Acoustic Telemetry 101
Acoustic telemetry is the process of measuring biologically relevant data for aquatic animals using transmitters called electronic tags. The tags are externally attached to the animal or, in the case of some fish species, implanted internally. The tags emit acoustic signals at fixed intervals that are detected by underwater hydrophones called acoustic receivers. The type of information obtained via acoustic tracking relates primarily to the animal’s geographic location, but can also include position in the water column, environmental data such as salinity and temperature, and biological information such as acceleration. There are two basic types of acoustic tracking: passive and active.
Example of an acoustic receiver (left) and tag (right)
Passive Tracking
Passive tracking occurs via receivers that are anchored fixed in space. Researchers place a number of receivers underwater (called a receiver array)—along migratory corridors, throughout a nursery or breeding area or in other areas that enable them to gather information about fish behavior, movement patterns, habitat use, and survival. Whenever an acoustically tagged fish swims within the detection range of a receiver, its presence is recorded on that receiver in the form of a code unique to that fish, along with time and date of detection. Detection ranges are strongly influenced by environmental conditions and background noise and are generally on the order of a few meters to over a kilometer. Tag batteries typically last from several months to 10 years. The data that are stored on the receivers must be physically downloaded which can be time and resource intensive.
Active Tracking
Researcher engaged in active tracking.
Active tracking consists of using a portable receiver to follow a tagged fish around in real time (usually from a boat) for short periods (typically 1 to 3 days). This is a labor-intensive and exhausting process for the researcher, but it provides more detailed information on fish movement, behavior, and habitat use than what might by obtainable from passive tracking. Active tracking can also be used in pilot studies to help determine the best placement of a fixed receiver array.
Acoustic telemetry is the type of biotelemetry predominantly used by the iTAG community, but it is not the only type. The illustration below gives an overview of biotelemetry technology and shows where acoustic telemetry fits in. More information about the other types of biotelemetry can be found below.