Abstract:Siltation of spawning gravels in upland rivers appears to be an increasing hindrance to salmonids' spawning success. River managers seek an eective and non-labour intensive means of loosening gravel and reducing ®ne material, so improving spawning success; this study compared three practical gravel cleaning techniques, applied at realistic (rather than intensive) levels, by assessing survival to hatching of buried brown trout, Salmo trutta L., ova at ®ve sites on four rivers with gravel substrate in southern England. Each site consisted of six reaches, of which three were cleaned by tractor rotovating, high pressure jet washing and pump washing; these were compared with adjacent, untreated reaches. Brown trout ova were buried in both ®ne mesh and coarse mesh boxes in each reach.Signi®cant improvements (at P 5 0Á05) in survival (number of live alevins) were found in three of the ®ve pump washed reaches, two of the ®ve tractor rotovated reaches and one pressure washed reach when the data were analysed by site. When data from all ®ve sites were analysed together, all treated reaches showed a signi®cant improvement (at P 5 0Á05) in egg survival to hatching compared with control reaches for ®ne mesh egg boxes; for coarse mesh boxes only pump washed reaches showed such an improvement.We feel that pump-washing provides the most eective, inexpensive and suitably non labour-intensive means of improving gravel, although ultimately it may be better to reduce the silt load of rivers. Freeze core bed samples taken before and immediately after cleaning were analysed for silt content; pump washing and high pressure washing may have reduced the amount of ®ne material.
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