“Sharktober” — a spike in shark bite incidents off the North American west coast during the fall — is real and 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 University of Hawai’i at Manoa’s Hawaii Institute of Marine Biology, 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 area during that period. Of the others, 33% were unidentified species and 16% were attributed to recluse sharks (Carcharhinus spp.)
Crucially, tiger sharks accounted for at least 63% of recorded bites during the month. What’s more, 28% of the October bites involved unidentified species, some of which may also have been tiger sharks, Meyer wrote in a study published Jan. 6 in the journal. Frontiers in Marine Science“The October increase appears to be due to 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 that the younger ones have, they are found worldwide in temperate and tropical waters, especially around the islands of the central Pacific.
Tiger shark numbers are highest in the waters off Hawaii during October according to ecotourism operators. This month also sees large, adult females migrate from the islands in the Pacific Northwest to the nearshore areas around the main Hawaiian Islands to give birth. The increased presence of large sharks is one key factor that could lead to more bites, Meyer said.
“The most likely explanation is seasonal reproduction: the partial migration of large adult female tiger sharks associated with the pupation period appears to increase their presence in coastal waters used by humans,” Meyer said.
Another 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 yolk sac. Sharks also give birth on average of about 30 cubs after 15 to 16 months of gestation.
That means females, when pregnant and after giving birth, will likely need to actively forage to replenish energy stores, Meyer said. Other ecological factors may also contribute to the increase in bites, he added, such as seasonal increases in the availability of preferred prey such as large reef fish. Attacks But it won’t be up to the mothers to protect their young – once born, tiger shark pups are independent and often stay in shallow areas to avoid being eaten by larger sharks, including their mother.
Available information suggests an increase in unprovoked shark bites around Hawaii, he said Daryl McPheean environmental scientist at Bond University in Queensland, Australia, who studies shark attacks but was not involved in this study.
“Any seasonal change in behavior that can increase the potential overlap of large shark species such as tiger sharks has the potential to increase the risk of being bitten,” he told Live Science by email, adding that regardless of the circumstances, the risk of being bitten remains low.
Meyer also noted that the overall risk of being bitten by a shark is extremely low. “The key takeaway is awareness, not alarm,” Meyer said. “Extra caution is advised during this month, especially for high-risk solo activities such as surfing or swimming in coastal areas.”
Although the October peak revealed in the study is specific to Hawaii and tiger sharks, there may be similar patterns elsewhere in the world, Meyer said. “When large coastal sharks show strong seasonal shifts in habitat use, bite risk can also become seasonal. Other regions and species may experience similar patterns, but the timing and drivers will vary depending on local ecology.”
globally, three large species of coastal sharks make up the majority of recorded unprovoked bites. These are great white sharks (Carcharodon carcharias), tiger sharks and bull sharks (Carcharhinus leucas), a species of requiem shark.
Bull sharks are believed to be behind a recent bite near Sydney, New South Wales, Australia four events in 48 hoursand this period roughly coincides with their summer breeding season in the southern hemisphere.
“Bull sharks along parts of Australia’s east coast are seasonally more abundant near shore and in rivers and estuaries during their breeding season in the austral summer,” McPhee said.
However, other factors may have had a greater influence on the recent bites in Australia, including a combination of increased summer water use by humans, environmental conditions such as storm runoff and reduced water clarity.
“There were a number of environmental conditions that concentrated bull sharks towards the mouth of Sydney Harbor and adjacent beaches,” McPhee said. “There was heavy rain in the basin that would have washed away the prey, and the water was cloudy. These are the best conditions for sharks to feed on.”
Despite considerable variation in the number of bites over time and in different locations, there is a general long-term trend increasing 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 picture is similar, according to data from the Florida Museum. There were a total of 157 attacks in the 1970s, but there were 500 in the 1990s and 803 between 2010 and 2019.
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