FI-I-3: Occurrence of thermophilic species in inland waters – case study | Umweltbundesamt

FI-I-3: 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 carp as a thermophilic species. 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.

Developments in freshwater fisheries still uncertain

So far, the impacts of climate change are still playing a secondary role compared to other factors impacting freshwater fisheries including aquaculture and pond farming. As far as catch yields in 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 which are of commercial interest to fisheries. This is why, rather than focusing on the potential impacts of global warming, the discussion is much more intense regarding 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 bodies. Protracted periods of drought – arising from progressive 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 production are water temperatures affected by climate change, the duration of ice cover on lakes in winter and water flow rates. Basically, fishermen/–women as well as aquaculture operators tend to be more worried about the proliferation of fish diseases and the presence of cormorants which has increased drastically in the course of the past two decades. Nevertheless, there is an increasing number of research projects looking into the impacts of climate change on the fish fauna in Germany’s inland waters.

Generally speaking, decentralised production structures and small-scale enterprises predominate in freshwater fisheries including aquaculture. 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 respect of changes in aquaculture.

As for the future, experts foresee however, that climate change will have increasing influence on fish stocks, yield conditions and proceeds in freshwater fisheries⁸⁸. For example, thermophilic species, distributed by shipping activities in canals via ballast water, 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 when temperatures rise⁸⁹.

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. The upper and to some extent also the lower parts of Lake Constance have in recent years become, and continue to become, nutrient- poor 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 usually 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 spring and summer of that year. In order to ensure successful reproduction, carp are in particular need of warm spring weather every year⁹⁰. 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 favourable conditions caused by higher temperatures 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, catch yields for carp oscillated around 800 kg per annum, while in 2007 more than 17,000 kg were caught. Post-2009 catch yields have settled around a distinctly higher level of approximately 4,000 kg annually. By contrast, the warm summers of 2014 and 2015 as well as 2018 and 2019 are reflected in a brief temporary increase in catch statistics. Nevertheless, the steep increase in carp yields experienced at the end of the record summer of 2003 did not materialise again. This may be attributable to various causes. On one hand, carp yield is an indicator which is not just influenced by the availability of this species in Lake Constance. The demand for carp is an equally strong influence that decides whether fishermen / -women target this species of fish. Besides, the number of professional fishermen / -women of Lake Constance has been regressive for years. In 2002, 152 fishing licences⁹¹ were issued, whereas in 2021 the number had dwindled to 66 deep-sea and 17 nearshore licences⁹². for fishing with set nets and pots. The carp yield achieved by professional fishing, as illustrated in the indicator, has been falling in parallel with the number of active fishermen / -women. Furthermore, there are other factors determining the survival chances of carp hatching in warm summers and consequently the carp yield in future years, such as the weather pattern in a subsequent winter and the number of cormorants – a natural predator of carp. Nevertheless, carp is considered a beneficiary of climate change.

^{88 - Fritsch U., Zebisch M., Voß M., Linsenmeier M., Kahlenborn W., Porst, Hölscher L., Wolff A., Hardner U., Schwartz K., Wolf M., Schmuck A., Schönthaler K., Nilson E., Fischer H., Fleischer C. 2021: Klimawirkungsund Risikoanalyse 2021 für Deutschland – Teilbericht 3: Risiken und Anpassung im Cluster Wasser. Umweltbundesamt (Hg.). Climate Change 22/2021, Dessau-Roßlau, 277 pp. https://www.umweltbundesamt.de/sites/default/files/medien/5750/publikationen/2021-06-10_cc_22-2021_kwra2021_wasser.pdf.}

^{89 - Basen T., Chucholl C., Brinker A. 2022: Auf schmalem Grad. Die Zukunft unserer Fische in der Klimakrise. Analysen, Vorhersagen, Handlungsmöglichkeiten. Stuttgart. Ministerium für Ernährung, Ländlichen Raum und Verbraucherschutz Baden-Württemberg. 120 pp.}

^{90 - Basen et al. 2022, cf. endnote no. 89.}

^{91 - Landtag von Baden-Württemberg (Hg.) 2013: Antrag der Abg. Reinhold Pix u. a. GRÜNE und Stellungnahme des Ministeriums für Ländlichen Raum und Verbraucherschutz. Gewässerschutz, Fischerei und Tourismus am Bodensee. https://www.landtag-bw.de/files/live/sites/LTBW/files/dokumente/WP15/Drucksachen/3000/15_3737_D.pdf.}

^{92 - Steiner P. 2022: Die Fischerei im Bodensee-Obersee im Jahr 2021. Gesamtbericht. Bundesamt für Umwelt (Hg.). Bericht zur IBKF, Band 2022. https://www.ibkf.org/wp-content/uploads/2022/06/IBKF-2022_Gesamtbericht.pdf.}