Basking sharks (Cetorhinus maximus) are large marine creatures that swim through the temperate zones of the ocean, with mouths wide open to filter the small prey they feed on, such as zooplankton.
A recent investigation, published in the journal ‘PLOS One’, now reveals new data about the life of this fish that can reach up to 10 meters in length, showing that, contrary to what was thought, it does not fast during its winter migrations and descends to much greater depths.
Between 2004 and 2011, a group of scientists led by the Woods Hole Oceanographic Institution (WHOI) attached tracking devices to 37 basking sharks off Cape Cod, a peninsula in the U.S. state of Massachusetts. Through the data collected, they tracked the movements of these fish, the second largest in the world (second only to the whale shark) during their migration from the east coast of the United States to the Sargasso Sea and the Caribbean, where they spend the winter in warmer waters.
The data show that basking sharks dive to depths of up to a thousand meters, where the ocean’s twilight zone, also known as the mesopelagic zone, begins at 200 meters depth, where sunlight has difficulty reaching, hence it is a world wrapped in twilight.
The researchers say the work makes clear the importance of the mesopelagic zone for these sharks, and also for other marine species, especially migratory ones. However, they emphasize that, although in this region there is more biomass than in any other area of the water column, little is known about the long-term impacts of increased fishing in this region or its role in the carbon cycle.
Although it is not yet known exactly what basking sharks feed on in the mesopelagic zone, Camrin Braun, a researcher at WHOI and the study’s principal co-author, says one hypothesis is that they are feeding on minuscule fish that are too small to be consumed by other predators. The fact that they are filter feeders helps the basking shark to exploit food sources that other sharks or fish do not have the capacity to access.
The ecologist and oceanographer says that one of the big questions to answer concerns the impacts that industrial fishing of fish and squid in the mesopelagic zone will have on basking sharks, a species classified as “Endangered” on the IUCN Red List. For centuries, basking sharks were hunted for their liver oils and their populations have not yet fully recovered.
Therefore, scientists say it is essential to deepen knowledge about their migrations and try to anticipate possible threats that could jeopardize their survival in the future.
“One of the greatest mysteries is their reproduction. We still do not know where basking sharks mate or give birth, and those long-distance migrations may be associated with reproductive behaviors that are taking place offshore or in the deep sea, where we cannot observe them directly,” notes Jaida Elcock, the first author of this paper.
