Sand Tiger Sharks Surprisingly Social in Open Ocean
While the apex predators were typically thought to lead mostly solitary lives, a new study finds that sand tiger sharks may be a lot more social.
Author Carson McCullers famously wrote about the heart as "a lonely hunter," but her description seems to apply just as well to a shark. They cruise the ocean on their own and generally have little contact with other sharks - or do they? While these apex predators were typically thought to lead mostly solitary lives, a new study finds that sand tiger sharks may be a lot more social than scientists had suspected.
Sand tiger sharks (Carcharias taurus) go by a number of common names, including grey nurse shark, spotted ragged-tooth shark, slender-tooth shark and ground shark. They swim in coastal waters in the western and eastern Atlantic, the Mediterranean, and near Australia and Japan, and measure about 10 feet (3 meters) long.
During the summer months, sand tigers that inhabit the waters off the coast of the eastern United States migrate to Delaware Bay, where they are in close contact with one another. However, scientists were uncertain about whether the sharks continued to interact socially when they dispersed back to the open ocean. [See Photos of a Sand Tiger Shark Nursery]
Previously, researchers had explored shark interactions in controlled environments like pens or laboratories, but this was the first study to investigate social behavior in sharks swimming in the open ocean. For the study, scientists attached acoustic tags to more than 300 sand tiger sharks, tracking their movements and recording interactions among the sharks for nearly a year.
The scientists conducted initial data analysis from two individual animals, and found that the sharks enjoyed an active social life year-round. They registered almost 200 encounters with other sand tiger sharks, and interacted repeatedly with the same individuals. The sharks also formed groups that varied in size depending on their location and the time of year.
And during late winter and early spring, the sharks took a break from their socializing and hardly encountered any other sharks, the scientists discovered. Danielle Haulsee, one of the researchers and a doctoral candidate in oceanography at the University of Delaware in Lewes, suggested in a statement that sharks might self-regulate their time in a group, depending on individual needs for certain activities that are best done alone, like finding food or mating.
Discovering that sharks are capable of making decisions associated with social networking casts these former "loners" in a new light, Haulsee said. "Our research shows that it is important for the scientific community to not rule out these types of behaviors in nonmammalian species," she said.
The findings were presented Feb. 22 at the 2016 Ocean Sciences Meeting, and the abstract was published online.
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Great whites and other sharks travel on "ocean highways," according to new research that is helping to define the routes. Mapping out the routes is enabling scientists to better understand the behavior of these apex ocean predators and to improve conservation tactics. Ongoing research also shows how busy both the sharks and these highways can be. "The sharks do show repeated travels along the same general paths, which is quite amazing since there are few 'road signs' in the open ocean," Mahmood Shivji, director of Nova Southeastern University's Guy Harvey Research Institute and the Save Our Seas Shark Research Center, told Discovery News. As for how the sharks know where to go, he added, "The main current hypothesis is that they use the earth's magnetic field to navigate, like some birds do." Great whites are among the most well-traveled sharks, swimming for thousands of miles with relative ease. In the winter, they migrate from the California and Baja coasts to a mid-Pacific open water area dubbed "The White Shark Café." Precisely why great sharks do this, however, remains a mystery.
For the research projects, Shivji and his team are affixing satellite tags to multiple shark species. The tagging itself can be arduous, since the scientists are so close to the toothy predators. "This type of tracking research needs to be long-term to really reveal and understand patterns of shark migrations," he said. "Such long-term studies are difficult and expensive to conduct, in part because working on the ocean and with big animals poses considerable challenges."
Once tagged with a satellite-tracking device, the sharks often need a bit of stimulation to get them back on their particular ocean route. This can be a dangerous hands-on process, as this researcher demonstrates.
Shortfin mako sharks continue to astound researchers, given the duration and miles involved in their migrations. "Our long-term satellite tracking data is showing that individuals can travel over 10,000 miles in a single year, and they also are starting to show indications of round-trip, repeated and predictable migration patterns," Shivji said.
Along their highway-like migration routes, sharks seem to primarily stop for three things: food, sex and babies. Here, lemon sharks stop for their babies. Research conducted in Bimini in The Bahamas, spanning almost two decades, shows that female lemon sharks that were born there returned 15 years later to give birth to their own young. Some sharks never seem to forget home sweet home, even if it is a temporary safe-feeling spot before traveling begins again.
Tiger sharks have been in the news lately because Shivji and his colleagues determined that these sharks undergo remarkable, bird-like migrations. The findings were published in the journal Scientific Reports. "The tiger shark migrations are bird-like in the sense that they seem to be following the same general round-trip, repeated migrations from warm over-wintering areas to temperate summer areas," he said. One tiger shark, named Harry Lindo, was tracked traveling more than 27,000 miles, possibly the longest-ever recorded distance traveled by any species of shark. "It is truly remarkable," Guy Harvey, a renowned marine artist, conservationist and fisheries ecologist said. Harvey has been working with Shivji on the shark tagging projects. Brad Wetherbee, another researcher on the team, noted that tiger sharks seem to have their own "café" as well, except they have given it a different name. "We joke that tiger sharks, not being media stars like great white sharks, wouldn't be comfortable in a 'café' and prefer to hang out in their 'truck stop' in the mid-Atlantic," Wetherbee, who is based at the University of Rhode Island, said.
As they travel, sharks may go solo or with companions. Here, an oceanic whitetrip journeys on its own. This species has been in decline around the world due to overfishing, the researchers said. Oceanic whitetip sharks used to be one of the most common sharks seen close to the water's surface, but that's no longer the case. In terms of its journeys, Shivji said, "The initial data are suggesting that, although they are highly migratory, they don't travel as far out into the open ocean as the shortfin makos and tiger sharks do in the Atlantic -- an unexpected finding."
This oceanic whitetip shark is definitely not traveling solo. Accompanying it are rainbow runners and pilot fish. The shark serves as a moving bodyguard for the much smaller, weaker fish. Oceanic whitetips, in turn, benefit from the fishes' presence, because pilot fish eat health-threatening parasites, dead skin and other undesirables off of the sharks. This kind of win-win arrangement is called "a mutualistic relationship."
Sand tiger sharks live very deep in the ocean, unlike more surface dwelling, or "pelagic," tiger sharks, shortfin makos and oceanic whitetips. In fact, sand tiger sharks prefer to stay close to the bottom of ocean floors. "Migrations are seasonal in the western Atlantic, with migrations along the coast into cooler waters in the summer and warmer waters in the winter," Shivji said.
Caribbean reef sharks also undergo migrations, with females traveling to "nursery" waters northwest of the Brazilian coast. These sharks, dubbed "the silent sea predators," are formidable hunters, but thankfully pose little threat to humans because they tend to avoid interactions with us. In this case, a bait ball containing fish and other edibles was intentionally dropped into the Caribbean reef shark's territory, leading to a feeding frenzy. Shivji said that tracking all migratory sharks poses major challenges, especially because sharks cover so much territory. This further poses conservation challenges, since waters off the coasts of multiple countries are involved, all linking up into the shark highways. "The shark long-term tracking will show where and when they spend the most time during their travels on a predictable basis," Shivji said. "Once we know this, fishery regulation and protection become more tractable since they provide geographic areas to focus management and conservation efforts on."