Acoustic telemetry is used to obtain a relatively continuous record of fish movements and is ideal for answering fine-scale questions. It can be done actively, in real-time, or passively, using a listening station. For both modes of telemetry the first step is to secure an acoustic transmitter or “pinger” to a fish. This pinger emits an acoustic pulse at a frequency from 32-300 kHz. This sound pulse is then picked up by a hydrophone. Where the hydrophone occurs varies between the two methods. The system best suited to a project will depend on the specific questions the researcher wishes to address.
|Acoustic tags come in a variety of different sizes, depending on the size of target species and depth-recording capabilities|
For passive tracking the hydrophone is mounted on a secured listening station. The listening station detects any tags within its range and logs the occurrence of the tagged fish. The range is generally 100-1000 meters and depends on the frequency of the transducer and the power output of the tag. Often a number of listening stations are used around islands, in bays, or in submarine canyons. Some listening stations communicate with and download data that has been collected by the tags. Others are simple and only indicate when a specific ID number is detected.
When conducting active tracking, a hydrophone is mounted to the tracking vessel and a sound pulse emitted by a sonic transmitter is used to follow the fish in real time. The intensity of the signal indicates the relative distance between the boat and the fish. The bearing to the fish is either determined by rotating a directional hydrophone around or by using multiple hydrophone elements, which do not need to be rotated to indicate direction. The vessels used for active tracking can range from a small skiff to an oceanographic research vessel. Active tracking is labor intensive and tracks typically only last on the order of hours to days. The ability to track generally depends both on the endurance of the crew as well as the sea conditions. Ideally tracks are of sufficient duration to enable the animal to recover from the tagging experience and resume its daily movement patterns. Based on previous data it has been shown that pelagic fish can recover from capture in as little as 4 hours. This, of course, varies between species and individuals and is highly dependent upon the degree to which the fish was stressed during the tagging procedure.
In addition to obtaining the path of the fish from the tracking vessel, transmitters may also encode data such as depth, temperature, heart rate, tail-beat frequency and compass heading. With this suite of parameters it is possible to determine not only where the fish is but also answer other interesting questions pertaining to the physiology of the animal. When coupled with additional sensors on the tracking vessel, the physical characteristics of the environment (e.g. temperature, oxygen concentrations, and prey abundance) can also be quantified and incorporated into a movement model. Active tracking is ideal for answering questions pertaining to the fine-scale movements of a fish; however the disadvantages are that the tracks tend to be short in duration and they are labor intensive.