Hydraulic and sedimentary characteristics of habitat utilized by Atlantic salmon for spawning in the Girnock Burn, Scotland

The Girnock Burn, a tributary of the River Dee, has been used as a monitoring site for Atlantic salmon, Salmo salar L., populations since 1966. The burn drains a catchment of 30 km² and is a third order stream at the catchment outlet; its width ranges from 1 m in the headwaters to around 15 m in its lower reaches. During the period 1966-1996, the mean number of fish (average lengths; males = 0.64 m and females = 0.68 m) entering the burn for spawning averaged 126 (range 38-293) with a mean male:female ratio of 0.96 (range 0.58-1.66). Typically, 85% of the adults were 2-sea winter fish. Redd counts in 17 spawning seasons showed that the number of redds constructed during the spawning season ranged from 13 to 141 with a mean of 79. Redds were generally 2-3 m long, 1-1.5 m wide and 0.2-0.3 m deep. Redd distributions showed that parts of the river were used more frequently for spawning than others. This paper describes the main physical characteristics of the river that influence spawning habitat. This characterization occurs at two scales; the overall Girnock system and individual river reaches. At the large scale, bed slope exerts a strong influence on spawning distribution by controlling the distribution of spawning-calibre sediment. Additionally, river flow during the spawning period appears to have a significant influence on accessibility and the subsequent distribution of redds. At the reach scale, local hydraulic (depth, velocity and Froude number) and sedimentary (particle size distribution) variables were found to be increasingly important. Mean utilized depth, velocity and Froude number for 93 redds were 0.248 m, 0.536 m s^-1 and 0.344, respectively. Samples of utilized sediment had a mean median particle size (D₅₀) of 20.7 mm and a mean fines (particles < 1 mm diameter) content of 5.4%. It was suggested that the Froude number may be a useful single descriptor of hydraulic habitat, as it is dimensionless and is thus comparable between different sizes of river and fish species.