A silver carp fitted with a metal jaw tag. Photo by IDNR.

May 1, 2023

TAG! (Those Fish)

Homonyms are words that have a multitude of meanings. For example, blind. Loss of sight due to illness or injury. A blind is also a structure used to hunt animals. Shoot is another homonym. To shoot is to discharge a projectile from a ranged weapon. A shoot is also a branch stemming from the base of a plant or tree. How about duck? A species of waterfowl. Or what animals do when hunters shoot from a blind. Context is key with homonyms.

Context also matters in tag. A childhood game for some, an exhausting retail task for others. In urban areas, a “tag” is also the signature left on graffiti by the artist. One of this author’s favorite pieces being a bridge on the Chicago River depicting Blinky, the three-eyed fish from The Simpsons. A cheeky nod to the river’s…muddy history.

A large dark green and brown fish held by one fisheries biologist against a wooden board while another fisheries biologist uses a teal gun to apply a tag near the top fin of the fish.
A flathead catfish is fitted with a FLOY tag. A device similar to a clothing tagger is used to affix the tag. Photo by Blake Ruebush/IDNR Fisheries.

If you look at the definitions, they may differ in meaning, but there is a common theme. To “tag” something is to mark it. Whether it be marking a player as “IT,” marking yesterday’s bread as “half off” or marking a wall with the phrase “WUZ HERE,” a mark is left on something for identification. In the Illinois Department of Natural Resources’ (IDNR) Division of Fisheries plenty of tagging is done. Tagging fish, that is.

Fisheries professionals have been tagging fish in the United States for 150 years. The first successful tagging took place in the U.S. in 1873, when Atlantic salmon were marked in Maine as part of a recapture study. Tales of tagging go back to the 1600s, however, with stories of anglers tying ribbon to the tails of salmon as they made their way out to sea. While ribbon has been replaced by metal and plastic, many tagging methods haven’t changed much in 100 years. The reasons behind tagging fish have also remained largely unchanged.

The goal of tagging is to mark a fish or group of fish for future capture and identification. Objectives of these tagging efforts are based around developing a better understanding of a fish’s life history and habits. Tagging studies can yield information about growth rates, movement patterns, population densities, mortality rates and habitat choice. This data gives biologists invaluable information about the fish’s life between the time of tagging and recapture. Ultimately, this data helps fisheries agencies better manage fish populations, control invasive species, and determine effective stocking strategies. Before any data can be collected, the proper tagging method must be used for the fish being studied.

Fish can be tagged or marked in several ways. The two main methods of tagging include mechanical tagging and physical marking. The following examples are by no means an exhaustive list. Instead, the following methods highlight some of the common tagging practices fisheries professionals utilize, many of which anglers may encounter on caught fish.


A close up of biologist holding a very small piece of blue vinyl tubing coded with numbers attached to a dark brown fish near its top fin.

FLOY tags are usually inserted near the fish’s dorsal fin, as seen in this flathead catfish. Photo by Blake Ruebush/IDNR Fisheries.

When it comes to external tags, the most common is the FLOY tag. Also known as a T-bar, anchor or spaghetti tag, FLOY tags have been used internationally since 1957 on a wide range of fresh (and salt) water fishes. Its popularity lies in its simplicity. Consisting of a small piece of vinyl tubing coded with numbers/letters, the tags measure around 2 inches long. They are lightweight, come in a variety of colors and can be easily transported. Due to their compact size and light weight, FLOY tags can be used on fish of all different species and sizes. They also can be used on fish of almost any life stage, an advantage over some of the other tags to be discussed. They are inexpensive, meaning large groups of fish can be tagged at a low cost.

Tags are inserted into the fish using a tool akin to a clothing tagger from a retail store. Various colors are used to differentiate fish from different waterways. Using different color FLOY tags is a common practice when working with a large sample size or sampling fish from multiple bodies of water. FLOY tags also display a phone number that anglers can call if they catch a tagged fish. This allows anglers to participate in data collection by providing biologists with information including, but not limited to, where and when the fish was caught.

Jaw Tags

A fisheries biologist holds up a large dark gray and silver fish. The fish has a metal loop attached to its bottom lip. The loop has numbers etched on its surface. In the background is a waterway.
Jaw tags are often used on large fish species, like this silver carp held by commercial fisher Gary Shaw. Photo by Kevin Irons.

Another commonly used tag is a jaw tag. Jaw tags are bands of metal, stamped with numbers and letters and perhaps have a phone number. These tags are clamped onto a fish’s jaw, usually in the corner of the mouth. The main benefit of a jaw tag is in its durability and placement. This type of tag is often used on large, predatory fish, such as pike, walleye and catfish, and firmly anchored into the fish’s lip. In the case of catfish and other species that utilize underwater structure, jaw tags are less likely to come loose. For example, a large flathead catfish might call a gnarly logjam home, a habitat that could easily tear out a FLOY tag or other external tag.

PIT Tags

Sometimes, tags aren’t as obvious as a colorful noodle or lip ring (like FLOY or jaw tags). Internal tags, normally inserted into the skin or belly of the fish, are also utilized by fisheries professionals. A commonly used internal tag is a PIT (Passive Internal Transponder) tag. PIT tags are a small microchip housed in a glass cylinder, shaped like a tiny pill. PITs are inserted into fish with a needle (for smaller fish) or through minor surgery (on larger specimens or for larger tags).

Like the previously mentioned tags, PITs do not rely on electricity to function. Rather, the microchip inside the tag is only “switched on” when it is near a scanner. When a fish of interest is captured, biologists can run a handheld scanner over its body to look for a tag. If the tag is detected, a code will be sent to the scanner. The code even includes the last time of capture for that fish, even if it is years later.

Like microchips used in dogs and cats, each PIT tag possesses a unique code. This makes them a great choice for performing studies on individual fish vs. large groups. Like FLOY tags, PIT tags can be used a variety of fish, although they are often reserved for species of interest, including large, predatory fish and endangered species.

Coded Wire Tags (CWT)

A tiny metal tube rests on a biologist's upturned index finger on top of a table.
Coded Wire Tags are tiny pieces of magnetized steel containing a series of letters and numbers corresponding to a year class of fish. Photo by Frank Sladek.

There is another method of tagging that often gets overlooked, mostly because of the size and placement of the tag. Like FLOY tags, coded-wire tags (CWTs) consist of letters and numbers, making up a code. But rather than unique codes for individual fish, CWTs have a single code for entire groups or year classes. For example, all 2023 steelhead trout born at Jake Wolf Memorial Fish Hatchery (Mason County) will have the same code. The steelhead trout from 2022 would also have one code assigned to the entire year class. Anglers who later catch these fish are rarely aware of this tag’s presence, and for good reason.

CWTs are made of tiny pieces of magnetized steel. Tiny is an understatement when it comes to these tags. At less than 1 millimeter in length and a ¼ mm in diameter (the size of a human hair), the tags go unnoticed by anglers. These small tags are inserted into the fish’s nose, completely invisible to the angler. CWT tagging has been automated in recent decades to allow large batches of fingerlings to be marked in the hatchery in a quick and efficient manner (~8,000 fish/hour).

A scanning device is used to scan the fish’s head when captured to look for the presence of a CWT. Unlike PIT tags, which stay in the fish after capture, a CWT tag must be removed from the fish’s nose for it to be read. Due to the tag’s teensy size, microscopes are often used to read codes from removed tags. To make identifying CWT fish easier, fish almost always receive another type of tag, a simple fin clip (e.g., adipose fin) when a CWT is first inserted. This external mark helps biologists differentiate wire tagged fish before a scan even takes place.

Fin Clipping

A very close up view of a biologist completing a fin clipping procedure on a small silver fish where one of the bottom fins of the fish is removed. The biologist uses blue handled surgical shears.
A young smallmouth bass has a pelvic fin clipped by IDNR biologists. Photo by Frank Sladek.

This method does not involve a tag but rather a physical mark put on the fish. Fin clipping is a common marking technique and one of the first recorded methods used to track fish populations. It involves the removal of one of the fish’s fins using anesthesia and surgical shears. The fin removed is usually an adipose fin (located between the fish’s dorsal fin and tail/caudal fin), caudal (tail) or pelvic fin.

Amazingly, this process was done by hand for most of the 20th century. With enough manpower, hundreds of thousands of fish could successfully be clipped over several days. Now, the process is largely done by automated trailers, owned, and operated by U.S. Fish and Wildlife Service. The multi-million-dollar trailers utilize computers and processing trays to move fish into a holding slot, perform the clip, and send the fish out of the trailer into adjacent holding tanks. Of course, like human hands, the system is not perfect and does require some calibration. Once dialed in, these trailers are capable of processing thousands of fish in an afternoon. Even with these advances in fin clipping, hand clips are still regularly performed.

The main goals of fin clipping are to measure movement patterns and survival rates in populations. A properly clipped fin will not grow back and, as a result, the clip identifies it as hatchery born. When biologists are assessing lakes with clipped fish, they look at the number of clipped fish vs. intact fish (fish with all their fins) in their surveys. Lots of clipped fish suggest that there is poor natural reproduction, while lots of intact fish indicate natural reproduction.

A close up of a biologist holding a small white and silver fish. This fish has one of its bottom fins removed. In the background is a net for catching fish.
The same smallmouth bass with its pelvic fin removed. Photo by Frank Sladek.

This influences stocking decisions for those waterways, with more stockings occurring when natural reproduction is poor. Fin clipping can also be used to measure return rates of fish in large or moving bodies of water, like rivers and the Great Lakes. One such study is being performed on Lake Michigan, with the assistance of some eager volunteers.

Role of Anglers in Tagging

In March of 2023, dozens of volunteers descended on Jake Wolf Memorial Fish Hatchery to participate in a salmon fin clip. Once an annual event (sidelined for the last few years due to Covid-19), the fin clip welcomed volunteers from the angling group, Salmon Unlimited, to perform fin clips on 100,000 coho salmon. Alongside hatchery technicians and IDNR staff from visiting offices, the entire process was completed, by hand, in less than 6 hours.

Studied by the IDNR’s Lake Michigan program, the clipped fish are used to determine salmon return rates at several Chicagoland harbors. If many clipped fish are captured, this suggests that the stocked fish are surviving and returning to the harbors where they were released as juveniles (fingerlings). Over the coho’s 3-year life cycle, many of those fish will be recaptured by the very same volunteers and IDNR staff who clipped them, providing valuable data, along with tasty salmon dinners for the lucky anglers.

Anglers are essential in the fish tagging process. While most anglers do not participate in the tagging of fish (unlike Salmon Unlimited members), they often catch fish that have been tagged or clipped when on the water. The angler’s observations about these tagged fish are important pieces of data that can be used to paint an even better picture of the fish’s habits. Data shared by anglers can not only be utilized in fisheries management by the state’s biologists, but by the anglers themselves to catch more fish.

When You Catch

A sign bringing attention to anglers that there is a catfish research project currently underway. The sign includes instructions for anglers if they do catch a tagged catfish. The instructions offer a phone number to call and information to provide over the phone.
Anglers are encouraged to read posted signs to learn more about tagging studies taking place. Photo by Frank Sladek.

When a tagged fish is caught, it can be quite a confusing experience for an unsuspecting angler. Some anglers shout “It’s being studied! Put it back!” while others say, “There’s dinner!” Thankfully, most marked or tagged fish can be kept. IDNR Division of Fisheries staff recommend anglers follow these steps if they catch tagged or marked fish.

  1. Before going out on the water, inspect the areas around the lake and boat launches. If a specific study is being done on that body of water, signs should be present with information on the study and what to do if a tagged fish is caught.
  2. If a fish with a jaw or FLOY tag is caught, inspect the tag for numbers. Depending on the tag, a phone number for a fisheries biologist may be found. If a phone number is not present, you can always call or email your region’s fisheries biologist for more information. Contact information can be found on www.IfishIllinois.org.
  3. Fish with FLOY and jaw tags can be kept by anglers, if the fish is otherwise legal to keep. If the fish is released, DO NOT remove the tag. If a fish is kept, anglers can mail or call in their tags. Anglers are usually asked to provide information such as the location, time, and date of capture. A quick length and weight can be helpful if you can accomplish in short order with no harm to a releasable fish.
  4. Fish with clipped fins can also be kept if size and creel limits are followed.

It is also worth mentioning that a tagged fish is not always part of an active study. In fact, IDNR fisheries biologists have found tags on fish 10 years after studies were completed. This goes to show the resilience of the fish and tags themselves.

Wrap Up

A line of individuals some seated some standing examine tiny fish in tanks. Three children stand by and learn about the process.
Volunteers from Salmon Unlimited assisted IDNR staff in fin-clipping 100,000 coho salmon that will be released into Lake Michigan. Photo by Kathy Andrews Wright.

In conclusion, tagging isn’t just a game for kids, tedious task for store clerks or creative outlet for urban artists. Tagging also happens to be a critical component in proper fisheries management. Not only does the practice allow biologists to track fish movement, growth, and habitat use, it also allows for state and federal hatcheries to develop proper stocking strategies for all types of fish. Data collected by biologists is reviewed and population assessments are made. With this data, biologists, fisheries managers, and hatchery managers work together to develop stocking goals and quotas for managed bodies of waters statewide. This ensures that the proper number of fish are being stocked, as over or understocking could negatively impact a fishery.

As fisheries projects develop and evolve, tagging will always be a preferred practice by fisheries professionals. With an increased availability of electronic devices like satellite and radio tags, biologists can get an even clearer view into the murky waters of fishes’ daily lives. Alongside tried and true methods like FLOY tags, jaw tags and fin clips, fisheries professionals have a great deal of monitoring tools at their disposal. Utilizing these methods, the IDNR biologists and managers can continue to tag and follow fish habits, leading to comprehensive, effective management strategies for Illinois fisheries.

Frank Sladek is the Natural Resources Coordinator at Jake Wolf Hatchery in Topeka. While he doesn’t tag fish, he tags plenty of photos on Jake Wolf’s Facebook page to highlight the awesome work done by IDNR fisheries. To learn more about guided tours and public programs offered at the fish hatchery, contact him at Frank.Sladek@illinois.gov.

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