Components Analysis of Instream Escapement Methods for Pacific Salmon (Oncorhynchus spp.)

Escapement counts of Pacific salmon (Oncorhynchus spp.) have played a critical role in the development of fisheries science. We describe the components of instream escapement counts which are (1) an estimate of the number of fish in the stream at specific times, (2) a method for interpolation between visits to estimate total number of fish present at any time and (3) an instream residence time parameter to determine total number of fish that spawned for the entire run. Estimation of number of fish present at any time depends on species, observation method, stream habitat type, and observer experience. Our generalized model accounts for all of these factors in a biologically meaningful manner. Data were collected by five methods for three species of salmon which were counted past a weir; however, the number of fish present was unknown to observers. Counts for walking found, on average, 20% of fish present, for rafting 43%, for swimming 63%, for fixed wing 85%, and for helicopter 100%. The reliability of different methods as an index of abundance was similar, despite variation in the proportion of the fish observed by each method. Subsampling of specific stream sections provided a good index of abundance.