Observer Accuracy and Precision in Aerial and Foot Survey Counts of Pink Salmon in a Southeast Alaska Stream

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Within- and between-observer variability and bias in aerial and foot survey estimates of spawning abundance of pink salmon Oncorhynchus gorbuscha were determined for survey counters from Chaik Bay Creek on southern Admiralty Island, Southeast Alaska, in 1992–1994. Multiple counts of pink salmon escapement in predetermined sections of the creek were made. At the same time, a mark–recapture experiment was performed to estimate the actual number of pink salmon and chum salmon O. keta. Observer counting abilities were also examined by using computer images that simulated aerially observed fish; four object types aggregated at predetermined numbers were tested. Observers tended to undercount the actual number of salmon present, and accuracy decreased nonlinearly with an increase in actual salmon escapements. On average, observers counted between 25% and 68% of the salmon present. Correction factors determined for each observer can be used to reduce bias in estimates of total abundance and make counts comparable between observers. Comparison of relative biases of observers making aerial surveys in both 1993 and 1994, years of nearly equal pink salmon abundance, suggested that learning occurred during the study.