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Decimal Coded Wire Tags™ (CWT)

The CWT is a length of magnetized stainless steel wire 0.25 mm in diameter. The tag is marked with rows of numbers denoting specific batch or individual codes. Tags are cut from rolls of wire by an injector that hypodermically implants them into suitable tissue. The standard length of a tag is 1.1 mm. For very small animals half-length (0.5 mm) are used. For larger specimens or improved magnetic detection, one and a half  (1.6 mm) or double length (2.2 mm) may be utilized.

Salmonid fishes are usually tagged in the snout, but "cheek" muscle and certain other tissue offers superior sites for many other species.

NMT has yet to encounter a fish, of sufficient size, that cannot be tagged. Arctic char Salvelinus alpinus as small as 22 mm total length have been successfully snout-tagged with half-length CWT (Champigneulle, et al. 1987 [Abstract]). Since body muscle also provides a suitable and much larger target than the snout of a char, it appears that smaller fish and other organisms can be successfully tagged. Although designed originally for small fish, coded wire tags have been applied successfully in a number of crustacean studies.

Advantages of CWT system:

• Can be used in very small animals.
• Minimal biological impact.
• High retention rates over the life of the host.
• Enormous code capacity (batch or individual identification).
• Tags are inexpensive.
• Potential for automatic scanning of large samples.

Limitations of the CWT system:

• Capital equipment is expensive (but can be rented from NMT or borrowed from other agencies).
• In most applications, tags must be excised, usually from dead animals, for reading decimal codes.
• Tags usually not externally visible.

Contents

• CWT Tag Formats
• CWT Injectors
• CWT Detectors
• CWT Readers

Additional resources

• CWT Video
• Coded Wire Tag Project Manual -- Guidelines on planning and conducting projects using CWT and associated equipment. (PDF 554K)
• NMT Decimal Coded Wire Tag Demonstrator  (Windows Application Installer 639K)

Northwest Marine Technology, Inc.                   (360) 468 - 3375

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Last edited: 04 Feb 2008