Individual dolphin calls used to estimate population size and movement

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.

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.”

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.

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