Tracking Trout

We are using two telemetry methods to track brook trout in Beaver Brook and connected waters: radio-telemetry and RFID ( for Radio Frequency IDentification) telemetry. From 2012 through 2013 we tagged and released 32 brook trout hatched and raised at the Cold Spring Harbor Fish Hatchery. Most of these fish were juveniles (< 1 year old) but a few were mature trout (approximantely 1.5 years old). This spring (2014) we started capturing,,tagging and releasing brook trout resident to Beaver Brook. These fish will serve as a “control group” so we can compare movements and habitat preferences of hatchery-reared fish to resident fish.  Since 2012 we have tagged and released almost 300 hatchery-reared mature trout with RFID tags (also know as PIT-tags) . In 2014 we started to tag all brook trout we caught during electrofishing surveys of Beaver Brook, 18 to date.  RFID telemetry will allow us to determine whether hatchery-reared and resident brook trout can move from Beaver Brook into Beaver Lake and possibly into the Mill Creek estuary.


It is 5x3x10 mm and weighs only .25 grams. The white part is inserted into the body cavity and the 12 cm antenna (covered by fingers) trails outside of body.

It is 5x3x10 mm and weighs only .25 grams. The white part is inserted into the body cavity and the 12 cm antenna (covered by fingers) trails outside of body.

In radio-telemetry, a tiny transmitter (“radio-tag”) is surgically implanted in the body cavity of an anesthetized fish. After recovery from surgery the fish is released into the study environment. The tag transmits a signal at a specific radio frequency that can be picked up by a special portable receiver from distances of up to ½ km or more. Each tag has its on specific code so it is possible to identify specific fish. The signals pulse at specific rate (for the tags in this study three pulses per minute). The signal is strongest when the antenna is pointed towards the tag and gets stronger as the receiver approaches the tag. In this way we can pinpoint the exact location of the fish. Once located, we record the location using a GPS unit along with water depth, water temperature, and other environmental characteristics such as presence of cover.


Searching for a radio-tagged fish in Beaver Lake. Notice the directional antenna. The receiver is resting on the deck of the canoe.

Radio-tags last only as long as their batteries. Because we are tagging very small fish the tags have to have very small batteries so they don’t last very long. The tags we are using are rated to last 40 days but we have had some last over 80 days. Larger tags can last years but they are bigger than the fish we are studying.

Video of surgical implantation of radio-tag.

RFID telemetry

12 mm HDX PIT tag from Oregon RFID.

12 mm HDX PIT tag from Oregon RFID.

RFID telemetry uses tiny tags about the size of a rice grain usually called PIT (for Passive Integrated Transponder) -tags. PIT-tags are also surgically implanted in the body cavity of anesthetized fish. Because PIT-tags don’t have batteries they can only be located when an external source of electrical energy temporarily “charges” the tag so it can emit a brief identification pulse. Therefore PIT-tagged fish are identified only when they pass by one of these sources, a big loop antenna attached to  a data logger and and powered by batteries. The antenna is stationary and encompasses a stream. Every few milliseconds it sends out an electrical pulse. If a tagged fish passes through the loop the tag is charged and send a pulse back through the antenna. The data logger records the identification code of the tag and the time of detection. The nice thing about these tags is that they last as long as the fish since they are not battery powered. However tracking is limited to the number and location of antennae. In this study we have two antennae, one between at the LIRR bridge between the Shu Swamp Nature Preserve an Beaver Lake and the other at the spillway between Beaver Lake and Mill Creek.

Video of surgical implantation of PIT-tag. After implantation the tag code is identified, and fork length and body weight is determined.