A coded wire tag (CWT) is an animal migration tracking device, specifically for tracking fish migration . It Consists of a small piece of magnetized wire injected into the snout or cheek of a fish so That It May be tracked for research or fisheries management .
The tags can also assist in studying the diet of birds that have eaten tagged fish.
The CWT is not visible once inside the fish; its presence is detected by means of a handheld wand or tunnel type detector that can sense the magnetized metal. A unique code is an individual fish that is exposed to the surface of the CWT, and can be removed and inspected under a microscope . Such insertion of information about the fish hatching date, release date, location, species, sex, and length are recorded along with the corresponding tag code in a database, so that the codes can be recovered. Coded wire tags have been used to research fish species from different families . 
Coded wire tags were first introduced in the 1960s as an alternative to late clipping.  The first CWTs used colored stripes and allowed about 5000 different color combinations to uniquely identify groups of tags. The next development was binary codes marked on tags by electrical machining discharge in 1971.  Binary codes allowed 250,000 unique code combinations. The sequentially coded wire tag was introduced in 1985, which allowed the tags to be sequentially increasing numbers, or codes, to be cut from the same spool of wire, so that individual or small groups of fish could be uniquely identified. 
With advancements in laser technology, it could be etched on tags, which caused the switch from binary to decimal codes. Currently, coded wire tags are marked using a decimal system , which is a row of numbers. The decimal system allows 1 million unique codes for batch tags, and 100,000 unique codes for individual sequential tags. 
Salmon fry weighing less than 1 gram can be successfully tagged,  and different sized tags are used depending on the length of the fish to be tagged. A small handheld injector is portable and can be used where small numbers of fish will be tagged. For large scale tagging projects, an automatic tag injector is used, which cuts and magnetizes the wire. In manual tagging, a juvenile fish is sneaked into a molded guide that will get you to the right place.  These automatic injectors can be integrated into a fully automated process of a mobile trailer called an AutoFish trailer, or autotrailer, where fish are mechanically sorted, adiposeend clipped, and tagged. Autotrailers do not require anesthetic or the fish to be removed from water.  When fish are tagged, their tag code and other information such as release date and location are entered into a database. In the Pacific Northwest , the Regional Mark Information System is used. 
Coded wire tags are used to track groups or individual fish for research and fishery management purposes. Commonly tagged species are coho , chinook , steelhead , chum , sockeye , and pink salmon .  In Alaska , British Columbia , Washington , Oregon , and California , the end adipose is removed from CWT tagged salmon as a visual indicator of the presence of the tag. However, in Washington the adipose end is removed from nearly all hatchery salmon in mass marking programs to distinguish them from wild fish. In particular, where marked fish hatchery are selected by fisheries rather than unmarked wild fish, some fish receive what is called a double index tag. In double index tagging, some hatchery fish are tagged with a CWT purpose but do not have their adipose end clipped. This means that the adipose end clip can not be used because it is a tag, because some fish with adipose end clips will not actually have tags, and some fish with intact adipose fins will have tags. In places where mass marking or double index tagging is practiced, an electronic detector such as a wand or tunnel must be used to determine if a tag is present the snout can be dissected to read the tag. Electronic tag detection is used in Washington, Oregon, and Idaho. Alaska and California depend on the adipose end clip to indicate the presence of a tag. 
Coded wire tags have been used to track the diet of fishers , because the fish is eaten, the tag passes through the bird’s digestive system.  CWTs have also been used in Japan. 
- Radio-frequency identification
- Acoustic tag
- Data storage tag
- Pop-up satellite archival tag
- ^ Jump up to:a b Vander Haegen, G. Blankenship, HL, Knutzen, D. McKenzie, JR Parsons, B. Seitz, AC, Kopf, RK, Mesa, Mr. Phelps, Q., 2011 . Advances in coded wire tag technology: meeting changing fish management objective , in: Advances in Fish Tagging and Marking Technology. American Fisheries Society, Symposium.
- Jump up^ Jefferts KB, Bergman PK, Fiscus HF. 1963 A coded wire identification system for macro-organisms. Nature198: 460-462
- ^ Jump up to:a b Nandor, GF, Longwill, JR Webb, DL, 2010. Overview of the coded wire tag program in the Greater Pacific Region of North America . PNAMP Special Publication: Tagging, Telemetry, and Marking Measures for Monitoring Fish Populations-A compendium of new and recent science for use in informing techniques and decision modalities: Pacific Northwest Aquatic Monitoring Partnership Special Publication 2, 5-46.
- ^ Jump up to:a b c Solomon, DJ, 2005. Coded Wire Tag Project Manual .
- Jump up^ Habicht, C., Sharr, S., Evans, D., Seeb, JE, 1998. Coded Wire Placement Tag Affects Homing Ability of Pink Salmon. Transactions of the American Fisheries Society 127, 652-657.
- Jump up^ Regional Mark Processing Center
- Jump up^ Mass Marking Fact Sheet, Washington Department of Fish & Wildlife.
- Jump up^ Hoffmann, A., Pattillo, PL, 2008.The practical application of mark-selective fisheries, American Fisheries Society Symposium. American Fisheries Society, p. 451.
- Jump up^ Review of 2008 Mass Marking and Selective Mark Fishery Proposals No. SFEC (08-2), 2008. Pacific Salmon Commission Selective Fishery Evaluation Committee.
- Jump up^ Sebring, SH, Ledgerwood, RD, Morrow, M., Sandford, BP, Evans, A., District, WW, 2012.Detection of Passive Integrated Transponder (PIT) Tags on Piscivorous Avian Colonies in the Columbia River Basin, 2011. Report of the National Marine Fisheries Service to the US Army Corps of Engineers, Portland District.
- Jump up^ Ando, D., Nagata, M., Kitamura, T., Shinriki, Y., 2004.Evaluation of loss rate of coded-wire tags in adipose eye tissue of salmon masur Oncorhynchus masou and effect on growth of tagged salmon. Fisheries Science 70, 524-526.