The Role of Humpback Whales in the Bazaruto Archipelago

This article is written by Iris Uijttewaal, with contributive information provided by Katya Kalashnikova.

July marks the start of humpback whale (Megaptera novaeangliae) season in the Bazaruto Archipelago, with two individuals already observed off Benguerra Island. As a cosmopolitan species, humpbacks travel thousands of miles, covering all oceans and undertake extensive annual migrations from high-latitude summer feeding grounds to low-latitude winter breeding grounds – and back. The whales travel to the warm waters of the Mozambique Channel as part of their annual migration route, as the warm sea temperatures make for ideal breeding grounds. They come to this area to mate, give birth and nurse, so they are often sighted with calves. 

The humpback whales’ behaviour monitored along the coast of east Africa indicates reproductive activities, which proves that the area is used as a breeding ground. The presence of new-born calves, observation of competitive groups and records of singing males are indicators that the Bazaruto Archipelago may play a crucial role in humpback whales’ ecology as a breeding habitat.

The role of humpback whales within the ecosystem

Whales play an important role within the ecosystem and provide some essential eco services, including the contribution to phytoplankton (a staple food source at the lowest trophic level for the entire aquatic food web) growth by dispersing their nutrient-rich faeces and urea during their migration, transporting iron and nitrogen from richer to poor areas.  

Phytoplankton absorbs carbon dioxide (CO2) and releases oxygen (O2), contributing up to 50% of all atmospheric O2 through the process of photosynthesis. Phytoplankton is being fed on by zooplankton, which once dies, takes carbon out of circulation and to the ocean floor, where it may remain for hundreds of years, as well as great whales when they naturally die and sink, as due to their impressive sizes and long lives they accumulate large amounts of carbon too. Even though the latter is considered as circumstantial in the carbon cycle in terms of the magnitude, whales’ contribution to the greenhouse gas reduction may be underestimated. Interestingly, both zooplankton and whale carcasses ended up on the seabed become an important food source for marine life at depths, shifting the carbon source to the sea floor, which trigger various 
processes including dominance events.

Our Chief Scientist, Dr. Mario Lebrato, has been studying these processes for a number of years and has published numerous papers, including the most recent one on how gelatinous carbon impacts benthic megafaunal communities, and you can read about it in our article

“I think it is fascinating how different carbon compartments end up all in the same place, but fill different ecosystem services, e.g., whale carcasses carbon provide food for months, or even years, while jelly-C is episodic, and vanishes in less than a month”
– Dr. Mario Lebrato


Like other large cetaceans, the whaling industry severely depleted the numbers of humpbacks by hunting, prompting a hunting ban in 1966 and their inclusion on the endangered species list in 1970 by the International Whaling Commission. Despite signs of increasing humpback whale numbers in both hemispheres, global population recovery may not represent the recovery of each of the sub-populations, and present numbers may still be far behind pristine abundance. Yet humpback whales face many threats, including plastic and sound pollution, unregulated tourist activities, marine traffic (including collisions with ships), oil and gas exploration on top of the global environmental changes. Yet, their importance for the climate change mitigation has been increasingly evident and underpinning pertinent research and protection measures.  

BCSS’s humpback whale data collection

From July until December, the BCSS team records opportunistic humpback whale sightings alongside spatiotemporal and environmental meta-data on expeditions, adding to the BCSS Ocean Observatory database of Theme 2 (Species ID & Habitat Mapping). To get more in depth insights we are currently working on setting up a project where data on the ecology of migrating humpback whales will be systematically collected, which aims to become a long-term cetacean monitoring program, embracing other marine mammals as well. We aim to open the program for volunteers and interns sometime during 2023.

As part of the Blue Economy model, Mozambique is moving towards a marine spatial planning process. If the model is to ensure that ecosystem health and function is the foundation of economic development, we need to address habitat use by species that are vulnerable, threatened, endangered and over-exploited. Conducting pertinent research to determine how the habitat is used by humpback breeding aggregations in the Bazaruto Archipelago provides a unique opportunity to evaluate large spatial zones. Results of this program will give Mozambique the opportunity to engage in broader regional cetacean research and monitoring projects, which will aid in better management and protection of the species.

About the researcher who supported this article

Katya Kalasnikova has been studying humpback whales along the coast of East Africa for several years. Since 2017, she has been collecting data on the whales’ ecology in Tanzania. She aims to establish a long-term marine mammals monitoring program in Mozambican waters with BCSS, and hopes to grow this project in a sustainable way with increased local participation, as she strongly believes in community-based conservation. Katya is a member of regional marine organizations such as WIOMSA and IndoCet. 

For scientific questions please contact:
Katya Kalashnikova, Bazaruto Center for Scientific Studies

For other questions about this article, please contact:
Iris Uijttewaal, Bazaruto Center for Scientific Studies

Bazaruto Center for Scientific Studies
Host of the first permanent Ocean Observatory focused on multi-ecosystem time series research in Africa, the Bazaruto Center for Scientific Studies (BCSS) was established in 2017 as in independent, non-profit organisation with a mission to protect and support the fragile ecosystems of the Bazaruto Archipelago, Mozambique. The research station is located on Benguerra Island, off the coast of Mozambique.

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