Off Oman's coast lives a small population of just over 80 Arabian Sea humpback whales (ASHWs). They are classified as endangered and thought to be the only humpback whale population that doesn't undertake seasonal migrations across the world's oceans. Now, to better understand movement patterns of this population, an international team of researchers has tracked their movements over several years. The results were published in Frontiers in Marine Science.
"We show ASHWs predominantly stayed within a very restricted home range along the coast of Oman," said first author Dr Andrew Willson, a marine scientist and founding director of Future Seas Global SPC. "Alongside very localized movements, we also document the first long-distance movement of an ASHW across the Arabian Sea, pointing to the possibility of other important habitats within the Arabian Sea."
Resident whales
Previous studies suggest ASHWs have diverged from southern hemisphere humpback whale populations around 70,000 years ago. Unlike their relatives, ASHWs usually only move within Arabian Sea waters. "This fundamentally challenges our understanding of humpback ecology. At some point in their history, ASWHs successfully adapted to a radical shift in their environment and foraging strategy," said Willson.
The study used 14 satellite tags that monitored whales' dives. Tags were deployed at two locations, Hallaniyat Bay and the Gulf of Masirah, and transmitted 53 days on average, sending a total of just over 1,800 locations for all tagged whales.
"Leading into this study we had so many questions about the daily life of the mysterious humpback whales found in the Arabian Sea, of which we would only get fleeting glimpses during long hot hours of boat surveys," said Willson. "Tagging these whales allowed us to peel back the lid of the sea and check in online to see where they were each day."
Of the whales tagged in the Gulf of Masirah, five remained in the area for the time during which tags transmitted locations, and two travelled south to Hallaniyat Bay. All six animals tagged in Hallaniyat Bay, moved between the Gulf of Masirah, Hallaniyat Bay, and northern Yemen. The Gulf of Masirah was the most important habitat for ASHWs, and 57% of locations were transmitted from there, followed by 18% transmitted from Hallaniyat Bay. Accordingly, home and core ranges of ASHWs reflected strong site fidelity between areas less than 400km apart.
Due to monsoons, the western Arabian Sea is one of the most productive upwelling zones globally and can support ASHWs year-round. "We think the movements of the whales in our study relate to their tracking of inshore prey, likely sardines, over the continental shelf. Deeper dives off the continental shelf could be related to searching for other food found in deeper water, such as krill," explained Willson.
Whale on a trek
Just one female whale known as Luban – named in Arabic for the frankincense-shaped pattern on her tail fluke – significantly deviated from highly localized travel. She traveled east across the Arabian Sea and was detected off the west Indian state of Goa. Luban covered around 7,000km on her return journey. While humpback whale song has previously been connected between Omani and Indian coasts, Luban's journey provides the first direct evidence of an ASHW crossing the Arabian Sea.
The area where Luban remained off the southern coast of India for up to a month is also known for its high productivity. The team believes that finding food or reproducing – key factors that motivate humpbacks to travel – may have been simultaneous drivers for her journey. Recently, Luban has been spotted again in the Gulf of Masirah. "It's always a relief when we make resightings of these amazing whales given the limited size of the population," said Willson.
Adaptation artists
The team hopes their work will help protect a small and isolated population of whales that's facing impacts from climate change and human activity. Data collected in this study can, for example, help ensure that fishing activities do not impede these animals in their habitat.
"Coastal fishing communities in Oman have revered and respected these whales for many generations," said Aida Al Jabri, a marine expert supporting the study with the Oman Environment Authority. "For rapidly modernizing societies in this region the study puts these whales more into view. This is critical to supporting their conservation."
The team pointed out that tracking data alone cannot answer all questions and future vessel surveys are needed to confirm how ASHWs use this habitat and how they respond to climate change and other threats.
"The Arabian Sea provides unique conditions allowing a once-migratory species to completely change its ecology. It's a testament to how extraordinary the region is," concluded co-author Suaad Al Harthi, the executive director of the Environment Society of Oman, the local research partner in the project. "We hope their adaptability will help ASHWs in uncertain times when their domain is influenced by accelerated climate change."
