Tracking fish on a flooded bog

Research theme: Tagging and telemetry

Understanding fish movement is important for fisheries management and gives us information on fish migration, their life history and the environmental pressures that they face. This information is vital for informing decision making and policy for fisheries conservation. Tagging data is used by scientists to study the lives and habits both of highly mobile fish species that migrate long distances (e.g. blue shark) and of those species which tend to live all their lives in one area (e.g. brown trout).

The migratory habits of the tagged fish are observed by measuring the distance travelled between tagging and recapture locations, and this can be linked to environmental variables and biological factors, such as spawning migrations, home ranges, movements around barriers, etc. Growth patterns may also be monitored if the fish can be recaptured, identified and measured again. Such observations also allow scientists to study the health of fish stocks and to assess whether geographically distinct populations are mixing with each other. This information is particularly important in our understanding of valuable game and sport fishes.

Tagging and telemetry are useful ways to monitor fish movement patterns. The simplest tagging method is the use of plastic tags with a unique code that can be attached to fish and read by anyone if the fish is recaptured. More specialised methods use electronic tags can be detected in some way using telemetry. Telemetry involves receivers or listening stations that collect information remotely from signals actively transmitted by tags attached to or implanted into fish. The two main methods of telemetry are acoustic telemetry and radio telemetry.

An alternative method is the use of passive tags that emit no signal but that can be detected at close range using an antennae. Additionally, some tags are capable of recording ambient environmental data, such as temperature, depth, salinity, etc., which can be later retrieved and used to build a detailed picture of the habitat used by tagged fish.

Ultimately, the tagging method used will depend on the research question to be answered, budget, the habitat (e.g. freshwater or saltwater), the size of the fish, the longevity required (battery size), and whether the tag will record data.

Below are some of the tagging methods used by Inland Fisheries Ireland:


Floy tags are very simple plastic tags with a unique code that can be attached to fish when it is caught. The tag also has contact details for reporting the tag. When fish are first captured, they are measured, tagged and then released. If the fish is later recaptured in a fisheries survey or by an angler, the unique code can be read off the tag to identify the fish, and the information can be sent to Inland Fisheries Ireland. This allows us to determine how far the fish has traveled from its tagging location and how long it has survived since tagging. As well as providing information about the range of the fish, comparison of length measurements and scale samples if available allows us to get valuable information about its rate of growth over time.

Some of the species that are tagged by Inland Fisheries Ireland are Atlantic salmon, sea trout, brown trout, eels, pike, bass and marine sportfish, especially sharks, skates and rays. Important advantages of floy tags are that they are simple to attach, cheap, last a long time and can be attached to a huge number of fish. For example, over 40,000 fish have been tagged by the Marine Sportfish Tagging Programme, with over 3,000 recaptures; many of the recaptured fish had made journeys of thousands of kilometres or survived in the wild for several several years later. This has allowed the programme to build up valuable information about highly migratory and long lived species, such as sharks and rays.

How can you help?

Have you found a tagged fish? Please look out for the tags below, and see our citizen science page for more details on how to report tagged fish.

Close up of a floy tag with code
Various jumbo and roto tags
Bass with a floy tag attached to its back. New Ross Port 2016.

Acoustic tags are transmitters that emit a burst of ultrasound, each with its own unique identifier. The acoustic transmitters are small enough to be attached to fish or surgically implanted inside them. The transmitters can be detected by a receiver unit up to 500m away, and this system is suitable for use in both freshwater and marine environments. Because the signals cannot transmit through the air, the receiving unit must be submerged underwater. Fixed listening stations in the water are used to pick up any signals from tagged fish as they pass by and record their movements. These receivers are often placed at strategic locations on structures including buoys, jetties and pontoons, and they may become completely covered in algae or barnacles if left out for long periods.

Different sizes of acoustic tags
Recovered acoustic receivers laid out along the ground
Acoustic Receiver post deployment

Radio tags emit radio signals at specific frequencies that can escape the water and travel through the air. This allows the signal to be detected by fixed aerials on the shore or by lightweight mobile receivers can be carried along riverbanks to locate tagged fish. It is even possible to conduct aerial surveys over lakes to find radio-tagged fish; see the Ferox Trout Study, for example. Radio tags are only suitable for use in freshwater as saltwater impacts the signal; their battery requirement also limits their size and consequently the size of fish that they can be used on.

Radio tag
Solar powered listening station

Passive integrated transponder (PIT) tags are tiny, glass-coated microchips that can be activated and detected at close range by a special handheld device or a fixed reader in a fish counter. They work on the same principle as the electronic tags used by vehicles at motorway toll stations. An important advantage of these tags is that they are inert and do not have a power source, so they are small enough to be injected into juvenile fish. They can last for the lifetime of a fish, which means they are particularly useful for tracking highly migratory species, such as salmon or sea trout passing through a fish counter. Occasionally, if a predator eats the fish and the tag is detected in its droppings, this gives even more information on the fish’s ecology.

PIT tagging a salmon smolt on the River Erriff.

Data storage or archival tags record data when they are attached to a fish. This information is usually ambient environmental variables, including the likes of temperature, depth and salinity, which are all useful when learning about the ecology of a particular species. The recorded information is usually not transmitted but simply stored on the device; therefore, the tagged fish typically needs to be recaptured for the information to be retrieved. This means that many tags need to be deployed to ensure an adequate return.

Pop-up satellite archival tags (PSATs) are an innovative type of archival tag that record data for a preset period, then detach from the fish and float to the water’s surface, transmitting their recorded data to an overhead satellite. This technology is continually improving, with “mini” versions (MPSATs) now available for use on smaller fish. See the EELIAD project for a PSAT study 

PSAT tag set up and ready for deployment
Silver eels tagged with PSATs.