Passive acoustic monitoring

What are Passive Acoustic Monitoring Methods?

Passive acoustic density estimation involves estimating animal density and/or abundance by detecting the sounds animals make, without necessarily seeing the animals themselves. We have developed acoustic monitoring and estimation methods for cetaceans (whales and dolphins), birds and many other species, including frogs, chimpanzees and gibbons. Cetacean species may spend large proportions of their time vocalising at depth, detectable by sound but unavailable for detection by visual surveys. Birds are often easier to hear than to see. Similarly, some frog species, chimpanzees and gibbons are difficult to see because of the dense habitat they live in, but easy to hear because they vocalise frequently. Such species are suitable for passive acoustic monitoring because they are difficult to detect by any means other than sound.

Methods of estimation from passive acoustic data include distance sampling and spatial capture recapture customised for acoustic detection. Detection might be by human listeners or microphones (on land) and hydrophones (in water). Digital passive acoustic recorders are rapidly replacing humans as detectors because they are relatively cheap, can operate or long periods and may be less sensitive to poor weather conditions. They do, however, generate great volumes of data that need to be scanned to identify target vocalisations. This has sparked research into developing machine learning methods to identify target vocalisations in acoustic recordings and developing statistical methods to estimate density and abundance using the machine learning outputs.

What species are monitored using these methods? 

The methods are suitable for any species that produce detectable sounds and are difficult to detect by other means. This includes many cetacean species, frogs, gibbons, howler monkeys, wolves, bats, some marine and freshwater fishes, and even insects. There might be many more species suitable for passive acoustic monitoring than those for which these methods have been used to date.

Who in CREEM works on these methods?

A few relevant publications by CREEM staff

Wang, Y., Ye, J. and Borchers, D.L. (2022) Automated call detection for acoustic surveys with hierarchically structured calls of varying length. Methods in Ecology and Evolution 13: 1552-1567.

Crunchant, A., Bochers, D.L., Kühl, H. and Piel, A. (2020) Listening and watching: do camera traps or acoustic sensors more efficiently detect wild chimpanzees in an open habitat? Methods in Ecology and Evolution, 11: 542-552.

Loveridge, R., Kidney, D., Ty S., Eang S., Eames, J.C. & Borchers, D.L. 2017. First systematic survey of green peafowl Pavo muticus in northeastern Cambodia reveals a population stronghold and preference for disappearing riverine habitat. Cambodian Journal of Natural History: 157–167

Carlén, I., Thomas, L., Carlström, J., Amundin, M., Teilmann, J., Tregenza, N., Tougaard, J., Koblitz, J.C., Sveegaard, S., Wennerberg, D., Loisa, O., Dähne, M., Brundiers, K., Kosecka, M., Kyhn, L.A., Ljungqvist, C.T., Pawliczka, I., Koza, R., Arciszewski, B., Galatius, A., Jabbusch, M., Laaksonlaita, J., Niemi, J., Lyytinen, S., Gallus, A., Benke, H., Blankett, P., Skóra, K.E. & Acevedo-Gutiérrez, A. (2018). Basin-scale distribution of harbour porpoises in the baltic sea provides basis for effective conservation actions. Biological Conservation, 226, 42–53.

Harris, D.V., Miksis-Olds, J.L., Vernon, J.A. & Thomas, L. (2018). Fin whale density and distribution estimation using acoustic bearings derived from sparse arrays. The Journal of the Acoustical Society of America, 143, 2980–2993.

Sebastián-González, E., Camp, R.J., Tanimoto, A.M., Oliveira, P.M. de, Lima, B.B., Marques, T.A. & Hart, P.J. (2018). Density estimation of sound-producing terrestrial animals using single automatic acoustic recorders and distance sampling. Avian Conservation and Ecology, 13, 7.

Measey, G.J., Stevenson, B.C., Scott, T., Altwegg, R. & Borchers, D.L. (2016). Counting chirps: Acoustic monitoring of cryptic frogs. Journal of Applied Ecology, 56, 894–902.

Kidney, D., Rawson, B.M., Borchers, D.L., Stevenson, B.C., Marques, T.A. and Thomas, L. (2016). An efficient acoustic density estimation method with human detectors applied to gibbons in Cambodia. PLoS ONE, 11, e0155066.

Stevenson, B.C., Borchers, D.L., Altwegg, R., Swift, R.J., Gillespie, D.M. & Measey, G.J. (2015). A general framework for animal density estimation from acoustic detections across a fixed microphone array. Methods in Ecology and Evolution, 6, 38–48.

Harris, D., Matias, L., Thomas, L., Harwood, J. & Geissler, W.H. (2013). Applying distance sampling to fin whale calls recorded by single seismic instruments in the northeast atlantic. The Journal of the Acoustical Society of America, 134, 3522–3535.

Marques, T.A., Thomas, L., Martin, S.W., Mellinger, D.K., Ward, J.A., Moretti, D.J., Harris, D. & Tyack, P.L. (2013). Estimating animal population density using passive acoustics. Biological Reviews, 88, 287–309.

Some relevant links

Gibbon in a tree with blue sky behind it.
Credit: Association Anoulak
Three people in a forest, looking up into the trees.
Deploying acoustic recorders in Laos.
Credit: Association Anoulak