Shark attacks in Hawaii spike in October, and scientists think they know why
“Sharktober” — the spike in shark bite incidents off the west coast of North America in the fall — is real, and it appears to be happening in Hawaii when tiger sharks give birth in the waters surrounding the islands, new research suggests.
Carl Meyermarine biologist at the Institute of Marine Biology at the University of Hawaii at Manoa, analyzed 30 years of shark bite data in Hawaii, from 1995 to 2024, and found that tiger sharks (Galeocerdo cuvier) represented 47% of the 165 unprovoked bites recorded in the area during this period. Of the remainder, 33% were from unidentified species and 16% were attributed to requiem sharks (Carcharhinus spp.)
Above all, tiger sharks accounted for at least 63% of the bites recorded during this month. Additionally, 28% of October bites involved unidentified species, some of which may also be tiger sharks, Meyer wrote in the study published Jan. 6 in the journal Frontiers of Marine Science“The October peak appears to be due to tiger shark biology rather than changes in human use of the ocean,” Meyer told Live Science by email.
Tiger sharks typically reach 10 to 14 feet (3 to 4.3 meters) long and weigh more than 850 pounds (385 kilograms). They are named for the dark vertical stripes that the young ones possess and are found throughout the world in temperate and tropical waters, particularly around the central Pacific islands.
Tiger shark numbers highest in waters off Hawaii in October according to ecotourism operators. This month is also when large mature females move from the Pacific Northwest islands to the near-shore areas around the main Hawaiian Islands to give birth. The increased presence of large sharks is a crucial factor that could lead to more bites, Meyer said.
“The most plausible explanation is seasonal breeding: a partial migration of large adult female tiger sharks associated with the pupping season appears to increase their presence in coastal waters used by humans,” Meyer said.
The other key factor is that childbirth is exhausting. Tiger sharks are ovoviviparous, meaning their eggs hatch inside the mother’s body and the embryos receive additional nutrition beyond what was in the egg’s yolk sac. Sharks also give birth on average to around 30 babies after 15 to 16 months of gestation.
This means that females, during pregnancy and after giving birth, will likely need to actively feed to replenish their energy stores, Meyer said. Other ecological factors may also contribute to the increase in bite numbers, he added, such as seasonal increases in the availability of preferred prey, such as large reef fish. Attacks However, this won’t depend on mothers protecting their young: once born, baby tiger sharks are independent and often stay in shallow areas to avoid being eaten by larger sharks, including their mothers.
Available information suggests an increase in unprovoked shark bites around Hawaii, said Daryl McPheean environmental scientist at Bond University in Queensland, Australia, who studies shark attacks but was not involved in this study.
“Any seasonal behavioral changes that may increase the potential overlap between large shark species, such as tiger sharks, can potentially increase bite risk,” he told Live Science by email, adding that regardless of the circumstances, bite risk remains low.
Meyer also noted that the overall risk of shark bites is extremely low. “The key implication is awareness, not alarm,” Meyer said. “Greater caution is advised during this month, particularly for high-risk solo activities such as surfing or swimming in coastal areas.”
Although the October spike revealed in the study is specific to Hawaii and tiger sharks, there could be similar trends elsewhere in the world, Meyer said. “When large coastal sharks exhibit strong seasonal shifts in habitat use, bite risk may also become seasonal. Other regions and species may experience similar patterns, but the timing and factors vary depending on local ecology.”
Globally, three large species of coastal sharks account for most of the unprovoked bites recorded. These are great white sharks (Carcharodon carcharias), tiger sharks and bull sharks (Carcharhinus leucas), a type of requiem shark.
Bull sharks are believed to be behind a recent spate of bites near Sydney, New South Wales, Australia. four events in 48 hoursand this period roughly corresponds to the summer breeding period of the southern hemisphere.
“Bull sharks along parts of the Australian east coast are seasonally more abundant near shore and in rivers and estuaries during their breeding period in the austral summer,” McPhee said.
However, other factors may have been more influential on Australia’s recent bites, including a combination of people’s increased water use in summer, environmental conditions such as storm runoff, and reduced water clarity.
“There was a set of environmental conditions that concentrated bull sharks towards the mouth of Sydney Harbor and adjacent beaches,” McPhee said. “There was heavy rain in the watershed which would have driven away prey and made the water murky. So ideal conditions for bull sharks to feed.”
Despite substantial variation in the number of bites over time and location, there is a general long-term trend toward increase in shark bitesespecially on surfers, McPhee added. In New South Wales, four bites were recorded between 1980 and 1999, while 63 bites were recorded between 2000 and 2019.
Overall, the situation is similar, depending on data from the Florida Museum. In the 1970s there were a total of 157 attacks, but they reached 500 in the 1990s and reached 803 between 2010 and 2019.
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