Sound-recording acoustic tags attached to marine animals are commonly used in behavioural studies. Measuring ambient noise is of interest to efforts to understand responses of marine mammals to anthropogenic underwater sound, or to assess their communication space. Noise of water flowing around the tag reflects the speed of the animal, but hinders ambient noise measurement. Here, we describe a correlation-based method for stereo acoustic tags to separate the relative contributions of flow and ambient noise. The uncorrelated part of the noise measured in digital acoustic recording tag (DTAG) recordings related well to swim speed of a humpback whale (Megaptera novaeangliae), thus providing a robust measure of flow noise over a wide frequency bandwidth. By removing measurements affected by flow noise, consistent ambient noise estimates were made for two killer whales (Orcinus orca) with DTAGs attached simultaneously. The method is applicable to any multi-channel acoustic tag, enabling application to a wide range of marine species.
The authors declare no competing or financial interests.
A.M.v.B.-B. conceived the study, processed and analyzed DTAG data, and wrote the paper. P.J.O.M., P.J.W. and F.I.P.S. contributed to data collection and interpretation. S.P.B. provided the code for measuring the coherence function, and input on flow noise theory. P.J.W. analyzed animal swim speed, and carried out the DTAG calibration. F.I.P.S. and A.M.v.B.B. performed manual audits of the DTAGs for detecting transient sounds. All authors contributed to the manuscript editing.
A.M.v.B.B. and P.B. were funded by The Netherlands Ministry of Defence. Fieldwork efforts and support for P.M. and F.S. was provided by the US Office of Naval Research [award numbers N00014-08-1-0984 and N00014-10-1-0355]. P.W. received a PhD studentship with matched funding from The Netherlands Ministry of Defence (administered by The Netherlands Organisation for Applied Scientific Research, TNO) and UK Natural Environment Research Council [NE/J500276/1].
Supplementary information available online at http://jeb.biologists.org/lookup/doi/10.1242/jeb.133116.supplemental
- Received October 13, 2015.
- Accepted May 22, 2016.
- © 2016. Published by The Company of Biologists Ltd