All words and images by biologists from WCA Partner Futurismo Azores Adventures and Cristina Marcolin. Header photo of two sperm whales going for a dive by Cristina Marcolin.
As you probably know, whale watching tours provide an excellent opportunity to collect information about cetaceans. At Futurismo, we usually record the location and some other notes about a species observation, and we often take photos valid for identification of individuals when possible. Both kinds of data are already widely used and collected by whale watching vessels worldwide. However, there is another kind of data, which is a bit more challenging to collect while with tourists – or at least that was what we thought before!
We frequently use hydrophones to look for sperm whales at sea and, more recently, to collect some audio recordings from them. Until now, these audio files were mostly used to share the amazing sounds with our guests, just enjoying the beauty of it, showing us a little bit more about the huge and unknown world below the water.
However, since last year, Cristina Marcolin (a student from the University of Genova) has been working with these acoustic files to estimate the size of the sperm whales recorded during our whale watching trips. Do you know how that is possible?
Sperm whales are a unique species among cetaceans and their most unique feature is their nasal apparatus. They have only one nostril, located on the top-left part of the head. A complex system for sound production is present inside their heads, as shown in the figure below:
Sperm whales produce a variety of noises through this apparatus, mostly click-type sounds. These are usually echolocation clicks, used by an individual during their deep dives to capture prey, and also codas, click patterns similar to songs that characterise each sperm whale clan around the world and are used for communication.
Each sperm whale click generally consists of several pulses. The time distance between two pulses (inter-pulse interval, or IPI) is related to the size of their sound generator organ, the spermaceti. The size of this organ can be used to estimate the size of the head, and thus, also the total length of the animal by applying some standardised formulas (Gordon et al., 1991; Growcott et al., 2011).
For her master thesis, Cristina implemented an updated and improved protocol on board our whale watching boats to record the sperm whale clicks after fluking. Together with the rest of the team, dozens of recordings were collected and analysed. She used different approaches to get the IPIs for each individual, to calculate the best possible value to use in the formulas for length estimation.
Cristina estimated more than a hundred body lengths! Most of the measurements were smaller than 12m (i.e. females or young males), while only 28% were larger than 12m (i.e. adult males). It’s worth noting that each measurement didn’t necessarily belong to a different individual, as audio files were not always possible to relate to identification.
Complementing our previous research, Cristina also managed to estimate the length of some of our well-known sperm whales. Do you remember our beloved female sperm whale called Orca? We’ve known her for a long time, and only now, we estimate her length as 9m… big girl! All of these findings are fundamental for conservation purposes, as the population size and age structure are important to assess the health of a species stock.
We must say that it wasn’t always nice and easy! Collecting useful data onboard whale watching vessels is challenging, and trial and error are regular parts of the process. However, we realised that with little methodological adjustments suitable to be done in regular whale watching trips, valuable acoustic data for scientific purposes can be collected. We would also like to recognise the extra effort made by the crew, both to collect the information and to learn and inform our guests about the ongoing research and the wonders of an acoustic ocean just below.
For sure, the chance to listen to one of the most incredible creatures of the world is something we won’t easily forget.
Gordon, J. C. (1991). Evaluation of a method for determining the length of sperm whales (Physeter catodon) from their vocalizations. Journal of Zoology, 224(2), 301-314. https://doi.org/10.1111/j.1469-7998.1991.tb04807.x
Growcott, A., Miller, B., Sirguey, P., Slooten, E., & Dawson, S. (2011). Measuring body length of male sperm whales from their clicks: the relationship between inter-pulse intervals and photogrammetrically measured lengths. The Journal of the Acoustical Society of America, 130(1), 568-573. http://dx.doi.org/10.1121/1.3578455