These adult fallback adjustment rates are designed as modifiers of the US Army Corps' visual counts in order to provide an estimate of total adult passage abundance.
The adult fallback adjustment (AFA) is expressed as the ratio of the number of unique PIT-tagged fish (NPIT) passing over the ladder(s) divided by the total number of their ascents, i.e.,
The annual values AFA by fish stock and hydroproject are reported at this website.
The can also be used to express other quantities associated with adult fallback. For example, the reciprocal of is an estimate of the average number of ladder ascents per fish, i.e.,
An estimate of the average number of fallback events per fish can be calculated as
This algorithm to determine ascension counts is based on radiotelemetry research at the Columbia dams performed by Brian Burke, Northwest Fisheries Science Center, NOAA, 2003-2004.
The following algorithm is used to determine a successful ascension at a dam: