Stream bed mobility and salmon spawning

Posted on September 22, 2010

The management and restoration of natural stream communities are often compromised by limited understanding of the connections between stream hydraulics, habitat features and local aquatic populations.  In the case of salmonid fish, the flow regimes and bottom substrates of particular locations used by spawners can be identified, but such detailed analysis is expensive in terms of the data required, and there is a need to develop indicators of spawning success at broader pool-riffle scales.   Working with Atlantic salmon in an headwater tributary of the River Tay, Scottish researchers related the frequency of spawning events observed in individual riffle/pool units to channel-forming parameters (stream power and shear stress).  Since the hydraulic measurements were made at the riffle/pool scale when the stream was flowing at bank-full height, they didn’t describe the microscale conditions of particular spawning habitats.  The best predictor of spawning frequency was excess shear stress (i.e., the degree of stream sediment mobility in excess of that used by spawning fish).  This measure was inversely related to spawning frequency and explained 71-90% of the variation in the data, depending on the section of stream examined.  Sediment mobility seems to be important through its impact on the availability of suitable spawning gravels and its relationship with scouring processes, which have negative effects on incubating salmon eggs.  This indicator of spawning success can be used to set ecological targets and assess the impacts of natural and human disturbance on salmon populations.

Reference:  Moir, H.J., Gibbins, C.N., Buffington, J.M., Webb, J.H., Soulsby, C. & Brewer, M.J.  2009.  A new method to identify the fluvial regimes used by spawning salmonids.  Canadian Journal of Fisheries and Aquatic Sciences 66, 1404–1408.