DART PIT Tag Adult Fallback Adjustment Rates

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  • The programming logic to determine a single passage event is currently being reviewed and updated.
  • The topic of fallback metrics is being evaluated, and we are seeking comments and alternative algorithms to implement on the site. Please submit your comments and ideas to web@cbr.washington.edu. Thank you.

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.,

NPIT = number of unique PIT-tagged fish detected at ladder(s),
ai = number of ascents made by the ith PIT-tagged fish (i = 1, ..., NPIT).

A seasonal ladder count (C) times the AFA provides an estimate of total adult passage abundance (N), 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:

  1. PIT-tag coil detections are sorted in chronological order for each tag code.
  2. Consecutive detections spaced farther than 6 hours apart are determined to be separate attempts of ascension.
  3. For each attempt, if the last two detections are in an upriver direction (i.e., the last coil the tag was detected on was upriver of the next unique previous coil), an ascension is determined to be successful. Same coil detections are ignored for this determination.
  4. Single coil detections are ignored.
  5. Once a fish has been determined to have successfully ascended at a ladder, any following successful ascensions are categorized as re-ascents.

Additional Notes:

  1. The estimates given for the BO4 site at Bonneville Dam are a combined BO2/BO3/BO4 system estimate. Since all fish detected at Cascades Island ladder (BO2) and Washington Shore ladder (BO3) must exit through the Washington Shore Vertical Slots detectors (BO4) and since nearly 100% of the fish passing BO4 are detected the estimate for BO4 can be taken as representative for the sytem as a whole. This does not apply to the 2004 migration year since the BO4 system did not go online until Feb. 2005.
  2. Fish PIT-tagged and released in the same year as detection are excluded from these analyses.
  3. On October 14, 2010, Tag "3D9.1C2C8E2CCB" detections were excluded from 2010 Summer Sockeye analysis at Lower Granite Dam Adult Ladder due to the uncharacteristic and repetitive nature of detections over the period: 15 July 2010 - 7 September 2010. View the full coil history for 3D9.1C2C8E2CCB at Lower Granite Dam Adult Ladder in 2010.