Salmon Escapement Monitoring in the Secesh River, Idaho, Using Dual Frequency Identification Sonar: an Endangered Species Act Application

Abstract:  Dual Frequency Identification Sonar (DIDSON) was used in the Secesh River to monitor adult escapement of a natural origin (wild) spring/summer Chinook salmon population.  DIDSON technology provided a passive means to enumerate adults that avoided incidental trapping and handling mortality of a threatened species.  High frequency DIDSON sonar files sampling the entire water column were continuously collected in the Secesh River during the salmon migration period.  The convolved samples over threshold (CSOT) motion detection algorithm reduced original file size by 53.5% to 89.4% prior to file reading, supporting a substantial decrease in reviewer effort.  Estimates of file reader error and file processing (CSOT) error were used to adjust raw observations to obtain unbiased estimates of escapement with associated uncertainty.  Estimated salmon escapement at the Secesh River DIDSON monitoring site ranged from 223 fish to 1,139 fish, and the associated coefficient of variation ranged from 1% to 10.8%.  Underwater optical cameras were used as the independent method for validation (species identification) of DIDSON target counts.  Of 2,638 optical camera recorded salmon passages that were observed in the validation zone, five of the corresponding DIDSON targets were incorrectly recorded as salmon during DIDSON file review (5/2638 = 0.19% positive error).  In this application, DIDSON proved to be a reliable and minimally intrusive means to generate unbiased and relatively precise estimates of adult escapement.  Estimates of natural origin salmon spawner abundance were derived and used to calculate the 10 year geomean abundance for population viability analysis.