The effects of low summer flow on wild salmon ( Salmo salar), trout ( Salmo trutta) and grayling ( Thymallus thymallus) in a small stream

• Published in: Freshwater Biology
• Main Authors: RILEY, W. D., MAXWELL, D. L., PAWSON, M. G., IVES, M. J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2009
• Subjects: Salmo salar
• Online Access: http://dx.doi.org/10.1111/j.1365-2427.2009.02268.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1365-2427.2009.02268.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2427.2009.02268.x

Summary 1. The effects of an experimentally imposed low summer flow on habitat use, displacement and survival of wild populations of juvenile salmon ( Salmo salar ), trout ( Salmo trutta ) and grayling ( Thymallus thymallus ) were investigated in a chalk stream. The habitat use and mobility of the fish in response to reduced flow was determined using passive integrated transponder‐tag detector systems. 2. Habitat use was compared to that available under different flow regimes. These consisted of an initial control phase of normal summer flow, an abrupt step change to 21 days of low flow, followed by a second control phase when normal summer flow was reinstated. First year (0+) salmon showed little change in their preferred substratum during low flow, whilst 1+ salmon increased their use of gravel and reduced that of mud during the day. Both 0+ and 1+ salmon were found in relatively deep water by day under low flow, whilst 1+ salmon preferred relatively shallow water at night. First year trout increased their use of gravel and reduced their use of mud and submerged tree roots under low flow, using relatively deep and fast flowing water by day. Older trout increased their use of gravel and occupied relatively deep, slow flowing water by day and relatively fast and shallow water at night. Grayling showed little change in their preferred substratum, but occupied relatively shallow water following the introduction of low flow. 3. The range of movement of juvenile salmon increased at night under low flow, and was greater by day under normal flow. There was also an initial increase in the range of movement of 0+ trout following the introduction of low flow. Older trout initially moved less under low flow. With reduced flow, the range of movement by grayling increased significantly during the day. 4. There was no net downstream displacement of any species with reduced flow, but the mortality rate in 0+ salmon, trout and grayling increased. This may be related to their small size and increased vulnerability to predation ...