Magnetic Signatures & Singing Stages: the mysterious connection between whales, sharks, and seamounts

On the bottom of the deep ocean lie seamounts, massive underwater volcanoes that are largely unexplored, and generally absent of marine protections or management. Most seamounts are located in areas beyond national jurisdiction; also known as the high seas, where their management and conservation require international cooperation which is particularly difficult to implement and enforce.

Whales and Sharks Are Drawn to Seamounts

Known as oases in the open ocean, seamounts show consistently high levels of marine biodiversity and are hotspots for life, including fish, sea turtles, seabirds and marine mammals. As it turns out, seamounts also play a key role in whale and shark migrations and life cycles. Whales in particular not only navigate with the help of seamounts, but also actively use them as important breeding and feeding grounds. Researchers have documented seamount visitation in humpback whales, sperm, sei, northern right, blue and several species of beaked whales. A recent study conducted in New-Caledonia revealed that humpback whales regularly visit seamounts during breeding season and spring migration periods in tropical and subtropical latitudes. Whales spend time close to seamounts, particularly seamounts in shallow depths.

But How Do They Find Seamounts in a Huge Ocean?

While it is not yet known exactly how whales and sharks use seamounts to navigate,  researchers have developed several strong hypotheses. First, seamounts are usually large, easy to locate landmarks in the often featureless open ocean. The change in ocean floor topography, the geomagnetic signature, and the effects on currents are detectable by whales and sharks over long distances. Differences in the geological history of various seamounts can cause individual seamounts to have strong and unique magnetic signatures. Whales and sharks are believed to use such signatures when navigating in the open ocean. Sensitivity to magnetic information has also been shown to be highly important to the navigation of great white sharks across the open ocean. Several other species of sharks have also been shown to use seamounts as reference points for navigation with great accuracy.

Seamounts are important for whales and sharks beyond their role as navigational beacons. Because they host exceptional biodiversity, seamounts are also important resting and feeding stops for whales and sharks. Moreover, they serve as areas for social aggregations during the breeding season, thus providing an essential function in the social lives of these animals. One remarkable service that seamounts may perform is serving as a ‘singing stage’ or a public address platform for male humpback whales. Seamounts are often quieter than the surrounding water column and provide better sound transmission towards the open ocean, providing an acoustically optimized stage for singing whales. As their songs attract other individuals, large whale aggregations can gather around seamounts—making for a full theater!

Protecting Seamounts Protects Whales and Sharks

Whales and sharks face several anthropogenic threats. In California alone, ten whales died from ship collisions in 2018, the highest number since 1982. They also increasingly face entanglement from fishing gear. Entanglement in fishing ropes (e.g., lines leading from surface buoys to bottom traps, or long line fishing gear, or abandoned gear floating in the water) can lead whales to quickly drown and, in other cases, to a slow and painful death through dragging heavy fishing gear. You can read NOAA’s most recent report on whale entanglement here, and learn more about the destructive effects of fishing on seamounts here. There is also strong scientific evidence that whales are highly vulnerable to military sonar and other sounds from military activities. Finally, deep sea mining on seamounts for valuable rare metals is a looming possibility.

To effectively protect highly migratory species such as whales and sharks, strong regulations are needed to protect them from fishing, collisions, and other disturbances. One straightforward way to protect seamounts and the rich marine life that congregates there is with fully or highly protected marine reserves.  Marine Conservation Institute advocates for strong protection of these fragile seamount ecosystems off the coast of California and around the world in areas beyond national jurisdictions where currently no legal mechanism allows for the establishment of MPAs.  We are hard at work with other organizations to facilitate a new United Nations treaty that will allow for protection of the high seas. With this new treaty, we can make real strides toward creating highly protected marine reserves at seamounts  where fishing, deep sea mining and other extractive activities are prohibited and where megafauna like whales and sharks can gather, breed, feed, and rest safely—beyond and between international boundaries.

Sources :

Derville, Solène, Torres, Leigh G, Zerbini, Alexandre N, Oremus, Marc, & Garrigue, Claire . (2020). Horizontal and vertical movements of humpback whales inform the use of critical pelagic habitats in the western South Pacific. Scientific Reports, 10(1), 4871.

Horton, Travis W, Hauser, Nan, Zerbini, Alexandre N, Francis, Malcolm P, Domeier, Michael L, Andriolo, Artur, Clapham, Phillip J. (2017). Route Fidelity during Marine Megafauna Migration. Frontiers in Marine Science, 4, Frontiers in Marine Science, 2017-12-21, Vol.4.

Garrigue, Claire, Clapham, Phillip J, Geyer, Ygor, Kennedy, Amy S, & Zerbini, Alexandre N. (2015). Satellite tracking reveals novel migratory patterns and the importance of seamounts for endangered South Pacific humpback whales. Royal Society Open Science, 2(11), 150489.

Letessier, Tom B, Mouillot, David, Bouchet, Phil J, Vigliola, Laurent, Fernandes, Marjorie C, Thompson, Chris, Meeuwig, Jessica J. (2019). Remote reefs and seamounts are the last refuges for marine predators across the Indo-Pacific. PLoS Biology, 17(8), E3000366.