Evaluation of utility of aerial overflight based estimates versus mark-recapture estimates of chinook salmon escapement to the Nicola River, B.C. / R.E. Bailey … [et al.]. : Fs70-1/2000-152E-PDF

Adult chinook salmon were surveyed in the Nicola River from a helicopter and the count data was examined to determine their statistical properties. Replicated counts were compared and variances estimated for reach counts using regressions and quartile approaches. Reach counts were highly repeatable. Expansions of peak counts indexed abundance well among years, however, when using the standard expansion factor, estimates were biased low for three of four years when compared to Petersen mark-recapture estimates. AUC (area-under-the curve) estimates of spawner-days described the spawner abundance in a more robust manner than peak counts. Apparent redd residence times were stable among years. Retrospective AUC estimates, calculated using observed spawner data and the mean apparent redd residence time, adequately estimated spawner abundances for all four years. While at this time, it is not definitively known what minimum number of overflights are required to produce scientifically defensible AUC escapement estimates, it is likely that four or more flights may be required on many systems. It is recommended that similar studies be conducted in other watersheds in order to examine: 1) the repeatability of aerial counts; 2) spatial and temporal variability in survey life; and 3) degree of bias associated with aerial overflight-based estimates.

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