Stream Restoration for the Spawning of Trout: How to Enhance the Egg to Fry Survival and to Monitor the Spawning Efficiency?

ABSTRACT 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 ( d g ) of the substrate must exceed 15 mm (Fredle Index, F i < 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 ( F i > 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 ( d g ) of the substrate was greater than 20 mm ( F i > 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.