“Sharktober,” the autumn spike in shark bites off the west coast of North America, is a reality, and in Hawaii, new research suggests the phenomenon appears to occur when tiger sharks give birth in the waters around the islands.
Carl Meyer, a marine biologist at the Hawaii Institute of Marine Biology at the University of Hawaii at Manoa, analyzed 30 years of Hawaiian shark bite data from 1995 to 2024 and found that tiger sharks (Galeocerdo cuvier) accounted for 47% of the 165 unprovoked bites recorded in the region during that period. Of the remainder, 33% were due to unidentified species and 16% were due to requiem sharks (genus: Lapis spp.).
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Importantly, at least 63% of the bites recorded that month were from tiger sharks. Additionally, 28% of bites in October involved unidentified species, some of which may have been tiger sharks, Meyer said in a study published Jan. 6 in the journal Frontiers in Marine Science. “The October surge appears to be driven by tiger shark biology rather than changes in human ocean use,” Meyer told Live Science via email.
Tiger sharks typically reach lengths of 10 to 14 feet (3 to 4.3 meters) and weigh more than 850 pounds (385 kilograms). Named for the dark vertical stripes found on young fish, they are found in temperate and tropical waters around the world, especially around islands in the central Pacific Ocean.
Ecotourism operators say tiger shark numbers are at their highest in the waters off Hawaii in October. This month is also the time when large adult females migrate from the islands of the northwestern Pacific Ocean to areas near the coasts around mainland Hawaii to give birth. Meyer said the increased presence of large sharks is one important factor that could lead to more bites.
“The most plausible explanation is seasonal reproduction. Partial migration of large female tiger sharks associated with the spawning season appears to increase their presence in coastal waters used by humans,” Meyer said.
Another important factor is that childbirth is physically exhausting. Tiger sharks are ovoviviparous. That is, the egg hatches inside the mother’s body and the embryo receives additional nutrients beyond what was in the yolk sac. Sharks also give birth to an average of about 30 pups after 15 to 16 months of gestation.
This means females likely need to actively forage during pregnancy and after giving birth to restore their energy reserves, Meyer said. However, he added that other ecological factors may also be contributing to the spike in bite numbers, such as seasonal increases in preferred prey such as large reef fish. However, the attacks are not caused by mothers defending their children. Tiger shark pups are independent at birth and often stay in shallow waters to avoid being eaten by larger sharks, including their mother.
Darryl McPhee, an environmental scientist at Bond University in Queensland, Australia, who studies shark attacks but was not involved in the study, said available information suggests that unprovoked shark bites are on the rise around Hawaii.
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“Seasonal behavioral changes that may increase the likelihood of overlap among large sharks, such as tiger sharks, may increase the risk of bites occurring,” he told Live Science via email, adding that regardless of the situation, the risk of bites remains low.
Meyer also pointed out that the overall risk of being bitten by a shark is very low. “The key is awareness, not vigilance,” Meyer said. “During this month, we advise people to be especially careful about high-risk solo activities such as surfing and swimming, especially in coastal areas.”
Although the October peak found in the study is unique to Hawaii and tiger sharks, Meyer said there may be similar patterns elsewhere in the world. “If large coastal sharks exhibit large seasonal variations in habitat use, bite risk may also be seasonal. Similar patterns may occur in other regions and species, but timing and factors will vary depending on local ecology.”
Globally, three large coastal shark species account for most of the unwarranted bites recorded. These are the great white shark (Carcharodon carcharias), the tiger shark, and the bull shark (Carcharhinus leucas), which is a type of requiem shark.
Bull sharks are believed to be behind a series of recent bites near Sydney, New South Wales, Australia, with four incidents occurring within 48 hours, a period that roughly coincides with the southern hemisphere’s summer breeding season.
“Seasonally, bull sharks along parts of Australia’s east coast are more abundant near the coast and in rivers and estuaries during Australia’s summer breeding season,” Mr McPhee said.
However, other factors may be more influential in recent stabbing incidents in Australia, such as increased summer water use by people, a combination of environmental conditions such as storm runoff and reduced water clarity.
“There was a set of environmental conditions that led to a concentration of bull sharks in the Sydney Harbor estuary and adjacent coastline,” Mr McPhee said. “Heavy rainfall in the catchment washed away prey and muddied the water, so conditions were perfect for bull sharks.”
McPhee added that despite large variations in the number of shark bites over time and location, there is a long-term trend of increasing shark bites, especially among surfers. In New South Wales, four bites were recorded between 1980 and 1999 and 63 bites between 2000 and 2019.
Globally, the situation is similar, according to Florida Museum data. In the 1970s, there were a total of 157 attacks, in the 1990s the number reached 500, and from 2010 to 2019 there were a total of 803 attacks.
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