A Coded Wire Tag (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 animals or improved magnetic detection, one and a half (1.6 mm) or double length (2.2 mm) may be used.
Fish, crustaceans, insects, gastropods, and many other animals have been successfully tagged with Coded Wire Tags. The small size of the tag and its benign materials means that lifelong retention with minimal effect on the host is the norm. They can be injected in very small animals and have an enormous code capacity for batch or individual identification.
Coded Wire Tags are non-transmitting (they are much too small for that), but their presence can be electronically detected. At recapture, the tag usually needs to be removed and the code read under a microscope. In some circumstances, the presence or absence of the tag in a particular body location can provide all the data that is required, or the tag can be removed without killing the host animal.
Regional Coded Wire Tag Programs
Coded Wire Tags are used extensively to manage Coho and Chinook salmon in the Pacific Northwest. Over 1 billion juvenile fish have been tagged with Coded Wire Tags since the mid-1970’s and over 5 million tags have been recovered. This program is a cooperative effort between the Canadian and US Federal governments, state governments and tribal governments. CWT data is essential for allocating Canadian and US total allowable catch, assessing compliance under the Pacific Salmon Treaty, calculating fisheries and stock specific statistics (i.e. exploitation rates, survival rates, maturation rates), monitoring marine survival, assessing fishing impacts, forecasting pre-fishery ocean stock abundances, and evaluating the effectiveness of hatchery production and experimental programs.
Similarly, managers in the Great Lakes Region of North America are using Coded Wire Tags to evaluate hatchery programs for salmonids in this very complex freshwater system.
If you are interested in learning more about these programs, the links below provide some resources.
- The Regional Mark Processing Center (RMPC) manages the Coded Wire Tag database for the US. Publications on this website include overviews of the regional program.
- The stock assessment methods used by Fisheries and Oceans Canada are described here. Fisheries and Oceans has hundreds of documents relating to the use of Coded Wire Tags on their website. Enter “Coded Wire Tags” in their search box to find them.
- Some sources describing regional sections of the Pacific Northwest Program in the US are below:
- The US Fish and Wildlife Service describes their marking and tagging program for the Great Lakes Region. Additional Field Notes provide interesting accounts of smaller portions of that program. Enter “Coded Wire Tag” in the search box to find them.