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:

Acoustic telemetry.

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.

References

Abecasis, D., Afonso, P., & Erzini, K. 2015. Towards adaptive management of coastal MPAs: The influence of different conservation targets and costs on the design of no-take areas. Ecological Informatics. DOI: 10.1016/j.ecoinf.2015.08.009

Bowles AE, Denes SL, Shane MA. 2010 Acoustic characteristics of ultrasonic coded transmitters for fishery applications: could marine mammals hear them? J. Acoust. Soc. Am. 128, 3223–3231. DOI:10.1121/1.3493438

Capello, M., Robert, M., Soria, M., Potin, G., Itano, D., Holland, K.,Dagorn, L. 2015. A Methodological Framework to Estimate the Site Fidelity of Tagged Animals Using Passive Acoustic Telemetry. Plos One, 10(8), e0134002. DOI:10.1371/journal.pone.0134002

Cooke SJ, Iverson SJ, Stokesbury MJ, Hinch SG, Fisk AT, VanderZwaag DL, Whoriskey F. 2011 Ocean Tracking Network Canada: a network approach to addressing critical issues in fisheries and resource management with implications for ocean governance. Fisheries 36, 583–592. DOI:10.1080/03632415.2011.633464

Cowley, P.D., Bennett, R.H., Murray, T.S. 2015. Acoustic Tracking Array Platform Status Report (2011-2014). South African Institute for Aquatic Biodiversity. Grahamstown. 22pp.

Cunningham KA, Hayes SA, Wargo-Rub AM, Reichmuth C. 2014 Auditory detection of ultrasonic coded transmitters by seals and sea lions. J. Acoust. Soc. Am. 135, 1978–1985.

Jefferson T.A., Curry B.E. 1996. Acoustic methods of reducing or eliminating marine mammal fishery interactions: do they work? Ocean Coast.Manage 31, 41–70. DOI:10.1016/0964-5691(95)00049-6

Ocean Tracking Network.

https://docs.google.com/file/d/0Bw4TSOL75rgcRmNnT2ZibGx1Mnc/edit?pli=1 (Last accessed: 09/02/2016)

Stansbury, A.L., Götz, T., Deecke, V.B., Janik, V.A. 2014. Grey seals use anthropogenic signals from acoustic tags to locate fish: evidence from a simulated foraging task. Proceedings of the Royal Society B 202: 20142595 DOI:10.1098/rspb.2014.1595

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