Restoration of spawning habitats of brown trout (salmo trutta) in a regulated chalk stream

Gravel bed spawning grounds are essential for the reproduction of salmonids. Such spawning grounds have been severely degraded in many rivers of the world because of river regulation and erosive land use. To reduce its effects on salmonid reproduction rates, river managers have been restoring spawning grounds. However, measures of effectiveness are lacking for the restored spawning sites of brown trout (Salmo trutta). In this study, two methods were used to restore gravel bed spawning grounds in the Moosach River, a chalk stream in Southern Germany: the addition of gravel and the cleaning of colmated gravel. Seven test sites were monitored in the years 2004 to 2008, focussing on sediment conditions. Furthermore, brown trout egg survival and changes in the brown trout population structure were observed. Both gravel addition and gravel cleaning proved to be suitable for creating spawning grounds for brown trout. Brown trout reproduced successfully at all test sites. The relative number of young-of-the-year brown trout increased clearly after the restoration. Sediment on the test sites colmated during the 4years of the study. In the first 2years, highly suitable conditions were maintained, with a potential egg survival of more than 50%. Afterwards, the sites offered moderate conditions, indicating an egg survival of less than 50%. Conditions unsuitable for reproduction were expected to be reached 5 to 6years after restoration.