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Writer's pictureJason G. Freund

Fisheries Management Topics: All About Fish Tagging

Fisheries managers and research scientists tag fish for any number of reasons. Most commonly, it is to estimate the population (a topic I am writing a post on for the future) but it is also used to examine movement (e.g. dispersal, migration, assessing fish passage), mortality, growth of individuals, and habitat use.


I am going to take a broad view of this topic and consider tags to include anything which can track individuals or groups of individuals over time. Tags can be unique to individuals or fish may be batch marked - a similar mark given to all fishes captured at a particular time and place. The type of tag used depends upon the goals of the study and the ability to pay for the study.


Types of Tags


There are many different methods for tagging fishes that differ in the information they provide, the cost and effort required, how invasive they are to the fish, and a host of other variables. Probably the simplest decision in a tagging study is, do we use tags that identify each individual or do we give all individuals the same mark? Batch marking is often used to identify individuals captured at a specific time and place and is generally simpler and less expensive. Batch marking might be used to assess movement between different streams or stream reaches or to signify that a particular treatment was given to individuals with a similar tag. I have used batch marks to assess movement between different sample sites and in a recent winter mortality study (look for a future post with details about that study) to signify which experimental treatment a fish received. Individual marks are probably more commonly used because they can provide more information than can batch tagged fishes. Batch tagging - or fin clipping - however is much simpler and cost effective when information specific to individuals in not necessary.


Categories of Tags / Marks (and links to information)


I might also have categorized these as internal and external tags - though biotelemtery can be either internal or external and some of the visible tags are partially internal or a subcutaneous and internal but are visible externally.

Biotelemetry is the tracking of fishes using a signal (frequency) unique to each individual. Telemetry is typically used to assess habitat use and movement. Chemical tags are a type of batch marking where individuals obtain through their environment (stable isotopes) or are given a chemical treatment (i.e. oxytetracycline). Visible tags are what most think of as fish tags. These can be individual (e.g. coded wire, t-bar tags, visible implant elastomer) or batch tags (e.g. visible implant elastomer, fin clips) - again depending upon the goals of the study. Passive integrated transponder (PIT) tags are among the most commonly used individual tags used by researchers. "Reading" of these tags requires a PIT-tag reader which returns a unique number assigned to each tagged fish.


Tracking Fish Movement and Habitat Use


Biotelemetry is the most common method use to assess fish movement and habitat use. There are two types of telemetry used in fisheries science - acoustic and radio. GPS telemetry is another option but is much more likely to be used in non-fisheries applications. Acoustic telemeters produce a unique sound signal that is captured by a hydrophone - essentially an underwater microphone (hydrophone). Radio telemetry is either low or high frequency and similar to acoustic telemetry, it produces a signal unique to each tag. Unlike acoustic telemetry, this signal is received out of the water by an antennae. The difference in the two signal types is how well they travel through water and air with radio signals transmitting more effectively in air.

Signals are received by an antenna or hydrophone and then a receiver is used to decipher the unique signal each tag sends. Most of the time, to save battery, signals are sent on time intervals. By knowing the strength of the signal (decibels for acoustic tags, signal strength in radio telemetry) and the angle of the strongest signal, distance can be inferred. By triangulating the signal from multiple locations, a more precise location can be determined. Signals are typically received using either passive (fixed-station) or active tracking. For my Masters, I tagged Largemouth Bass (Micropterus nigricans) in the Ohio River (Freund and Hartman 2002, 2005) by actively tracking them and currently, we have a project on the Mississippi River using the vast array of stationary (fixed) hydrophones to track Grass Carp (Ctenopharyngodon idella).

While less commonly used for movement and habitat studies, a number of other tag types can be used. We did a small study in graduate school looking at Walleye (Sander vitreus) and Sauger (S. canadensis) populations in the Monongahela River in Morgantown, WV and tagged them with T-bar tags. One Sauger had moved at least 60 miles after spawning and was captured by an angler. However, this is often a bit like finding a needle in a haystack. Passive integrated transponders (PIT) tags can be used to track movement and assess habitat use but typically in shallower streams or in specialized fish passage situations (Center 1990). Like telemetry, PIT tags can be received actively or passively. However individual tags are much less expensive, allowing for a greater sample size, a limitation of most telemetry studies. Lastly, chemical tags and stable isotopes can be used to assess movements from natal (spawning) grounds.


Demographic Studies


Demographic studies - that is the study of population parameters such as population size, fecundity, age structure, mortality, and sex ratios (among other parameters) - often utilize batch marks. This is because we are typically interested in what happens to a cohort of fishes over time. It also means that the tag needs to be retained and readable for a number of years.

Brook Trout to be tagged
A West Virginia Brook Trout and a syringe with visible implant elastomer used to tag fishes with individual marks.

By tagging and recapturing fishes, we can estimate parameters such as growth rates, mortality and surviorship, production to biomass (P:B) ratios, emigration and immigration rates, and other parameters. Study design allows for a number of different parameters to be estimated at once. In one study I was part of (Petty et al. 2005), we gave individual tags using VIE and found that:

Our results suggest that spatial and temporal variations in spawning, survival, and movement interact to determine the distribution, abundance, and size structure of brook trout populations at a watershed scale.

By marking Brook Trout and sampling them seasonally, we were able to track demographic changes across seasons.

I have seen tags used in many different fisheries demographic studies. For example, we might assess movement around a barrier like a beaver dam - to measure immigration and emigration - by giving fishes above and below the barrier a different mark. Tags that persist - like T-bar tags - may be used to estimate longevity. Part of why we know so much about waterfowl ecology and population dynamics is that we have decades of demographic data collected by researchers aided by waterfowl hunters. Demographics and factors like angler harvest are often studied using T-bar tags that identify individuals. And tagging and recapturing fishes provides the most direct method to estimate the growth of individual fishes which is an important piece of information for fisheries biologists.


Population Estimation


A much more detailed post about capture-mark-recapture (CMR) and its use for estimating populations, probably the most common reason for tagging fishes, is upcoming. But for now, one of the most common methods to obtain population estimates for fishes is by capturing them, giving them a tag, releasing them, and recapturing them at a later date. Then a proportional relationship, the number of recaptures / number of individuals captured in the second sample = number of individuals captured and tagged in the first sample / the total population size is used to estimate the unknown - the population estimate. As I'll write more about in a later post, there are a number of important assumptions that are made but among the most important are that individuals do not lose their tags and those tags remain visible to researchers. As you can see in the formula below, reducing the denominator - the number of recaptures - would give an artificially high population estimate.

Of course, population estimation is often more complicated than a single marking session and a single recapture session. Some state and federal agencies support "tag and release" programs where anglers do the tagging. These are particularly common for saltwater fisheries. These are examples of studies with many different capture and marking sessions as well as recapture sessions. This is much akin to the federal duck banding program mentioned above. Often, these are demographic studies that may also provide some ability to estimate populations but those estimates may be secondary to other population estimation methods.

There are many other methods of population estimation that are often used, sometimes in conjunction with tagging studies. And quite often information about movement, demographics, and population estimation can all be addressed by a single tagging study. However, if the main goal is to better understand movement or demographics, the methods used will probably need to change.


Limitations of Fish Tagging Studies


As with any study method, fish tagging experiments have limitations. For example, biotelemetry studies are often limited by sample size as individual tags are expensive - often a few hundred dollars per transmitter tag. Other limitations are the tracking effort, "blind spots" in fixed array systems, external noises or radio signals, how and where fishes were captured, the environment in which they are tracked, and a host of other factors. On the other hand, it is the most reliable method to understand habitat use and movement in lakes and large rivers.

Tagging studies require a good bit of thought and planning. Considerations need to be given to information the study is able to obtain, the type of tag used, the length of the study and how long the tags need to be visible, the amount of time and money dedicated to the project, and other factors. Because we are often dealing with open systems - that is there is the potential for immigration and emigration - we need to understand that our tagged individuals may leave the study reach. This will alter our ability to estimate population size but can provide us with interesting information. For example, in the Kickapoo River watershed, the WDNR had recaptured a PIT-tagged fish that was recaptured by electrofishing in a reach over thirty miles away. Talk about recapturing a needle in a haystack.


Tagging is one of the more commonly used fisheries science and management methods because of the wide applications of tags and the information that they can provide.


Links

Literature Cited / References / Reading List













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