Northwest Fisheries Science Center

Display All Information

Document Type: Contract Report
Center: NWFSC
Document ID: 6535
Title: Evaluation of a full-flow PIT-tag interrogation system at McNary Dam, 2002
Author/Editor: Gordon A. Axel, Earl F. Prentice, Benjamin P. Sandford
Publication Year: 2003
Publisher: National Marine Fisheries Service
Contracting Agency: U.S. Army Corps of Engineers. Walla Walla, Washington
Contract Number: W68SBV10333831
Pages: 35
Date: 09/01/2003

In 2001, the National Marine Fisheries Service evaluated the technical feasibility of interrogating PIT tagged fish in large–diameter juvenile fish bypass pipes (full flow bypass system) at McNary Dam.  Objectives of this evaluation were:

  1. to develop a PIT tag interrogation system that could function year round rather than only seasonally,
  2. to provide greater flexibility in the operation of the juvenile fish facility without disrupting operation of the present juvenile interrogation systems, and
  3. to develop a PIT tag interrogation system that can be used with the existing juvenile fish bypass pipe that leads from the powerhouse to the juvenile fish collection and monitoring facility.

Based on information from the evaluation conducted in 2001, the new interrogation system was installed in 2002 and its performance evaluated.  The system exceeded the goal of 95% tag reading efficiency for four antennas.

Two methods of determining tag reading efficiency were used:  direct and indirect.  For the direct evaluation, we released a known number of tagged fish at a specific location upstream from the detectors.  That number was then compared to the number of unique tag codes read.  Using this method, over 98% reading efficiency was obtained under all test conditions.  For the indirect evaluation, we conducted a statistical comparison of tags read between antennas.  This method showed tag reading efficiency of the system ranged 99.5-100% during the season.

Comparisons of tag reading efficiency were also made between the new full flow detection system and the existing PIT tag interrogation system.  The new system consistently showed higher tag reading efficiencies than the existing system.  Differences between systems can be explained in part by the possible sudden passage of large numbers of tagged fish from the wet separator, which causing tag collision on the existing system, or by fish being bypassed to the river without being interrogated.

A contributing factor to the success of the full flow system was the use of radio frequency clamps to shape the electromagnetic field generated by the antenna.  This concept was developed in 2001 and deployed for the first time on this project.  Shaping the electromagnetic field enabled a narrow tag reading "window" to be formed, thus reducing the likelihood of more than one tag being read at any one time.  This reduced the loss of information from tag-code "collision."