Coded Wire Tagging


The binary coded wire tag (CWT) is a tiny length of magnetized stainless steel wire usually 1.1 mm long x 0.25 mm diam. For very small animals tags half this length are used, and for larger specimens tags half again or double length may be utilized. These tags are marked with rows of factory-etched notches denoting specific batch or individual codes. Single tags are cut from rolls of wire by a device/injector that hypodermically implants them into suitable tissue. Salmonid fishes are usually tagged in the snout, but "cheek" muscle and certain other tissue offers superior sites for many other species.

Batch codes, identifying a group of animals sharing one or more characteristics, are most commonly used for fisheries management purposes. In this case, the same code is repeated over a specific length of wire. Sequential CWTs are designed to identify individuals or small groups. The use of these sequential tags is somewhat more complex in requiring the "filing" of CWTs that bracket the implanted tag or tags.

CWTs are injected into tissue using automatic tag injectors (Figure 1), capable of implanting up to 1000 tags/hour; hand operated injectors achieving rates of several hundred tags per hour; or by using a single shot "muzzle loaded" syringe (useful for only small numbers of specimens and requiring pre-cut/magnetized tags).

Due to their small size, and usual position in opaque tissue, CWTs must be located and recovered using specialized magnetic detectors that come in various forms (e.g., Wand, tunnels, and desk top/Portable detectors). Once excised, CWTs are visually decoded under magnification of about 10x. CWTs are normally dissected from dead/harvested fish (see Data Recovery), but the method offers considerable opportunity to recover data from living specimens as well:

Simply knowing a fish is tagged, as determined by the CWT detector, can provide a simple "yes or no" answer (e.g., identifying hatchery origin fish in a mixed population, etc.).

Specific location (e.g., left cheek, right cheek, nape, origin of dorsal fin, etc.) of the implanted CWT, as determined by a Wand Detector, can provide a number - increasing with animal size - of different "codes" (Tipping and Heinricher, 1993).

Shallow, and often visible implants of CWTs, may be excised without significant damage to the animal (Oven and Blankenship 1993).

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" CWTs (Champigneulle, et al. 1987). 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.

Advantages of CWT system:

Limitations of the CWT system:

A list of selected references, and other information, relating to use of the CWT system in various kinds of animals.


Northwest Marine Technology, Inc
P.O. Box 427, Ben Nevis Loop Road, Shaw Island, Washington 98286
Telephone (360) 468-3375 Fax (360) 468-3844