Fish restocking does not save the biodiversity of Swiss waters

The systematic introduction of fish into Swiss lakes and waterways must be abandoned as soon as possible: the latest FOEN report says so

Fish restocking: a balance sheet
The point on the effectiveness of 40 years of fish restocking in the waterways and lakes of Switzerland (Photo: Envato)

THEFederal Office of the Environment of Swiss Confederation has just published the summary of the checks on the effectiveness of the strategies fish restocking in the basins of Switzerland. The report, which analyzes data from 40 years of practice, reaches a very clear conclusion: restocking not only does not provide real support to wild fish stocks, but at best it simply increases fishermen's catches.

Rather, we need to restore the habitat functionality, and do it as soon as possible: despite its impressive wealth, the biodiversity of Swiss fresh waters is in serious trouble, and introducing young trout fry, explain the authors of the survey, is not enough to support wild populations.

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Fish restocking in Switzerland: it is not effective for trout
The introduction of trout fry into Swiss basins is not effective in supporting wild populations: the results of the FOEN report (Photo: Envato)

The biodiversity of Swiss waters, a heritage in danger

La Switzerland is one of the European regions with the greatest diversity of freshwater fish: despite constituting only 0,4 percent of the continent's land surface, it hosts almost 20 percent of the known fish species in Europe. “Projet Lac”, created from 2010 to 2020 bySwiss Federal Institute of Water Science and Technology (EAWA extension) and the University of Bern, identified in the Swiss basins 106 species of fish. In total, the fish species that populate the Old Continent are 550.

The rich biodiversity of Swiss waters, however seriously threatened: over the last 150 years, explains the FOEN, a large part of the streams, rivers and lakes have been canalised and "90 percent of wetlands, lakes and ponds have been drained”. At present, 71 percent of the types of aquatic environments found in Switzerland are listed in the Red Lists drawn up by the Institute.

The deterioration of the aquatic environment is primarily due totemperature increase, to the use of pesticides in agriculture and the numerous works of hydraulic arrangement scattered along the course of rivers and streams. The most seriously threatened species are crayfish, aquatic plants, amphibians, fish and cyclostomes: 9 species of fish are already considered extinct and 58 percent of them are on the Red List. In 2020, the Federal Council was forced to increase the level of protection of 25 species, including the eel, the Doubs trout and the grayling.

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Fish restocking in Swiss lakes and rivers
Lake Blausee, in the heart of the Bernese Alps: the FOEN report mostly concerns watercourses: only 3 percent of the data analyzed concerns lake habitats (Photo: Envato)

Fish restocking in Switzerland, a controversial tradition

To support the wild fish populations in Switzerland, the Cantons have been carrying out artificial fish restocking, in some cases for more than a century. As stated in the latest FOEN report, “even today this activity constitutes an important part of the fisheries management work carried out by the state services with the help of numerous voluntary fishing companies”. However, the Institute underlines, the fish restocking remains a controversial topic: it is a very expensive procedure and there are still several doubts about its real effectiveness.

Already in 2002 the then Federal Office for the Environment, Forests and Landscape (SAEFL) had published a report on the effectiveness of fish restocking in Switzerland which did not leave much margin: according to the efficacy tests carried out from the 90s onwards, the success of fish releases gave very irregular results.

In particular, the FOEN recalls today, “the more functional natural reproduction was and the older the restocking fish, the less effective the restocking was”. The 2002 report therefore suggested giving priority to repopulation with fry in waters where natural recruitment is non-existent or deficient.

Today the FOEN has updated that report from twenty years ago, summarizing the new results available in light of increasingly systematic and precise tests thanks to new knowledge on genetics and conservation.

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A pike swims in the waters of a lake
The fish repopulation in Switzerland has involved various species in addition to trout: among these pike and Arctic char (Photo: Envato)

The results of four decades of fish restocking in Switzerland

The latest FOEN report summarizes the data relating to 400 test of monitoring the effectiveness of fish restocking carried out in Switzerland in the last 40 years, of which 388 in watercourses and 12 in lake environments. L'81 percent of the tests relate to rivers, 7 percent to streams.

The tests on trout represent 88 percent of the total, and highlight the doubts of technicians and scientists about the real effectiveness of repopulation"The analysis of the tests carried out”, the report reads, “shows that while the proportion of stocked trout in the stream population may be very high immediately after introduction, it decreases dramatically later, as individuals become older”. Fish populations that reach adulthood, in essence, still come from the natural recruitment.

The data also suggests that fish from the restocking, “once you reach adulthood or a catchable size, they tend to replace each other rather than adding to wild populations".

The results of tests on populations of grayling (Thymallus thymallus), salmon (Salmo Salar) e pike (Esox Lucius) are of the same type: for none of these species was it possible to detect a significant number of adult specimens from repopulation.

In the lakes the situation is somewhat different. In some cases stocking fish outnumber them 90 percent of the total catches even years after its introduction, as happens with i whitefish in Lake Hallwil in the canton of Aargau and for the Arctic char (Salvelinus Umbla) in Lake Geneva, but the data on trout and pike confirm the picture drawn for the watercourses.

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Fish restocking in Swiss lakes and rivers
The Scuol stream, in Switzerland: according to data from the Federal Environment Office, only the reintroduction of disappeared species justifies the release of fish into waterways (Photo: Envato)

Is the repopulation of aquatic fauna really effective?

Forty years of checks on the effectiveness of fish restocking, we read in the conclusions of the document, “reveal that in waterways no example of repopulation with juveniles (0+ and 1+) has made it possible to support wild populations of trout or grayling capable of reproducing naturally”. Only the reintroduction of disappeared species justifies an introduction of fish”if, and only if, the problems that caused the extinction are solved and if natural recolonization is not possible".

As far as the lake habitats, although it is not yet possible to draw definitive conclusions, the results suggest that repopulation is capable of supporting the professional fishing of some species in bodies of water with water quality severely compromised, while providing dubious benefits in functional environments.

Various scientific research underlines that the potential benefits of repopulation, from the point of view of protection of biodiversity, are even lower thanimpact on wild populations, which may suffer from the transmission of diseases or the effects of "genetic pollution" caused by the introduction of farmed fish.

"The functional restoration of the habitat is the only action that lastingly improves the situation of aquatic fauna”, concludes the FOEN, “accordingly, and given the risks it poses to biodiversity, ideally any repopulation action should be stopped”, unless you are working on the reintroduction of a vanished species.

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Fish restocking: a practice to be abandoned: for the FOEN in Switzerland
Fish restocking is a practice that should be abandoned: for the FOEN in Switzerland "the functional restoration of the habitat is the only action that lastingly improves the situation of aquatic fauna" (Photo: Envato)