Trout catches in Switzerland have fallen by 60% since 1980. At the same time, health problems were identified for fish in different streams. These findings were viewed by the Federal Institute for Environmental Science and Technology (EAWAG) and the Agency for the Environment, Forests and Landscape (SAEFL) as sufficient grounds for launching the „Fischnetz“ (fish net) project in 1998. Its objectives were to document the health status and the decline in catch, to track down their reasons and to suggest measures for correction. All 26 cantons, the Principality of Liechtenstein, the Swiss Fisheries Association (SFV), the Swiss Society of Chemical Industries (SGCI) and the University of Bern joined the project and around CHF 3 million were invested.
The investigations conducted in 77 sub-projects were based on a dozen separate hypotheses. Was the fact that fewer fish were being caught attributable to a decline in angling activity? Are cormorants and goosanders devouring all the fish? Are the fishes' habitats progressively disappearing? Are the fish being poisoned with chemicals? Has the supply of food declined or is climate warming to blame? The „Fischnetz“ project has looked into all of these possible causes.
The catch statistics clearly show that fewer anglers are buying permits, which has reduced the angling effort. This does not, however, fully explain the decline in total catch. A more detailed analysis of catch statistics reveals that the fish stocks have also declined. The project leadership concludes that the principal reasons for the decline in stocks are poor quality of habitat and the proliferative kidney disease PKD. Poor habitat quality includes deficiencies in both morphology (for example missing shelter in constructed stream courses or uniform river banks) and water quality (chemical pollution).
In some cases, the straightening of rivers, the artificial reinforcement of riverbanks and the destruction of riparian vegetation took place decades ago. And yet these measures still have consequences (in the form of monotonous and fragmented habitats, for example) today. They hinder the efforts of fish to escape adverse conditions and block their access to spawning grounds. The isolation of habitats also restricts the genetic diversity of populations.
The pollution of the watercourses by chemicals has decreased markedly over the past 30 years. Nevertheless, after heavy rain the concentrations of nitrogen compounds such as nitrite and ammonia reach peaks which are dangerous for aquatic wildlife. Pesticide inputs are still too high in intensively cultivated areas of the Central Plateau. Furthermore, effect concentrations of natural and synthetic hormones are attained in densely populated zones of the Central Plateau. Probably, fish health is impaired by the combined effects of these substances („chemi-cocktail“).
Proliferative kidney disease (PKD) – first identified in Switzerland in 1979 – has been intensively investigated in the course of general research into fish health. In 2000 and 2001, this infectious disease was detected at 190 out of 462 locations investigated – most notably in the waters of the Swiss Central Plateau. PKD leads to swollen kidneys and is often fatal. Catch figures are lower in PKD positive river stretches than in waters without PKD. PKD therefore might be one of the main factors having led to the observed decline in catch.
The decline in fish stocks is greatly influenced by the combined impact of the various contributory factors. Examples of this are the temperature-dependent outbreak of PKD, which becomes manifest only when the water temperature remains above 15°C for longer than two weeks. Between 1978 and 2002, the temperatures in Switzerland's watercourses rose by around 1°C. Apart from promoting the spread of PKD, the rise in temperature also leads to a reduction in the availability of habitats suitable for trout, since the waters of the Central Plateau are becoming too warm for this cold-water adapted species.
It is important to note that the relative importance of the causes varies from one water body to another. Measures must, therefore, be tailored suit the local conditions. First and foremost it is necessary to improve the habitats. Connectivity must be enhanced along the entire longitudinal course of a running water, riverbank vegetation is to be promoted, and steps must be taken to ensure that water levels are always sufficient. Quality standards need to be defined and met for all relevant substances. Enforcement and monitoring of the Water Protection Law must be more consistent. Furthermore, water-body management must be optimized. Fish from PKD-infected waters must not be released into PKD-free or non-inspected waters. Restocking must only be undertaken in a planned fashion. Systematic surveillance of fish stocks is also necessary so that long-term development and the effectiveness of measures can be monitored.
Follow-up project and advisory service
The measures necessitate more comprehensive information, training and support. „Fischnetz“ will therefore support the cantons and fisheries organizations in their implementation of the measures by means of the follow-up project "Optimisation of fish catch yields and water quality". As of April 2004, the anglers will also be able to call upon the angling consultancy, FIBER, which will be jointly funded by EAWAG, SAEFL and the SFV.