While most people are afraid of great white sharks (Carcharodon carcharias), Yuuki Watanabe was drawn to them since childhood. “Personally, I love white sharks. Since I was a child, I have been fascinated (for unknown reason) by something big, fast, and looking cool, including space shuttle, bullet trains, and white sharks,” Watanabe, from the National Institute of Polar Research in Japan, said over an e-mail. White sharks are definitely big, fast, and cool-looking. Measuring up to 20 feet (6 meters) in length, they are famous for their bursts of speed as they chase fast prey in the water… and sometimes out of it !
Watanabe grew up to be interested in the energy expenditure of these sharks, which tells scientists a lot about these animals. As a rule of thumb, if you swim faster you end up consuming more energy. Commented Watanabe: “In general, the lifestyle of organisms has a full spectrum ranging from slow type (typically with low body temperature and large body mass) to fast type (with high body temperature and small body mass). This determines food requirement, locomotion performance, growth rate, life span, and more.” That means the energy expenditure seen for these sharks gives scientists a glimpse of the shark's life style and their effects on the ecosystems they inhabit.
One of their secrets to being such an efficient top-level predator even in cold water is a network of blood vessels called rete mirabile (an elaborate network of tiny blood vessels, in which the arteries of the animal lie next to their veins in tightly packed arrays). Also known as the "wonderful net," it is a countercurrent heat exchanger where vessels that are carrying warm blood transfer their heat to the cold blood in vessels that are coming back from the extremities. “Tunas (like bluefin and yellowfin) and some sharks (such as the great white shark, salmon shark, and shortfin mako shark) are endothermic fish. Due to warmed muscles, they can sustain high swim speed and migrate great distances,” explained Watanabe.
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However, the researchers found that great white sharks don’t have as “great of a swimming performance” unlike many people- including researchers- believe. According to Watanabe, the impression that these sharks have a great swimming performance comes from the fact that they eat seal, their impressive ability to breach, and their high body temperature. "Although we showed that they can sustain high speed when they traveled between islands, we also found that they save energy by swimming slowly when aggregating near seal colonies." Watanabe said, believing that the slow swimming that great white sharks partake in makes them analogous to cheetahs in savanna. This slow swimming behavior is not detrimental and can actually help them hunt by allowing the great whites to just ‘sit-and-wait’ when swimming near seal colonies. That doesn’t mean actually sit— great white sharks are one of those species that need to continuously keep swimming to breathe! Nevertheless, Watanabe commented, "our research showed that white sharks slow down when hanging around seal colonies and accelerate when they find a seal. In the energetic perspective, [that is how a] white sharks' strategy is similar to cheetahs' strategy."
The scientists also found that the white sharks in the Neptune Islands Group (Ron and Valerie Taylor) Marine Park off South Australia were repeatedly diving and believe this behavior was due to animals searching for prey to eat. Long-nosed fur seals (Arctocephalus forsteri), a preferred prey animal of the local white sharks, are repeatedly diving to the sea bottom to search for their own prey but also to escape a sneak attack from the ambush predator. Watanabe believes it to be reasonable for the sharks to behave similarly to fur seals to increase their chances of encountering them… and eating them.
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Great white sharks are listed as Vulnerable by the International Union for Conservation of Nature (IUCN). This shark species is currently protected in the Australian Exclusive Economic Zone (EEZ) and state waters, South Africa, Namibia, Israel, Malta and the USA (specifically the states of California and Florida states, and directed fisheries are prohibited off all coasts). There are strict, protective laws in place but sadly many take advantage of the loopholes and inadequate enforcement including promoting the black-market for high-value products of these sharks such as jaws, teeth and fins.
While this study is not directly linked to the conservation of white sharks, commercial shark cage diving boats are operated where this study took place. Some scientists believe that these operations could potentially affect the natural behavior of white sharks, and Watanabe's team is currently working with a diving boat in the area to try to understand the potential impacts and to mitigate them.