Passive Integrated Transponder (PIT) tags are commonly used in fisheries science to monitor fish distribution and movement. A PIT tag is similar to a microchip and resembles a grain of rice (typically 9 mm or 12 mm in length), that can be inserted into an adult or juvenile fish. Each PIT tag has a unique tag code (a series of numbers and letters) that allows for individual fish identification. In adult fish, the tag is typically inserted into the back of the fish near the dorsal fin, whereas the tag is inserted into the body cavity of juveniles. Tag insertions are done through the use of a sterile needle with a PIT-tag injector. Once the tag is inserted into the fish, data such as species, length, and weight is recorded, and the fish is then released and tracked by its unique code.

Tracking fish with PIT tags and PIT arrays is different from tracking big game animals with radio collars. Radio collared animals can be tracked at most locations using satellites or handheld receivers. PIT tags do not have a power source, rather, they require activation from an instream antenna, which are referred to arrays.  Thus, PIT tagged fish must pass within a few feet of an array in order to be activated and detected. Therefore, relocations of PIT-tagged fish are dependent on array locations. Once the fish is detected, the fish’s unique PIT-tag code, date, and time is uploaded into a program called Columbia Basin PIT Tag Information System (PTAGIS).

The PTAGIS database stores information for all PIT-tagged fish in the Columbia River basin. Upon PIT tagging a fish, the biologist will upload all information associated with that unique PIT tag. This includes species, the location of where the fish was tagged, the release location (may or may not be different from tagging location) and the length and weight of the fish when tagged. This information will be associated with the fish every time the fish is detected at an array

In-stream arrays are the most common way to detect a PIT-tagged fish but there are other methods. If a PIT-tagged fish is captured in a rotary screw trap, during electrofishing, or gillnetting, the biologist will use a handheld PIT-tag reader to scan the fish. Typically, the recaptured fish will be measured for length and weight and that information along with the PIT-tag code will be uploaded to PTAGIS. The PTAGIS database will associate the PIT-tag code from the day the individual fish was tagged with all sequential detections at arrays or recapture events. Therefore, each PIT-tagged fish can have a lengthy history of locations and times it was detected that allows biologists to track fish over long time periods and at great distances from their original tagging locations.

PIT arrays are distributed throughout the Pacific Northwest so that biologists can track Chinook Salmon and steelhead back and forth between freshwater and saltwater environments. This includes large rivers, small streams, and in side channels. Arrays can be used to observe fish passage at large dams, as well as monitoring fish screens that are designed to bypass fish around irrigation diversions.

PIT array locations are selected so that fish distribution and movement can be monitored for a variety of management reasons. A prime example of a system with a network of arrays is the Lemhi River basin, located in the upper Salmon River watershed. The Lemhi River has four arrays installed in the main-stem river and additional arrays installed at the mouth of its various tributaries. Fish detections at these arrays provide biologist with information that can be used to assess fish survival, movement and distribution, spawn timing, abundance, and specific habitat use. In the Lemhi River basin, PIT arrays are commonly used to better understand where juvenile fish overwinter and how habitat in those specific locations is related to overwinter survival. Overall, PIT tagging fish is a common practice that provides useful information that is used for fisheries management.  

For more information on PIT tagging, please see the following links:

https://idfg.idaho.gov/blog/2021/04/smolt-monitoring-project-week-6-pit-tagging

https://idfg.idaho.gov/blog/2020/04/wild-adult-steelhead-returns-detections-new-tributary-pit-array