waters – case study

Despite a substantial reduction in the phosphorus content of Upper Lake Constance, the hot summer of 2003 led to an explosive increase in thermophilic carp. In particular during spawning and the development of larvae, warm weather conditions provide carp with competitive advantages. The warm summers of subsequent years bestowed record yields on commercial fisheries.

The line graph shows the development of the phosphorus content in Lake Constance in milligrams per cubic metre from 1970 to 2017. There is no trend.
FI-I-2: Occurrence of thermophilic species in inland waters – case study

The line graph shows the development of the phosphorus content in Lake Constance in milligrams per cubic metre from 1970 to 2017. From 1970 to 1980, the phosphorus content rose from 45 to 87 milligrams per cubic metre. After that, the value fell almost continuously to below 10 milligrams until 2003 and has been hovering in this range ever since. The trend is significantly downward. Another line shows the carp yield in Lake Constance's Upper Lake. Between 1970 and 2002 the curve oscillates around 1,000 kilograms. Between 2003 and 2007, the curve rose steeply to yields of 17,000 kilograms in 2007 and dropped again to below 4,000 in the following years until 2014. For 2016, it was almost 6,000 kilograms again. There is no trend.

Source: Landwirtschaftliches Zentrum Baden Württemberg / Fischereiforschungsstelle (Catch statistics; commercial fisheries Upper Lake Constance)

Developments in freshwater fisheries still uncertain

So far, the impacts of climate change are still playing a secondary role to other factors impacting on freshwater fisheries. As far as catch results of lake and river fisheries are concerned, these are subject to the general conditions governing fishing operations and cost-covering marketing opportunities as well as the availability of selected fish species of commercial interest to fisheries. This is why, rather than focussing on the potential impacts of climate change, the discussion is much more intense on conflicts arising from increasing tourist exploitation of lakes and rivers, losses of fish to hydropower plants, restrictions on fisheries from conservation-based constraints or changes in the nutrient contents of water. Protracted periods of drought, arising from advancing climate change, pose increasing and clearly visible threats to populations of mussels, crabs and small fish species which occur in small and minute lakes and rivers. The situation is similar in aquaculture, although in this case, the most important influencing factors impacting on production are water temperatures affected by climate change, the duration of ice cover on lakes in winter and water flow rates. Basically, fishermen tend to be more worried about the distribution of fish diseases and the presence of cormorants which has increased drastically in the course of the past two decades.

Generally speaking, decentralised production structures and small-scale enterprises predominate in freshwater fisheries. This explains why there is a dearth of nationwide data which would facilitate a systematic identification of climate-related changes regarding shifts in species composition of the fish fauna, not just in lakes and watercourses but also in aquaculture.

As for the future, experts do not rule out the possibility that climate change will have increasing impacts on fish stocks, income sources and proceeds in freshwater fisheries. For example, thermophilic species, distributed by shipping activities in canals, will have better opportunities to become established as water temperatures rise. Thermophilic species such as carp might benefit in terms of competing for habitats, whereas brown trout and other species which can exist only where temperatures are low, are likely to suffer restrictions to their habitats.

Using the example of Lake Constance for which longterm catch statistics exist from commercial fisheries, it is possible to demonstrate that particularly warm years can entail changes in the fish fauna. Upper Lake Constance and to some extent also Lower Lake Constance have in recent years become nutrient-poor again as a result of water pollution control measures. The phosphorus content of Lake Constance, which in the late 1970s and in the early 1980s amounted to more than 80 mg per cubic metre of water is now settling at around 6-8 mg. It is not to be expected that sizeable quantities of carp would exist in such lakes.

Therefore, the surprisingly strong presence of carp in 2003 is obviously due to particularly warm conditions in the summer of that year. Especially in Lake Constance it is rare to have early and prolonged warming of the lake water at the time of carp spawning and subsequent development of carp larvae. In most years, a warm period in early summer is followed by a cooler phase associated with the lake water cooling. Such conditions are not conducive to the emergence of young carp. As a result of those favourable conditions prevailing in 2003, subsequent years saw the highest carp yields ever recorded since the compilation of statistics on commercial fisheries in Lake Constance began. Between 1970 and 2003, carp catch results oscillated around 800 kg per annum, while in 2007 more than 17,000 kg were caught. After 2009, the yields have settled around a clearly higher level of approximately 4,000 kg, while the yield level rose again after the warm years of 2014 and 2015. As far as longerterm impacts are concerned, especially after the warm year of 2018, it is not possible to make any assertions at this point in time.



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