Chelonia Limited

Wildlife Acoustic Monitoring

Frequently asked questions

Q. What does 'passive acoustic monitoring' mean?

Passive acoustic monitoring is the detection of cetaceans by listening for the noises they make. Active acoustic monitoring means sending out very loud sounds and listening for the weak echoes from animals, and is used in fish finders and depth sounders. PODs, however, are PAMs and are silent when logging.

Q. What does 'train detection' mean?

Toothed cetaceans make long series of similar clicks - called trains - made with similar time gaps between successive clicks. The gaps usually show continuous variation - getting shorter or longer, and dolphins also vary the sound characteristic of the clicks. Trains are also produced by boat sonars and can arise by chance from random sources of clicks. The software filters that extract cetacean trains are essential to the POD's ability to accurately identify the presence of the animals and work by identifying coherent trains - those in which the variation in timing and character between successive clicks is lower than might occurs by chance among the many background noise clicks that are logged.

Q. Can PODs distinguish porpoises from dolphins?

Yes. Porpoises are among the minority of species that produce rather long clicks that contain only high frequencies within a narrow band, usually centred near 130 kHz. C-PODs identify enough of the sound characteristics of each click to distinguish these narrow band high frequency (NBHF) clicks from the shorter, more wideband clicks of dolphins.

Q. Can PODs distinguish dolphin species?

Generally no, but a distinction may be possible between larger groups, e.g. distinguishing beaked whales from smaller dolphins.

Q. Should I use a hydrophone or a POD?

If you want to study the frequency spectrum of vocalisations, use a hydrophone and suitably fast recording system. You need a sampling rate at least three times the upper frequency of interest, preferably more. So porpoise clicks, for example, need 500k sample/s. On a 16 bit system this will require about 80 GB storage/day and dedicated software to analyse these large volumes of data.

If you want to monitor animal activity or study their echo-location behaviour over periods of time, C-PODs are easier as the logistics are much simpler, the data volumes much more manageable and accurate automated and standardised post-processing analysis is included.

Q. Can I test PODs by immersing them over the side of a boat in the presence of animals?

Yes, but this can give very misleading results as the animals may be silent in this situation. You can easily check that a POD is working on land - see the section Starting the F-POD in the F-POD User Guide.

Q. Do I need to look at every click train?

No. Some visual validation is a good idea, and it is a quick task to look at ten times in a file and view the first ten trains following each. This generally shows that the false positive rates is too low to be a concern. But if you are studying an area where there are no or few porpoises, false positives may have a significant effect on your data and examination of each train classified as a cetacean train may be needed, but will be quick as there will be few of them.

Q. Can the POD be used for large whales?

No. All large whales, apart from the sperm whale, are baleen whales (as opposed to toothed whales) and do not use ultrasonic echolocation. Sperm whales do produce prodigiously loud clicks at around 9 kHz which is below the range covered by C-PODs, but it may be that there is enough higher sound for PODs to log them - this hasn't been tested yet.

Q. What other species may be difficult to study with PODs?

All odontocetes that have been tested, apart from the sperm whale, are logged and there is nothing in what is known to suggest that any species would not be logged, but some species may be silent more often than they echo-locate.

Q. Is it OK to use 'black boxes' in scientific research?

Yes. Your voltmeter, oscilloscope, sound card, camera, scales, DNA analyser, any AI system, etc., all use crucial internal digital signal processing that is not accessible to you. You use these devices because you are aware that their transfer function is good enough for your purposes. POD performance cannot be validated by studying the design and logic but has been empirically validated in various tank and sea tests as well as thousands of deployments.

Please contact us if you have any other questions.