Response of juvenile rainbow trout to turbulence produced by prismatoidal shapes

Water velocity is a common measure of habitat suitability for salmonids. It is often noted that salmonid focal positions occur in low-velocity areas adjacent to high-velocity ones. Such locations are associated with increased turbulence. Turbulence in the stream regions typically occupied by salmonids is produced by flow separation around large objects such as rocks or on the riverbed. Turbulence has also been shown to increase the energetic cost of swimming for salmonids. Despite the link between cover and turbulence, no measurements of turbulence at the focal positions of juvenile salmonids have been reported in the literature. We measured turbulence at the focal positions of juvenile rainbow trout Oncorhynchus mykiss in a laboratory flume and found that those focal positions were characterized by low turbulence levels across a range of average velocities. Fish occupied highly turbulent locations if excessive average velocities were present. Eddy size, measured as the autocorrelation function of the velocity time series at a point, did not result in obvious correlations with focal position establishment, but it was correlated with turbulence levels. Turbulence provided some additional discriminatory resolution, allowing differentiation of potential focal positions of equal average velocities but different turbulence levels. Thus, accounting for turbulence levels may allow more accurate prediction of focal positions.