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Freshly Baked Science

Navigating the Open Ocean

by Jessica Bolin

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28th February 2019

Whenever I’m exploring a new city for the first time, you can guarantee I’ll have my smartphone’s navigation app or a satnav on hand. Not only does it tell me where I am, but also where the closest food outlets are, if there’s a park, shopping centre, or bus station nearby, and the distance and time it will take to travel from one location to another. Humans have invented incredible technology to assist with navigation, which helps us get to places quickly and efficiently… especially for people like me, with no sense of direction!

However, humpback whales don’t have the luxury of a smartphone to help them navigate through the open ocean. Humpbacks in Eastern Australia migrate up to 5000 kilometres each year, which is one of the longest migrations of any animal on Earth. In the Austral summer, they are foraging in the cold waters of Antarctica, stocking up on krill to prepare for the long journey ahead. In winter, these ocean giants leave their foraging grounds and travel up the east coast of Australia, eventually stopping to breed in the warm waters of the Great Barrier Reef. Interestingly, humpbacks exhibit high route fidelity, meaning they use the same route with extraordinary accuracy, and travel in near-straight lines, year after year. So, how do they know where they’re going, and more importantly, do they ever get lost?

Earth’s magnetic field

Migrating animals are believed to use the Earth’s magnetic field to navigate. For instance, homing pigeons have tiny particles of magnetite in their upper beaks, which scientists have found allows the pigeons to ‘see’ the Earth’s magnetic field, and is responsible for their impressive navigation abilities. Whales have a small quantity of magnetite in their skull and are also believed to use the magnetic field for navigation. However, in the open ocean, the Earth’s magnetic field varies widely, so it’s more likely humpback whales use a combination of the magnetic field, sun’s orientation, and perhaps the position of the moon and stars, to make an ‘internal map’ of the ocean that helps them find their way.

 

Seamounts

Humpbacks love mountains! Recently, scientists in the western Pacific have found humpbacks near New Caledonia make ‘pit-stops’ at giant undersea mountains - called seamounts - during their southern migration, with one humpback spending a whopping 22.5 days at a seamount between New Caledonia and New Zealand. Seamounts are characterised by shallower water relative to the surrounding deep ocean and are highly productive. It’s possible that seamounts represent an anomaly in the magnetic field, and the whales can sense their location. Or, the whales may use them for resting and snacking, or even as a breeding location.

 

Thermal fronts

 

Opposed to seamounts, which are confined to their position on the seafloor, thermal fronts can occur anytime, and anywhere, all throughout the ocean. Thermal fronts are produced when two water bodies of different temperatures meet. At the meeting point between the cold and warm water, there is a steep change, or gradient, in temperature. Depending on their size, fronts can persist for days, weeks or even months, and marine predators seem to use them to navigate, including basking sharks and loggerhead turtles. On the Gold Coast, Australia, humpbacks associate with strong temperature gradients, and scientists believe East Australian humpbacks use thermal fronts generated by the East Australian Current (yep, from Finding Nemo!) and even the current’s inner edge, to navigate during their migration.

 

Of course, these are just three out of many, many factors that humpbacks use to navigate. The fact that humpbacks use near-identical routes, and don’t seem to get lost during their lengthy migratory journeys, is incredible. We still have so much to learn about these magnificent ocean giants and the true extent of their navigational skills. I know that if my navigation abilities were as precise and accurate as humpbacks, I’d never need a satnav again!

Definitions

Austral: relating to the southern hemisphere
Fidelity: the degree of exactness with which something is copied or reproduced.

References

1. Mora C. V., Davison C. V., Wild J. M. & Walker M. M. 2004. Magnetoreception and its trigeminal mediation in the homing pigeon, Nature, 432, 508-511.


2. Horton, T.W et al. 2011. Straight as an arrow: humpback whales swim constant course tracks during long-distance migration, Biology Letters, 7(5).


3. Garrigue, C. et al. 2015. Satellite tracking reveals novel migratory patterns and the importance of seamounts for endangered South Pacific humpback whales, Royal Society Open Science, 2(11).


4. Scales, K. L et al. 2015. Oceanic loggerhead turtles Caretta caretta associate with thermal fronts: Evidence from the Canary Current Large Marine Ecosystem, Marine Ecology Progress Series, 519, 195-207.


5. Reinke, J. et al. 2016. Coastal Fronts Utilized by Migrating Humpback Whales, Megaptera novaeangliae, on the Gold Coast, Australia, Journal of Coastal Research, 1(75), 552-556.

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