Over the course of five years, the BCSS marine research station had monitoring effort of 3860 hours in the open sea of the Mozambique Channel. Its growing database now boasts records of at least nine cetacean species, documented through over 1000 sightings, totalling over 6000 animals encountered. Notably, more than 1000 of these individuals are migrating humpback whales. These remarkable counts stem from visual observations, highlighting species richness of the area, and underpinning further protection of this critical habitat.
As the humpback whales prepare to leave Mozambique for their Southern Sea feeding grounds, we find ourselves pondering the intricacies and complexities of this annual blue migration in the Mozambique Channel.
Humpback whales embark on a seasonal migration between their high-latitude summer feeding areas and low-latitude winter breeding grounds. When southern hemisphere humpbacks are in tropical waters during the southern hemisphere winter, their northern hemisphere counterparts are feasting in high-latitude regions during the northern hemisphere summer. This pattern reverses every six months, when the seasons invert. Every year we witness a northward migration and the southward migration of humpbacks within the WIO region.
However, migratory timing varies, influenced by age, sex, and reproductive status. This results in different peaks in migration for various reproductive classes. During the journey to breeding grounds, lactating females are the first to migrate north. They do so not for breeding but to provide their calves with favourable conditions, including protection from killer whale predation at low latitudes. Immature whales and non-pregnant, non-lactating females follow, with pregnant females migrating last. The return journey to summer feeding grounds occurs in reverse order, with females and calves being the last to leave the breeding grounds.
Humpback whales embark on a seasonal migration between their high-latitude summer feeding areas and low-latitude winter breeding grounds. When southern hemisphere humpbacks are in tropical waters during the southern hemisphere winter, their northern hemisphere counterparts are feasting in high-latitude regions during the northern hemisphere summer.
Satellite tracking studies have unveiled the details of this departure process, showing similarities in migratory patterns across the WIO region, to which Mozambique belongs. However, despite evidence of southward departure beginning in late October, some individuals may linger in the breeding grounds until December.
When it comes to feeding, some researchers suggest that humpback whales rely on stored fat and typically do not feed during the austral winter. However, there is evidence of opportunistic feeding at stop-over areas along the migratory route, outside their primary feeding grounds in the Southern Sea.
In the Antarctic, during the austral summer, humpback whales are known to inhabit the waters of the Southern Ocean, often near marginal sea ice and regions of Antarctic krill density. In June and July, they begin their northward migration. Yet, some individuals, likely non-pregnant, non-lactating females, may not always migrate. Acoustic observations have revealed that not all Southern Hemisphere humpback whales head toward the equator at the end of the Antarctic summer. Some remain in the eastern Weddell Sea in the Antarctic region throughout the winter, leading to the hypothesis that young, non-pregnant females skip the energetically costly migration.
These variations in migratory patterns, timing, and other parameters show that feeding and breeding periods may overlap. Imagine a scenario around this time of year, with a young female humpback whale (who never left the feeding grounds) in Antarctica, foraging alongside a mature male who has successfully returned from the breeding grounds. Meanwhile, a mature non-pregnant female opportunistically feeds en route along the west coast of Madagascar, and other gulps plankton at the Crozet Islands while making her way to the Southern Sea. Simultaneously, a mother and her calf revel in the warm waters of Bazaruto, with her escort patiently awaiting her readiness for departure. These individuals are spread across various locations, each engaged in distinct activities yet interconnected by the same migration.
For those who share our passion for whales and dolphins, we invite you to join our diving expeditions during the 2024 migration season. Please note that space is limited due to the size of our research station, so securing your spot early guarantees participation during migration peaks and under the best conditions. Alternatively, consider enrolling in one of our scientific training programs to learn about data collection by our scientists, contributing to the expanding knowledge of one of the most awe-inspiring blue migrations.
Photo credits: Bazaruto Center for Scientific Studies, Orlando Miranda, Salvador Colvee
For questions about this article, please contact:
Ekaterina Kalashnikova, Bazaruto Center for Scientific Studies
Ekaterina.Kalashnikova@bcssmz.org
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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