The establishment of marine reserves is an increasingly important planning tool for conservation. To maximize the effectiveness of such protected areas, decision makers need to detect critical habitats for over-exploited species, a task which requires knowledge of their spatial ecology. Tracking an animal on land is a challenge, but in the marine environment, where humans are physically limited in time and space, it seems a virtually impossible task. In the 1980's scientists found a powerful solution:
Watch this video to see how acoustic telemetry works
In acoustic telemetry animals are tagged with a transmitter, which produces ultrasonic frequencies (inaudible to fish) and constantly emits a unique barcode. A number of receivers are mounted on the seafloor and form listening lines, which can be up to 50km long. When a tagged animal passes that line, the signal is received and logged (together with salinity and water temperature) at the nearest listening station. Once the researchers retrieve the receivers they obtain the logged information and are able to make conclusions about:
- Where animals are
- How fast they travel
- Where they die
The use of acoustic telemetry has helped to track vertical movements and to reveal spatial patterns for a number of iconic marine species. Although the majority of scientists is using acoustic telemetry for research on bony fish and other marine organisms, the tagging of charismatic Chondrichthyan species, such as Great White Sharks (Carcharodon carcharias), has received significant attention in public media.
Acoustic tags - Picture by www.htisonar.com (modified)
Acoustic telemetry has revealed a lot of baseline information on species that urgently need protection. But recent studies also showed that there might be reason for concern about the method: Although the signals that are emitted by acoustic tags are inaudible to fish, some marine mammals are able to perceive them. It was shown that seals actively use signals from acoustic fish tags to seek out prey (feeding signal = dinner bell effect). Another study suggests that it can warn seals of a predator (early warning system = cat bell effect). This could potentially implicate unintended consequences on acoustically tagged sharks, which interact with marine mammals as predators (e.g. seals and dolphins) as well as prey (e.g. orcas).
Watch this video to see how a shark is tagged
Does this mean acoustic telemetry is causing more harm than good? It is very unlikely. The effects of the emitted sounds on predator-prey dynamics remain largely unstudied in the natural environment. Undoubtedly acoustic telemetry has delivered essential information for the protection of species and improved our understanding of conservation planning. It is important to comprehend that acoustic telemetry is a tool, which should be used with care and for the purpose of answering a research question.
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