Baleen whales* are large whales with a filter-feeding system (called baleen) in their mouths. Also known as “mysticetes”, they are famous for undertaking some of the longest recorded migrations amongst mammals. In fact, gray whales hold the record for the longest distance traveled by any mammal ever! Back in 2011, a gray whale was tracked swimming from Sakhalin island, Russia, to Cabo San Lucas, Baja California, and back – a whopping 22,511 kilometers1, and that too without any pitstops for feeding (see Figure 1)! Most Baleen whales, like blue whales and humpback whales, migrate seasonally within a single ocean basin (e.g. Atlantic, Pacific, etc) between the polar regions (where they feed), and tropics (where they mate and breed). Although some, like bowhead whales, prefer cooler climes and only travel between the Arctic and the sub-arctic regions.

Cold water is richer in nutrients than warmer, tropical water. Across our oceans, cold water from the depths moves upwards to the surface through a process called upwelling, bringing with it plenty of nutrients. At the poles, this nutrient-rich water can support lots of phytoplankton (microscopic marine algae) and an enormous amount of krill (small crustaceans which are a diverse group of invertebrates that include crabs, lobsters, shrimp, krill, copepods, amphipods, and more sessile creatures like barnacles) that some baleen whales love to feast on. Did you know that some whales eat up to 4 tonnes a day – about the same as the weight of an adult elephant? No wonder, so many whales flock to the poles on an annual basis to fill up before migrating to their breeding grounds! Baleen whales are creatures of habit. They almost always visit the same breeding and feeding grounds, year after year after year. Some scientists even believe that this strong preference for feeding grounds (like preferring to eat at your favourite restaurant) plays a role in the decline in population numbers of North Atlantic Right Whales. In spite of their habitat being cut through by extremely busy shipping lanes, North Atlantic right whales keep going back. It doesn’t make sense to return to a place that is dangerous, does it? BUT if that is where all the food is, then where else WOULD you go? 

In the Northern Indian Ocean (NIO) however, things are different. Populations of both pygmy blue whales and humpback whales do not undertake typical poleward migrations but remain in the NIO year-round. Strange, isn’t it?

The pygmy blue whales of Sri Lanka have been observed feeding and pooping in the warm tropical waters just 5 degrees above the equator. This discovery led Oceanswell’s founder Dr. Asha de Vos to launch the Sri Lankan Blue Whale Project, now the longest-running blue whale project in the NIO, because she wanted to understand WHY. Why do they not leave the warmth of these waters to areas that are typically considered better feeding grounds? Turns out not only are they homebodies, but Sri Lankan blue whales also have a unique palate! They feed primarily on sergestid shrimp rather than krill (the top food choice of blue whales in other ocean basins)2, which are present abundantly in the NIO within a depth of 300 m. Since the whales have to dive deeper to catch sergestid shrimp, Sri Lankan blue whales lift their flukes up before a deep dive more often than other populations around the world!2

The unique population of humpback whales off the coast of Oman, known as the Arabian Sea Humpback Whales (ASHW), are also non-migratory and stay within the NIO all year long. The ASHW stay near Oman for the most part, occasionally traversing the Arabian Sea to the west coast of India. Apart from their songs, the ASHW population differs from the humpback whale populations of the Southern hemisphere in when they breed. Breeding cycles are the months in which the whales mate and give birth. The breeding cycle of the ASHW is the same as that of other humpback whale populations of the northern hemisphere (like the North Pacific humpback whales or the North Atlantic humpback whales) which extends  from December to March. On the other hand, the breeding season of the whales of the southern hemisphere extends from July to October.3

What is so special about the Northern Indian Ocean that unique populations of not just one, but two different baleen whale species have made it home? Well, home is where the food is! The Indian Ocean is the only ocean basin with no polar end – it is capped by the Eurasian landmass. Migrating all the way to Antarctica from the central part of the NIO uses a lot of energy. So why migrate when you don’t have to? Especially when the NIO is actually quite productive. Food is in enough supply throughout the year that the whales can thrive thanks to the unique monsoon circulation and how it interacts with the seafloor and surroundings. 4 

The ability of these resident whales to feed year-round depends on the intricate oceanographic dynamics of the NIO. A major cause for concern is climate change affecting these oceanographic conditions, which may lead to shifts in the amount and locations of prey available for baleen whales.5 With rapidly warming oceans, the temperature of the NIO has been rising consistently.6 Increasing temperatures have caused a decrease of up to 20% in phytoplankton over the past six decades which can disrupt the food chain and cause a shortage in food availability for the whales.6 The ASHW population is listed as Endangered while the NIO pygmy blue whale population is listed as Data Deficient on the IUCN Red List of Threatened Species, and both face many man-made threats.3,5 Although they are such enigmatic animals, we know very little about the baleen whales of the NIO. Understanding them better will help us come up with the best course of action to protect these amazing creatures. 

Stay tuned as we continue to unearth (or should I say un-ocean?) the mysteries of these amazing, gentle giants!

 

* Check out this poster for more on baleen whales!

REFERENCES:

  1. https://www.nationalgeographic.com/animals/article/150414-gray-whale-pacific-migration-endangered-ocean-animal-science
  2. de Vos, A., Faux, C.E., Marthick, J., Dickinson, J. and Jarman, S.N., 2018. New determination of prey and parasite species for northern Indian Ocean Blue Whales. Frontiers in Marine Science, 5, p.104.
  3. https://iucn-csg.org/csg-special-projects/arabian-sea-humpback-whales/
  4. Prasanna Kumar, S., Narvekar, J., Nuncio, M., Gauns, M. and Sardesai, S., 2009. What drives the biological productivity of the Northern Indian Ocean?. Washington DC American Geophysical Union Geophysical Monograph Series, 185, pp.33-56.
  5. de Vos, A., Pattiaratchi, C.B. and Wijeratne, E.M.S., 2014. Surface circulation and upwelling patterns around Sri Lanka. Biogeosciences, 11(20), pp.5909-5930.
  6. Roxy, M.K., Modi, A., Murtugudde, R., Valsala, V., Panickal, S., Prasanna Kumar, S., Ravichandran, M., Vichi, M. and Lévy, M., 2016. A reduction in marine primary productivity driven by rapid warming over the tropical Indian Ocean. Geophysical Research Letters, 43(2), pp.826-833.

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