Estimating Chinook Escapement to Medium Sized Rivers Using Combined Resistivity and Multibeam Sonar Fish Counters

Chinook Salmon (Oncorhynchus tshawytscha) have been enumerated on the Chilcotin River since the 70’s, historically by visual methods through periodic helicopter counts. A recent project attempts to validate the accuracy of this method through the use of a resistivity counter validated for efficiency by the use of imaging sonar. During the summers of 2008 and 2009, flat pad sensors were attached to a Logie 2100C resistivity counter which was set up at a site on the lower Chilcotin River near Redstone, BC. A BlueView Proviewer E, 2D high-definition imaging sonar was used to evaluate counter efficiency through visual confirmation of fish movement over the sensors.  Review of this data indicated the resistivity counter to be over 90% efficient for upstream Chinook salmon movement with few downstream migrants observed. Total escapement for 2008 and 2009 was calculated to be 2327 and 1192 fish respectively.  In comparison the estimates produced by the helicopter overflights  were 2001 in 2008 and 998 in 2009 when corrected peak counts were applied. Although there are certain variables that need to be taken into account, for example water conductivity, river discharge and species composition, this method may be duplicated in other river systems where no counts are undertaken or where validation of existing count methods is required.