A10. Radio Frequency IDentification (RFID) - PIT Tag

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Mobile prospection on the Valserine river - France © Scimabio Interface

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1. Objectives

Pass-through antenna © Scimabio Interfaceg

The Radio Frequency IDentification system has several purposes but is mainly used to:

  • Assess fishway accessibility and passability.
  • Enable the identification of individuals and track migratory fish.
  • Estimate population size via the mark-recapture protocol.
  • Assess fish movements, behaviour, habitat and micro-habitat use relating to hydrology, water temperature, and other environmental parameters.
  • Estimate impact of cross sections, weirs, obstacles, and culverts (Norman et al., 2009; Macleod and Gagen, 2018).

2. Method summary

The principle of Radio Frequency IDentification (RFID) is to identify fish that have previously been tagged and released. RFID uses a range of antenna shapes, depending on the mobility of the device, which are adapted to fish passage characteristics or study purposes. Antennas are made of loops of copper wire plugged into a tuner (used to set the detection range) which in turn is connected to an antenna reader (by a twinax wire), that hosts the reading program. When fish pass near or through the antenna, the antennas magnetic field wirelessly powers the capacitor of the passive PIT-tag (Passive Integrated Transponder) inside the fish. This transfers the fish’s identification number (10 – 15 digits numbers) alongside the date and hour to be registered. The tag does not have a battery, making it light and operational throughout the entirety of the life of the fish. It is composed of a copper wire and electronic circuit (including a capacitor) contained inside a glass capsule. The maximum detection range is 1.5 m, but is usually around 0.8 m. Settings are complex and the detection range depends on the shape of the antenna, the number of cooper wire loops inside it, reader settings and much more.

2.1 Portable antenna prospection

Surveys using portable antennas detect the location of tagged fish in a river. Surveyors carry the antenna by hand while the reader is carried in a backpack and plugged into a battery. If there are two or more antennas, then readers are wirelessly synchronised. In this scenario the entire width of the stream is surveyed. The detection range of the antenna reader is approximately 80 cm. A sound is made when a tag is detected, allowing the surveyor to obtain an accurate record of its location. Data must be saved onto a computer after the survey. Some antennas are not easily portable but can be moved. This is the case for rigid floating antennas, inflatable floating antennas, and raft mounted floating antennas, which can all be used for mobile surveys.

2.2 Fixed antennas

Pass-through antenna © Scimabio Interface

Fixed antennas are usually built on fishways, streams trail, or culverts and placed on the riverbed to assess habitat use. There are a range of antenna styles to choose from, with differing forms and features to suit a variety of situations. These include: pass-through rectangle, circular loop, pass-over loop, pass-by loop or stick antennas and several others. Fish are detected by passing through or near the antenna (0 – 1.5 m, depending on the device). A single antenna can be used, or multiple antennas if synchronized wirelessly or with a cable. To check the antenna is functioning correctly, it is advisable to put a marker tag near the antenna, which has a device inside with an on-off function. The marker tag can be switched on and off periodically to test if the antenna is able to detect every time it is activated. All data is recorded inside the antenna reader, which is located in a safe place, preferably away from potential flood risk. Data can either be sent to a distant server or be downloaded directly.

2.3 PIT tags

Two types of PIT tags exist, full-duplex (FDX) and half-duplex (HDX). Both operate at a frequency of 134.2 kHz. The FDX simultaneously receives energy and transmits its ID, whereas the HDX transmits its ID and then receives energy. Therefore, FDX antennas constantly transmit the magnetic charge field whilst listening to the emitted tag information, but HDX antennas stop emitting their magnetic field whilst processing the emitted information. FDX has a rate of 30 reads per second whereas HDX has a slower rate of up to 14 reads per second. However, the HDX tags are bigger (23 x 3.8 mm) and have the capacity to store energy, leading to a more powerful signal and greater detection range. FDX tags are smaller (8 mm minimum) and can be inserted into fish with a minimum size of 4.5cm, with a hypodermic injector. Their detection range is lower than the HDX tags and are more suitable to restricted fish passages, like culverts or fishways. A few other accessories are needed to properly complete the installation, such as a Piezo buzzer, a tuning indicator, and other devices.

3. Advantages

Mobile prospection on the Valserine river - France © Scimabio Interface

RFID method and tags

  • Very small fish can be tagged (Kelly et al., 2017).
  • No surgical operation – just a fast and light incision in the skin that quickly heals.
  • Tags do not require batteries, so can outlive the fish, and become 'ghost' tags but still detectable in sediment.
  • Cheap mass tagging.
  • Limited handling (only once for tagging).

Mobile surveying

  • Can be used in translocation surveys.
  • Allows detection of non-mobile fish.
  • Allows detection of lost tags from dead fish or rejected tags to deduce efficiency.

Fixed antennas

  • Allows automatic detection of fish.
  • Ideal for remote applications by running on batteries or solar power, as systems operate on 12 – 24 volts DC power.
  • Works in high flow conditions, when electrofishing is not possible.
  • Can be adapted to various environments, fishways and urban facilities.
  • Data is recorded in situ but can be transferred wirelessly with a modem, enabling remote processing.

4. Limitations

Pass-over antenna © Scimabio Interface

RFID method and tags

  • Do not have a large detection range.
  • Can be expelled.
  • If a study spans several years care must be taken to avoid confusing data entries for individuals if a tag is reused several times, i.e. using the same tag with the same identification number for several fish may result in confusion.
  • Detection range from the antenna depends on tag size (Kelly et al., 2017).

Mobile surveying

  • Cannot be deployed during all hydrological conditions due to safety reasons.
  • Hard to access some remote river habitats.
  • Battery lasts one day.
  • Detection range has to be tested several times a day to make sure it does not reduce
  • Detection range depends on number of wire loops and wire diameter.
  • Conductivity can affect detection range.

Fixed antennas

  • Hard to set-up properly.
  • Detection range depends on number of wire loops and wire diameter.
  • Cannot be installed in all locations due to potential magnetic field interference.
  • Technological potential for micro-habitat survey with multiple antennas and one reader – technology becoming more common but still rare.
  • Conductivity can affect range.

5. Recommendation for method application

Caution during tagging process is essential for guaranteed fish survival and unmodified behaviour due to handling stress. Follow a well-defined protocol and check for ethical considerations and permits.

5.1 Mobile surveying

Pass-over antenna © Scimabio Interface
  • Wirelessly synchronise the antennas before starting.
  • Get the position of each detected tag by taking a waypoint on a GPS. Alternatively, synchronize watches with a GPS to the second (taking a waypoint each second), so you can link the data registered by the reader and GPS position at any moment.
  • Note ‘ghost’ tags (fish dead, tag expel) (O’Donnell et al., 2010).
  • Make sure to save your data at the end.
  • Do not delete the data in the portable reader, just in case.

5.2 Fixed antennas

  • Follow a guide and refer to a professional for your study design and material. Some advice can be found in Steinke and Anderson (2011).
  • Properly and securely anchor and protect the antenna to prevent damage during flooding.
  • Contact or collaborate with a consulting company or the manufacturer, for the installation process.
  • It is better to synchronize the reader with a synchronising cable, as wireless synchronisation can be less reliable.
  • Do not install it near pieces of metal or electric wires, as it will disturb electromagnetic field and lead to problems.
  • Install in a location with enough water flow to avoid stationary fish behaviour, which may inhibit reading of other PIT-tags.
  • In order to be sure of there are no sources of disturbance nearby, do several tests before installing the final equipment.
  • Multiple tags can be in the magnetic charge field at the same time, leading to tag collision. Tag monitoring systems can be designated to limit this effect.

6. Costs

All material for a simple antenna system is usually around £3,000 – £4,000, but cheaper systems can cost c. £1,000 (Bond et al., 2007). Supplementary costs for more complicated installation can be several thousands of pounds. A tag costs between £1.50 and £3.00.

7. Data analysis

Data analysis can be complex depending on the number of fish tagged, their mobility, and the purpose of the study. Micro-habitat analysis, for example, requires skills in software coding. The distance travelled and speed per of an individual can be analysed depending on the survey objectives, as well as mapping and analysis of fish behaviour/position with GIS data collected during mobile survey. Analysis of data from mobile survey and fixed antennas is not the same. For data analysis, refer to references and see Norman et al., (2009).

Furthermore, evaluation of detection efficiency has to be performed to properly analyse data (Connolly, 2010). Detection probability can be negatively influenced by several parameters such as stream width, depth, distance from the middle of the stream (Richer et al., 2017) and also by tag size, orientation, proximity with other antennas (Zydlewski et al., 2006; Fetherman et al., 2014), tag collision, time of day and age of fish (Axel et al., 2005; O’Donnell et al., 2010). These factors directly impact the number of times an individual passes and is detected by the antenna array. For fixed antennas, regardless of accurate installation, the detection probability can vary due to stream hydrology, vibration and ambient radio frequency noise, fish speed and individual behaviour (Aymes and Rives, 2009). Assessing these parameters is necessary to measure detection efficiency.

8. Survey

See references.
For system construction and operating procedure see : https://www.fws.gov/aftc/abernathy/sopinterrogationsystemconstructionsop.pdf


  1. Axel, G. A. et al. (2005) ‘PIT-Tag Detection System for Large-Diameter Juvenile Fish Bypass Pipes at Columbia River Basin Hydroelectric Dams’, North American Journal of Fisheries Management, 25(2), pp. 646–651. doi: 10.1577/m04-071.1.
  2. Aymes, J. C. and Rives, J. (2009) ‘Detection efficiency of multiplexed Passive Integrated Transponder antennas is influenced by environmental conditions and fish swimming behaviour’, Ecology of Freshwater Fish, 18(4), pp. 507–513. doi: 10.1111/j.1600-0633.2009.00373.x.
  3. Bond, M. H. et al. (2007) ‘A New Low-Cost Instream Antenna System for Tracking Passive Integrated Transponder (PIT)-Tagged Fish in Small Streams’, Transactions of the American Fisheries Society, 136(3), pp. 562–566. doi: 10.1577/t06-084.1.
  4. Connolly, P. J. (2010) ‘Guidelines to Indirectly Measure and Enhance Detection Efficiency of Stationary PIT Tag Interrogation Systems in Streams’, Tagging, Telemetry and Marking Measures for Monitoring Fish Populations—A compendium of new and recent science for use in informing technique and decision modalities: Pacific Northwest Aquatic Monitoring Partnership Special Publication, 2871(January), pp. 119–125.
  5. Fetherman, E. R. et al. (2014) ‘Raft and Floating Radio Frequency Identification (RFID) Antenna Systems for Detecting and Estimating Abundance of PIT-tagged Fish in Rivers’, North American Journal of Fisheries Management, 34(6), pp. 1065–1077. doi: 10.1080/02755947.2014.943859.
  6. Kelly, B. B. et al. (2017) ‘Detection efficiency of a portable PIT antenna for two small-bodied fishes in a piedmont stream’, North American Journal of Fisheries Management. Taylor & Francis, 37(6), pp. 1362–1369. doi: 10.1080/02755947.2017.1388886.
  7. Macleod, I. R. and Gagen, C. J. (2018) ‘New Applications of Radio Frequency Identification Stations for Monitoring Fish Passage through Headwater Road Crossings and Natural Reaches’, 72.
  8. Norman, J. R. et al. (2009) ‘Application of a Multistate Model to Estimate Culvert Effects on Movement of Small Fishes’, Transactions of the American Fisheries Society, 138(4), pp. 826–838. doi: 10.1577/t08-156.1.
  9. O’Donnell, M. J. et al. (2010) ‘Use of Portable Antennas to Estimate Abundance of PIT-Tagged Fish in Small Streams: Factors Affecting Detection Probability’, North American Journal of Fisheries Management, 30(2), pp. 323–336. doi: 10.1577/m09-008.1.
  10. Richer, E. E. et al. (2017) ‘Incorporating GPS and mobile radio frequency identification to detect PIT-Tagged fish and evaluate habitat utilisation in streams’, North American Journal of Fisheries Management. Taylor & Francis, 37(6), pp. 1249–1264. doi: 10.1080/02755947.2017.1374312.
  11. Steinke, K. and Anderson, J. (2011) Aquatic PIT-tag interrogation system construction & standard operating procedure.
  12. Zydlewski, G. B. et al. (2006) ‘Remote monitoring of fisgh in small stream a unified approache using pit-tag’, pp. 492–502.
  13. https://fishbio.com/field-notes/fish-monitoring/pit-tag-the-full-story
  14. https://www.oregonrfid.com/
  15. http://www.scimabio-interface.fr/en/techno-rfid/