This study was designed to define the microhabitats selected in summer by juvenile Atlantic salmon (Salmo salar). Curves were developed describing the preference of 880 young salmon for water velocity at the fish's position (nose velocity), mean water column velocity, total water depth, and stream substrate size. Study sites were chosen in six morphologically diverse streams in Nova Scotia and New Brunswick during 1982–84. Of the four variables measured, only nose velocity chosen by both fry and parr was not significantly different among years or rivers. Atlantic salmon fry (< 65 mm) most frequently selected nose velocities between 5 and 15 cm∙s−1, small parr (65–100 mm) between 5 and 25 cm∙s−1 and large parr (> 100 mm) between 5 and 35 cm∙s−1. Apparently, juvenile salmon utilized water depths and stream substrates which varied within tolerable limits according to their availability in conjunction with preferred water velocities. Significant differences in the body shape and size of the pectoral fin of Atlantic salmon parr in different rivers did not influence the selection of nose velocities within the range of flow conditions sampled.

Water velocity is an important component in aquatic studies but is subject to alteration by the presence of an underwater observer. Tests using two types of water velocity meter and a snorkeler showed the effects to be a function of the diver's distance from the probe. Lateral positions generally caused an acceleration of flow at the measurement point of up to 150%, while downstream positions resulted in a decrease in flow of up to 65%. It is recommended that where underwater observers are employed, they position instruments that can be read remotely and then move well away from the point of measurement.