An Investigation into the Factors Influencing Escapement Estimation for Chinook Salmon (Oncorhynchus tshawytscha) on the Lower Shuswap River, British Columbia

The area-under-the-curve (AUC), method based on helicopter visual enumeration, is a commonly used technique to estimate escapement of chinook salmon (Oncorhynchus tshawytscha) for the Fraser River and its tributaries. Two key factors associated with this method are survey life and observer efficiency. The survey life currently assumed by Fisheries and Oceans Canada (DFO) for Thompson Basin streams is seven days and for most counting flights the assumed observer efficiency is 100%. Telemetry and tower observations were used to estimate annual (2000-2002) survey lives for male (telemetry) and female (telemetry and tower observation) chinook salmon in the Lower Shuswap River. Mark-recapture based survey lives were directly estimated by dividing the mark-recapture escapement estimate by the AUC estimate of total spawner days. Observer efficiency was estimated by comparing observer counts from helicopters to counts from photographs. Various combinations of total fish counts, observer efficiencies, and survey lives were used in AUC escapement estimates, and compared to mark-recapture and peak count estimates. Survey lives varied annually. Many of the estimated mean survey lives were significantly less than seven days; mark-recapture based survey lives were generally less than those estimated from telemetry and observations from towers; and mean survey lives for males were higher than for females. The mean observer efficiency of helicopter enumeration was 97% (95% CI is * 6%), this is not significantly different than the 100% assumed by DFO.