Basking sharks are found in temperate coastal waters across our planet.
These sharks are big — second in size only to whale sharks. In summer, they’re easy to spot, swimming lazily at the ocean surface. But every winter basking sharks just . . . disappear. Scientists thought they were hibernating. It took only a few months of research to prove otherwise. David Sims is with the Marine Biological Association in England. Beginning in February, 2001, he put satellite transmitters on 20 basking sharks.
Mini-computers inside the tags showed how deep the sharks dive and how far they swim, along with water temperature and light level. Every few months, one of these tags detaches automatically, floats to the surface, and transmits data to a satellite . . which in turn sends email to Dr. Sims’ computer. In almost two years he’s heard from 12 sharks.
Eight more are still wearing tags. The data are clear — basking sharks are as active in winter as in summer. But in winter they spend more time in deep water.
Basking sharks are an endangered species. The Asian market for shark-fin soup targets them. A single fin — sometimes as big as a tabletop — can be worth a small fortune. Future tagging studies might help.
A trail of circumstantial evidence led scientists half a century ago to conclude that basking sharks hibernate. There are a few threads to the story, explains Dr. David Sims. The first one was that basking sharks weren t seen at the surface during winter. During summer, of course, they can be seen quite regularly, feeding on swarms of zooplankton. Since they were not seen in the winter, that suggested they must go somewhere else.
The other bit of evidence, Sims continues, was that basking sharks have very large livers, maybe one-fourth of the total body weight. Fisherman who caught basking sharks noticed that their livers weighed less in spring than in the fall. Scientists concluded the sharks must use the liver as an energy reserve organ in winter much the way a hibernating bear is sustained by its layer of fat.
The final bit of circumstantial evidence in support of hibernation had to do with the sharks gill rakers. Gill rakers are comblike structures, quite long, Sims explains. They act as a filter, to extract zooplankton (mostly calanoid copepods) from the water. Basking sharks, when they re feeding, swim forward through the water with their mouths wide open, and the gill rakers strain out zooplankton; the seawater flows through the mouth and out of the gills.
Reports in the scientific literature suggested that basking sharks shed their gill rakers in autumn and then slowly re-grow them over the winter. The deal was that without the rakers they couldn’ t feed, Sims says. To scientists of the day, these three factors sharks disappeared in winter, their livers shrunk over the winter, they seemed incapable of feeding in winter–suggested that basking sharks must hibernate to conserve energy.
Thanks to the satellite tag studies, says Sims, we now have very good data sets showing basking sharks are very active during the summer, the autumn, AND the winter. What s more, their behavior and habitat in the winter is not very different from their behavior and habitat in summer.
What about the missing gill rakers? Explains Sims, “If you go back to those old papers, the chronology of them losing gill rakers and re-growing them is based on three animals. If you look at the entire data set of winter-caught basking sharks which those authors had at their disposal you find that, in fact, half of the animals DO have gill rakers, AND they have food in the stomachs.” Basking sharks do shed and re-grow their gill rakers, but they don t all shed their rakers at once, in the fall, and they don t go all winter without eating.
Besides disproving the hibernation hypothesis, the satellite tag study has revealed new information about basking shark behavior. Some of their dives were VERY deep, says Sims. They can go from the surface to over 850 m depth over a matter of hours and back again that s the deepest vertical range of any shark yet measured.
This deep-diving ability means sharks can feed, not just on zooplankton clustered in surface waters, but on zooplankton communities that occur in deep water 200 m to 1000 meters down. For baleen whales such as blue and fin whales, the maximum diving depth is 450 m, notes Sims. So basking sharks can clearly utilize zooplankton communities beyond the depth of air-breathing mammals.
The satellite-tag study is an important first step toward a reliable estimate of basking shark numbers. These sharks have been hunted intensively around England and Ireland for 200 years, mostly for the oil from their livers. We just don t know how many basking sharks are left, Sims says. An often-quoted statistic that 20,000 sharks remain worldwide is not reliable. “I don t know where that number is coming from it s not the result of any scientific analysis that I am aware of,” he notes.
But now, with the tags we’ve been able to show where basking sharks go, how far they move, and what their preferred habitat is, Sims continues. Once you ve established their preferred habitat over several annual cycles, it becomes possible to target areas for surveys. What is required, for a reliable population estimate, is for a number of surveys to be undertaken in areas of known shark concentration throughout the world.
Right now, basking sharks are listed as vulnerable on the IUCN red list. This means they are considered at risk of becoming extinct not in the immediate future, but in the medium future. Over the past two centuries, an estimated 250,000 mature basking sharks were caught in the waters around Europe and Ireland. We don t know what effect that has had on the population, Sims notes.
Fishing may not be the only reason sharks now seem scarce in these waters. The distribution of zooplankton is constantly changing; Sims says, the sharks may have simply moved away in search of better feeding grounds. What we found from the satellite tracking is, basking sharks are fantastic at finding zooplankton hot spots, he says. We ve tracked them and related their movements to remote sensing images of plankton, and the sharks are able to find hot spots over ranges of about 500 km. We don t know how they do it, whether they have learned to go to particular areas, or whether they can actually perceive plankton prey at long distances. But what our work does show is, they can do it.
The following articles were used in preparing this script:
- Ahuja, Anjana. Satellites help to solve shark mystery. London Times, Oct. 20, 2001, pg. J 8.
- Viney, Michael. Scientists get their teeth into sharks. The Irish Times, August 24, 2002, pg. 56.
- Highfield, Roger. Call to extend safety zone for the gentle giant of the British seas. The Daily Telegraph (London), Sept. 16, 2002.
- Sims, David W. Threshold foraging behaviour of basking sharks on zooplankton: Life on an energetic knife-edge? Proceedings of the Royal Society of Biological Sciences, Series B. 266(1427):1437-1443; 1999.
- Sims, David W. Can threshold foraging responses of basking sharks be used to estimate their metabolic rate? Marine Ecology Progress Series, 200: 289-296 (2000).
- Sims, David W. Congruent trends in long-term zooplankton decline in the north-east Atlantic and basking shakr (Cetorhinus maximus) fishery catches off west Ireland. Fisheries-Oceanography 11(1):59-63; 2002.