Frequently asked questions

Q. What does 'passive acoustic monitoring' mean?

A. 'Passive acoustic monitoring' is a method of detecting and logging cetaceans by listening to the noises they make. Active acoustic monitoring is achieved by sending out loud sounds and listening for the echo from the animal. This is the method used in fish finders and depth sounders. PODs, however, are silent when logging and are therefore not invasive. Neither will they adversely affect the animal's behaviour.

Q. What does 'train detection' mean?

A. 'Train detection' means recognising the time pattern of clicks that are characteristic of the long series of click trains produced by toothed cetaceans. 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.

Q. Can PODs distinguish porpoises from dolphins?

A. Yes. If one or more scans is set to log clicks centred below 100 kHz, these will reliably identify non-porpoise encounters.

Q. Can PODs distinguish dolphin species?

A. Generally no, but if the species mostly use different frequencies, a distinction could be made.

Q. Can you replay the click sounds?

A. No, but a completely synthetic click could be played in a time sequence logged by the POD.

Q. Should I use a hydrophone or a POD?

A. 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 you will need to find 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, use PODs, as they capture useful click information and store it in files that are smaller by a factor of around 0.5 million and additionally includes the results of standardised analysis.

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

A. This, and towing, can give very misleading results as the animals may be silent, or directing their interest elsewhere, or there may be echoes from the hull or other sources of noise that impede train detection. If you are a new user and want to verify that PODs work, the best thing to do first is to moor them in an appropriate area at sea for a few days.

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

A. Not unless you are studying very low densities of porpoises. 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. If nearly all your detections are classified as 'low probability' with very few associated 'high probability' trains then caution is needed. Ursula Verfuss and colleagues studying porpoises in very low density areas of the Baltic do make visual inspection of all detections, and this is clearly justified for their project. We are continually improving the specificity and sensitivity of the analysis software.

Q. Can the POD be used for large whales?

A. 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 and it is possible that the POD could monitor them, but this has not been tested and might prove too difficult because of the number of echoes of these very loud clicks that will be received, and could block the recognition of trains.

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

A. Orcas may to be difficult to record because they are quite often silent and when they do click the frequency spectrum is so wide that there may be difficulties in getting clean enough data (that is, sufficiently free from non-orca clicks) to enable the train detection to work well.

Please contact us if you any other questions.