Individual dolphin calls used to estimate population size and movement

Mother and calf bottlenose dolphins - often mothers and calves use signature whistles to stay in contact (Credit: Simon Elwen)

An international team of scientists has succeeded in using the signature whistles of individual bottlenose dolphins to estimate the size of the population and track their movement.

The research, led by the University of Plymouth and Stellenbosch University, marks the first time that acoustic monitoring has been used in place of photographs to generate abundance estimates of dolphin populations.

Writing in the Journal of Mammalogy, researchers say they are excited by the positive results yielded by the method, as the number of dolphins estimated was almost exactly the same as estimated through the more traditional photographic mark-recapture method.

They are now working to refine the technique, in the hope it can be used to track other species – with a current focus on endangered species such as humpback dolphins.

Quicker information processing and advances in statistical analysis mean in the future that automated detection of individually distinctive calls could be possible. This can generate important information on individual animals and would be particularly useful for small, threatened populations where every individual counts.

The paper’s senior author Dr Tess Gridley, Co-Director of Sea Search and the Namibian Dolphin Project and a postdoctoral fellow in the Department of Botany and Zoology at Stellenbosch University, said:

"The capture-recapture of individually distinctive signature whistles has not been attempted before. The dolphins use these sounds throughout life and each has its own unique whistle. Therefore, by recording signature whistles over time and in different places we can calculate where animals are moving to and how many animals there are in a population.”
Working with Dr Simon Elwen of Stellenbosch University, the Namibian Dolphin Project has been researching Namibia’s resident bottlenose dolphins for the past 12 years, and built up a catalogue of more than 55 signature whistles dating back to 2009.

This particular study was led by Emma Longden, who began the project during her BSc (Hons) Marine Biology degree at the University of Plymouth. As an undergraduate, Emma completed an internship with the Namibia Dolphin Project for a month in 2016, and returned again in 2018 to complete work on the mark-recapture project.

She analysed more than 4000 hours of acoustic data from four hydrophones positioned along the coast south and north of Walvis Bay, Namibia, during the first six months of 2016.

All in all, they identified 204 acoustic encounters, 50 of which contained signature whistle types. From these encounters, 53 signature whistle types were identified; 40 were in an existing catalogue developed in 2014 for the Walvis Bay bottlenose dolphin population, and 13 were newly identified.

Of the 53 signature whistle types identified, 43% were captured only once, whereas the majority (57%) were recaptured twice or more.

Emma, whose work on the project was also supervised by Dr Clare Embling, Associate Professor of Marine Ecology at the University of Plymouth, said:

“One of the great things about bioacoustics is that you can leave a hydrophone in the water for weeks at a time and collect so much data without interfering with the lives of the animals you are studying."

Hearing the calls of the wild

Emma Longden started working in Namibia as part of her BSc (Hons) Marine Biology degree and the research she has conducted has resulted in her being lead author on a piece of scientific research
Read more about Emma's experiences

Dr Embling, part of the University's Marine Vertebrate Research Group, added:

“This work is incredibly important as it allows us to track and count the number of dolphins in small vulnerable populations. It builds on our previous research looking at the impacts of noise on marine organisms and monitoring vulnerable marine mammal populations. It also showcases the fantastic level of research that our marine biology students are able to achieve, and the opportunities available to them through our partnerships with conservation organisations such as the Namibia Dolphin Project and the Ocean Giants Trust."

<p>Bottlenose dolphins (CREDIT: TESS GRIDLEY)</p>
(Image: Tess Gridley, Namibian Dolphin Project)
<p>Emma Longden capturing dolphin data</p>
<p>Namibia dolphins</p>
(Image: Simon Elwen, Namibian Dolphin Project)

The full study - Longden et al: Mark–recapture of individually distinctive calls—a case study with signature whistles of bottlenose dolphins (Tursiops truncatus) - is published in the Journal of Mammalogy, doi: 10.1093/jmammal/gyaa081.

Collaborating on ocean conservation

Through an ongoing partnership facilitated by the Ocean Giants Trust, the University is working with Sea Search to enable students to get hands-on experience of ocean conservation. A series of scholarships are available to those enrolled on marine biology and conservation undergraduate courses at the University, and two students are currently working on three-year associations with Sea Search.


Read more about our work with the Ocean Giants Trust

Marine biology

Plymouth has an unrivalled location and reputation as a centre for marine biology.

A diverse range of rich coastal habitats – estuaries, sandy beaches, and rocky shores – are readily accessible for field trips and your own projects.

Find out more about marine biology