We know that animals' migration habits are often easily explained. For example, they would head south in hopes of avoiding the chilly winter weather and head back north just in time for the summer's warmth. However, as it turns out, basking sharks are a lot more complex than what scientists initially believed.
Understanding Migration Behavior of Basking Sharks
Kerstin Menke of the German Oceanographic Museum in Stralsund, northern Germany, poses next to a basking shark's mouth replica on January 8, 2013. The nearly ten-meter long model should, according to the museum will, be placed in the entrance area of the North Sea aquarium. AFP PHOTO / STEFAN SAUER /GERMANY OUT
Paul Mensink, a Western biology professor, and his colleagues from Queen's University, Belfast, stumbled upon a surprising discovery while observing the basking shark's seasonal movements. The team tracked four basking sharks reaching three to six meters in length off of Ireland's northernmost tip to find where the enormous fish spends its winter days and why.
In a study published in the journal Environmental Biology of Fishes, titled "Cool runnings: behavioral plasticity and the realized thermal niche of basking sharks," the team affixed tags to measure and record the location, depth, and water temperatures of where the basking sharks lingered in migration.
Mensink explains that the team was working under the premise that the sharks would swim to the coastal waters of Ireland during the summer since it was a productive feeding area, then migrate south during the winter days where it was warmer. Adding that, the team believed that changes in water temperature were the driver for the migration of basking sharks.
This is why researchers were shocked at their discovery that completely blew their premise out the window.
Two of the four basking sharks being observed surprisingly stayed put in Ireland. While the other two sharks migrated south to subtropical and tropical waters on the African coast, which was as expected. However, instead of meandering in the warm waters, the two basking sharks spent a large chunk of their time in deeper, colder waters that rivaled Ireland's cold shores.
Each morning, data showed that the migrating sharks dived to depths ranging from 200-700 meters, returning near-surface around midday, only to submerge back to the chilly depths of the waters in the evening.
Puzzling Migration Behavior of Basking Sharks
Mensink, also a marine ecologist specializing in education technology and teaching fellow at Western's Faculty of Science, says that the sharks moved like clockwork, similar to the dive patterns of animals that come up to the water's surface for a breath of air, but of course, that's not why the sharks do it.
During the shark's dive cycle, they place themselves through an extreme range of water temperatures that range from 27 degrees Celsius near the water's surface and seven degrees Celsius in the depths. This left scientists puzzled as to why sharks would behave in such a way.
Mensink says that the team suspects that the sharks are behaving unusually to feed in what is known as the deep scattering layer, which is filled with gelatinous zooplankton and many other options for feeding. At midday, however, sharks come back up near the surface to bask in the water's warmth.
Meanwhile, the sharks that have remained in the shallow waters near Ireland's coasts were experiencing extremes of warmth or cold as the winter water temperatures ranged from nine degrees Celsius to 17 degrees Celsius. Unlike their migrating counterparts, these stay-at-home sharks had little observable variation in depths in the six months the basking sharks were tracked.
As to why this pair of basking sharks decided to linger while the other two headed south' Mensinks team hasn't been able to put the pieces together just yet.
Jonathan Houghton, a co-author of the study from the Queen's University Belfast, explains that the study tempts them to think about the basking sharks as oceanic species that can aggregate in coastal hotspots from months rather than coastal species that are reluctant to head out into the vast ocean when decreasing water temperatures forces the animal to do so, reports PhysOrg.
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