Decline of fish catch in Switzerland

Abstract.

Fish catch, especially of brown trout, has decreased by about 50% over the last 15 years in many Swiss rivers and streams. In addition, the health status of brown trout has been reported to be impaired in many streams. In order to evaluate the causes of these phenomena, the project "Fishnet" was launched in 1998. We present an overview of the concepts and approaches of this interdisciplinary nationwide project, our experience to date of identifying problems and filling data gaps, and the first results from selected sub-projects.¶ Fishnet has a planned duration of 5 years and aims to (i) document the decline in catch, abundance and impaired health status of fish populations in selected rivers, (ii) elucidate the causes of these problems, and (iii) develop measures to improve the situation. To achieve these aims, a wide variety of research projects are being undertaken. In addition to co-ordinating existing projects, Fishnet initiates and launches new projects that investigate these issues. Monitoring projects are being conducted over a geographically broad area to investigate selected aspects of the decline (e.g. the incidence and prevalence of diseases). Other studies aim at detailing the relationships between suspected causes and observed effects. Twelve hypotheses were initially formulated to identify probable causes. These include reproductive failure, fish health, habitat degradation, reduced quality of habitat, and climatic changes (such as increased water temperature and shifts in the seasonal occurrence of floods). Results of initial studies point to the differential impact of factors, depending on the specific geographic region, the fish species and other factors. For example, most of the hypothesized causes are currently found in the Swiss midlands.¶ By integrating the results and conclusions of existing and new sub-projects, solutions for the complex problem of fish decline and impaired health will be proposed. Practical measures to address the problem will be developed and disseminated to the public and policy groups. The various stakeholders will also be involved during the whole project. Financial and other forms of support are provided by these different parties.