Coded Wire Tags (CWT) are fish tags. A Coded Wire Tag is a length of magnetized stainless steel wire 0.25 mm in diameter – about the width of a human hair. Rows of numbers on each tag identify a group of fish or an individual animal. Coded Wire Tags are produced as a spool of wire that is loaded into an injector. The injectors cuts individual tags from the spool and hypodermically implants them into the fish. The standard length of a tag is 1.1 mm. Half-length tags (0.5 mm) are used for very small fish. For tagging larger fish or improved magnetic detection, one and a half (1.6 mm) or double length (2.2 mm) tags may be used.
Along with fish tagging, 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 into very small animals and have an enormous code capacity.
Coded Wire Tags do not transmit a code (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. This is quick and easy with practice and a few special tools. Sometimes, 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
Fish managers in the Pacific Northwest rely on Coded Wire Tags to manage Coho and Chinook salmon. Over 2 billion juvenile fish have been tagged with Coded Wire Tags since the mid-1970’s and over 10 million tags have been recovered. The Canadian and US Federal governments, state governments, and tribal governments throughout the region cooperate on this program. Among other uses, the data collected 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
- 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.
More information about Regional Programs
The links below provide some resources for learning more about these programs:
- 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.
- Michigan Department of Natural Resources describes their long-standing Coded Wire Tagging program here. They have tagged over 15 million salmon with CWT since 1990 and recovered over 75,000 tags.