Publications

Stream restoration measures in Swiss rivers have included fish passage construction, creation of shelters and water quality enhancement. Nevertheless, the effectiveness of these actions on brown trout populations has not been elucidated through long-term studies. The initiation of stream restoration measures in the Boiron of Morges, a tributary of Lake Geneva, Switzerland, provided an opportunity to (1) determine the effectiveness of stocking, (2) assess the impact of pollution, and (3) evaluate the influence of stream restoration measures on a potamodromous brown trout (Salmo trutta) population, which was suspected to increase in population density. Electrofishing has been used to survey the population biannually since 1997. Surveys revealed that stocking was ineffective, whereas recolonisation after pollution was rapid in sections accessible by potamodromous spawners migrating from the lake, but was delayed by 2 years in sections upstream of an impassable barrier. Overall, water quality improvement and fish migration were the main restoration actions essential for enhancing brown trout population viability.

Today, salmonid management is largely based on river restoration. One of the fundamental elements is the creation of functional spawning sites. Key values of sedimentological characteristics must be defined in terms of granulometry, permeability, and oxygen concentration to ensure the best possible success of the operations. These values for the egg-to-fry survival were estimated in three countries (Slovenia, Sweden, and Switzerland) on three salmonid species (brown, marble, and sea trout) across 22 streams and 109 sites. The permeability of the substrate was contingent upon the concentration of fine sediment. The presence of fine particles (with a diameter of less than 1–15 mm) in the substrate at a proportion of 38%–47% resulted in a permeability reduction by a factor of 10. In order for permeability to be significant, the geometrical diameter (dg) of the substrate must exceed 15 mm (Fredle Index, Fi < 7). To obtain the same oxygen concentration inside the gravel as at the surface itself, permeability must exceed 5000 cm/h. In order to observe at least 80% of the oxygen concentration observed at the surface in the interstitial water, the diameter of the substrate must be at least 17.0 mm (Fi > 5.0). The interstitial oxygen concentration may decline to below 80% of the value observed at the surface when the substrate contains 30% or more of fines sediments smaller than 4 mm, 26% of fines smaller than 2 mm, and 22% of fines smaller than 1 mm. The impact of sediment on the interstitial oxygen concentration was found to be greater the finer the sediment was. A reduction in the interstitial oxygen concentration to below 7.5 mg/L (68% of saturation) resulted in a survival rate of less than 30% for eggs at the eyed stage. When the granulometry (dg) of the substrate was greater than 20 mm (Fi > 7.0), the survival of the eggs to the eyed stage was at least 60%. Therefore, it can be concluded that when restoration programs are planned, it is essential to consider these values in order to increase the success of natural spawning of the trout.