• boulders;
  • habitat heterogeneity;
  • velocity refuge;
  • GIS;
  • viewshed analysis


Few studies investigate the behavioural response of organisms to stream enhancement schemes. One behavioural process that is rarely examined in enhancement studies is the visual isolation created by adding boulders on the river bed. The objective of this research is to use a Geographic Information Systems (GIS) viewshed analysis to determine if the increase in density of Atlantic salmon observed in boulder-added zones is related to visual isolation or, alternatively, the presence of a velocity refuge. Eight study sites were established on Catamaran Brook and Little Southwest Miramichi River (New Brunswick, Canada). Each reach was divided into three quadrats of 3 m × 2 m. In one of the quadrats, 36 boulders (D50 = 0.20 m) were added to increase visual isolation. Boulders were removed from another adjacent quadrat whereas a third quadrat was left in a natural state. A detailed Digital Elevation Model (DEM) was created for all the sites with a total station. Atlantic salmon were observed by snorkelling on several occasions during two summers. Their position was recorded with the total station and the snout and average velocity was measured. A GIS viewshed analysis was performed to determine the visible area for each fish and to verify whether the surrounding fish were visible or not. Results suggest that the primary mechanism responsible for the observed increase in Atlantic salmon population density in the experimental quadrats is a reduction in the field of view of individuals, through an increase in habitat heterogeneity, which is consistent with the visual isolation hypothesis. There was also no change in the snout velocity of salmon among the three treatments, suggesting that the increase in density is not consistent with the velocity-refuge hypothesis. Copyright © 2007 John Wiley & Sons, Ltd.