A Modified Downrigger for Detecting Radio Transmitters in Deep Water

The advantages and disadvantages of aerial (i.e., through the air–water interface) and underwater tracking of radio-tagged free-swimming fish are compared. A novel device is described that can be used to detect radio signals from depths exceeding the detection range of aerial antennas. This device can be used in small, confined bodies of water to track seasonal movements of fish into deep water, to identify locations of lost or expelled transmitters, to monitor relatively immobile benthic species, and to determine swimming or suspended depth while retaining several other advantages associated with the use of radio (versus ultrasonic) transmitters. The device is inexpensive, simple to construct, and easy to use with any commercially available telemetry receiver.