Optimizing Aerial Count Frequency for the Area-under-the-Curve Method of Estimating Escapement

Abstract

The area-under-the-curve estimate of salmon escapement is computed from aerial counts of fish and estimates of the residence time of spawners. Imprecision in both of these factors leads to error in the final escapement estimate and, thus, affects its reliability as an index of escapement over time. The population of Chinook salmon Oncorhynchus tshawytscha in the Nechako River, British Columbia, was chosen for a case study, and this paper illustrates how simple simulation models can be used to explore the increases in precision about escapement estimates associated with increasing the frequency of aerial flights. Results show that confidence in escapement estimates for the Nechako River Chinook salmon population decreases as the frequency of survey flights decreases; results also show that this pattern is robust to the typically small annual differences in mean residence time in the Nechako River. Such models can help managers optimize the frequency of aerial flights by comparing the benefits of frequent flights (i.e., increased precision in escapement estimates) with the costs of those flights.