An Evaluation of the Use of Resistivity Counters in the Touchet River in Washington and the Imnaha River in Oregon

The Nez Perce Tribe Department of Fisheries Resources Management (NPT DFRM) and Washington Department of Fish and Wildlife (WDFW) operated resistivity fish counters on the Imnaha River from 2008-2010 and Touchet River from 2004-2008; respectively, with the primary purpose of counting adult steelhead.  Generally, the counters worked as advertised and recorded fish passage, both up and downstream.  However, using video for counter validation indicated multiple fish species, besides steelhead, were using the counter ramp.  It was difficult to determine fish species from the resistivity counter output data without video confirmation.  During clear stream conditions, video generally resolved the fish species issue, but turbid stream flows were common during the spring months, leaving many fish unidentified.  Further, a majority of the fish moved over the counter ramp at night, limiting the video capability to identify species.  As such on the Touchet, about 30-40% of the fish were unidentifiable on an annual basis.  Unidentified fish were eventually assigned a species based on trap records during the same time period.   On the Touchet, over the four years, estimated fish numbers were: 126 summer steelhead, 22 spring Chinook, 30 bull trout, 9 brown trout, 2 whitefish and 7 bridgelip suckers passed upstream over the counter ramp.  Due to the inability to positively identify all fish, WDFW has discontinued using the resistivity counter on the Touchet River.  In the Imhaha River, data from one year of resistivity counter data and several years of single pass redd counts in Camp Creek suggest a return of adult steelhead estimated to be greater than 230 adults per year.  After multiple years attempting adult spawner estimation in the Imnaha, with only one successful year to date, NPT DFRM has yet to make a decision about continuation of this technology.