ESO Completes Arabian Sea Humpback Whale Acoustic Analysis: A Sound is Worth a Thousand Words

الحوت الاحدب، Hupback Whales

Humpback whale off the coast of Oman (copyright Environment Society of Oman).  Male humpback whales sing complex songs underwater. Passive recorders stationed in various locations off the coast have confirmed that Arabian Sea humpback whales sing different songs to their Southern Hemisphere Indian Ocean counterparts, and provided insight into the seasonal variations in humpback whale song in different locations.  One surprise finding includes the detection of Southern Hemisphere singers at one location during the Omani summer, when Arabian sea whales were not singing.

The following is a press release from the Environment Society of Oman:

The Environment Society of Oman (ESO) in partnership with New England Aquarium (NEA) have recently finalised their findings on a two-year acoustic dataset on the Arabian Sea Humpback Whales (ASHW). The aim of the project was to document spatial and temporal distribution of Arabian Sea humpback whales in the region, investigate singing behaviour and geographic variation, as well as assess potential threats to the population posed by anthropogenic noise. The project was conducted off the coast of Oman in Hallaniyats Bay and the Gulf of Masirah from 2011 to 2013.

Currently listed as Endangered by the IUCN Red List, the Arabian Sea humpback whale has a population estimated to be less than 100. The study was undertaken as a matter of urgency and as a means to identify conservation solutions by acoustically assessing the presence and seasonality of whales, and monitoring the amount and effects of ambient noise on whales. Measuring sound is a critical factor for cetaceans as hearing is their primary sense used for foraging, migration and reproduction, and impairment of communication and hearing can have serious population consequences. The study involved three components, seasonal and geographic detections of humpback whale vocalizations off Oman, characterizing ambient noise in the monitored regions, and comparing song structure variation across the western Indian Ocean, with ground-breaking results.

Suaad Al Harthi, Executive Director at the Environment Society of Oman, stated “I would like to thank our sponsors Shell Development Oman for funding and supporting this project. The research and the acoustic analysis has revealed valuable information about the Arabian Sea humpback whales, confirming the areas that are important to them, and the potential impact of noise pollution. Our conservation program is ongoing and with the support of local and international partners we are able to develop and address conservation concerns.”.

The detection of whale presence involves the highly effective method of Passive Acoustic Monitoring (PAM) for assessing distribution across broad spatial and temporal scales. From continuously recording at three sites for two years, and a total of 1,369 acoustic recording days, it was discovered that the population utilizes both the Gulf of Masirah and Hallaniyat Bay for breeding. However, the Hallaniyats, had more frequent singing of Arabian Sea Humpback Whales, suggesting it may be a more important breeding area. Nevertheless, the study clearly indicated that both locations are considered “hot spots” for the population. There was also a shift in distribution from south to north into the Gulf of Masirah towards the end of the breeding season, apparently as the population shifted into the non-breeding season.

Muna Al Shukaili, General Manager of External Relations and Social Investment Lead at Shell Development Oman said, “We are proud to sponsor this initiative and help raise awareness on the impact human activities have on the Arabian Sea humpback whales. This is the reason we have collaborated with ESO and we hope that a lasting solution can be found for these vulnerable mammals. Preserving the ocean biodiversity is part of Shell’s commitment towards the environment and sustainable development in Oman.”

Andrew Willson, Senior Marine Consultant at Fives Oceans Environmental Services, said, “This study has been a break-through for the team’s work in the Arabian Sea given that the sophisticated equipment and subsequent analysis has allowed us to monitor for the occurrence of whales within their critical habitat almost year round. This significantly extends knowledge gained from conventional small vessel surveys conducted along the coast of Oman over the last 18 years. The technique has capitalised on one of the key traits of marine mammals; that they are highly dependent on communication through acoustics for their survival”.

Considered the most consequential finding of the overall study in the context of conservation, the analysis of ambient sound sought to determine potential threats in each location.  Using sophisticated standardized analyses, biological, physical and anthropogenic noise sources were assessed to provide a profile of the “soundscape” over time and across frequencies.  Areas around Port Duqm with elevated anthropogenic noise appeared to have a decreased level of humpback whale singing activity, suggesting that whales were either disturbed or their song displays were masked in vicinity of the port. This indicates that further assessment and action in the preservation of the Arabian Sea humpback whale is highly advisable.

Dr. Salvatore Cerchio, the Project Lead Researcher from the New England Aquarium said, “In-depth analyses and findings around the world on the impact of loud anthropogenic noise sources on whales are compelling. This type of noise pollution is widely recognised as having a negative effect on the marine life and marine ecosystems alike. Our findings off Oman indicate that the same processes are likely at work in the Arabian Sea, and could be a contributor to the low number of Arabian Sea humpback whales living off the coast of Oman. What we need to do now, is work together with governments towards putting in place a marine noise policy and learn how to better monitor and manage it.”

The final section of the acoustic analysis involved an oceanic comparison of humpback whale songs that benefited from a large-scale international collaboration, with contributions from researchers with song samples from several regions, including National Institute of Ocean Technology, India; Globice, Reunion Island; and Accademia del Leviatano, Italy, working off the Comoros Islands.  The song structure of humpback whales off Oman were compared to samples from the west coast of India and from the Southwest Indian Ocean (SWIO) to assess isolation of the population from Southern Hemisphere populations, and to describe their singing behaviour. Analysis indicated several key findings, revealing that songs from Oman and India across the Arabian Sea appeared to be very similar, but that the ASHWs song remained distinct and consistently different from the SWIO song across the years. This finding reinforces our understanding of isolation of the ASHW as previously indicated by genetic and photographic studies.

However, two surprise findings were also of great interest. Unlike humpback whale song around the world, which changes progressively each year, the song of whales off Oman changed very little across three years, remaining virtually the same during the study period.  Moreover, it was found that some Southwest Indian Ocean animals moved into the Arabian Sea during the Omani summer when the ASHWs were not singing; however, the ASHWs did not learn or pick up there song as happens elsewhere in the world.  The overall conclusion drawn from this study of singing behaviour is that behavioural isolation mechanisms may exist to prevent the mixing of the Arabian Sea population with Southern Hemisphere animals.

In summary, the acoustic study of the Arabian Sea humpback whales has provided many new insights into the behaviour and conservation of the population. Only through in depth studies, and large-scale international collaborations as demonstrated in this project, will the recovery of the Endangered Arabian Sea humpback whale population be assured.

press coverage for this report includes:

You can download copies of the reports on this study that were presented to the Scientific Committee of the International Whaling Commission in 2016 and 2018.