Hawaii Sees an Annual Shark Attack Spike. Now Scientists May Know Why

It can be dangerous to venture into Hawaiian waters in the month of October: Shark bite incidents spike that month. This isn’t exactly new knowledge; according to Hawaii’s Department of Land and Natural Resources, “Early Hawaiians recognized this, and cautioned against going in the water” between October and December. But now, Live Science reports that scientists think they may have figured out why. 

According to a study published this month in Frontiers by Carl G. Meyer, a marine biologist at the University of Hawai'i at Manoa’s Hawaii Institute of Marine Biology, an analysis of shark bite data from 1995 to 2024 revealed 165 bite incidents over that period. Of those incidents, unidentified species accounted for 54 bites (33 percent) and requiem sharks (Carcharhinus) accounted for 27 bites (16 percent). 

One Shark Species Pinpointed

The shark with the greatest number of bites in that period, however, was the tiger shark (Galeocerdo cuvier) with 77 bites, or 47 percent of incidents. Tiger sharks are some of the largest sharks in the world: They can typically grow up to 14 feet long (although up to 18 feet has been recorded) and are found in temperate and tropical oceans around the globe; they’re known to eat pretty much anything they come across, including sea turtles and even garbage, according to Oceana. 

Digging down even deeper into the data, Meyer found that 20 percent of bite incidents occurred in the month of October, “a frequency 2 to 4 times higher than in any other month” even though there aren’t more people in the ocean at that time. Tiger sharks made up at least 67 percent of bites in October, and some of the nine bite incidents involving unidentified species might also be attributed to the species. 

“This seasonal trend aligns with peak tiger shark sightings at ecotourism sites and the partial migration of mature females from the Northwestern Hawaiian Islands,” Meyer writes in the study, “suggesting a potential link to reproductive cycles.” 

Tiger sharks give birth to live young after a gestation period of 13 to 16 months. Rather than feed their young via a placenta, however, the young feed off the egg yolk and then uterine fluid before birth, according to the Florida Museum of Natural History. Litter sizes range from 10 pups to 82, though one study showed that around 30 seems to be the average.

As Meyer told Live Science, large females come in closer to the shore to give birth. (This is where the pups remain until they’re big enough to avoid being a meal for larger sharks, including mom.) These females may also be more actively foraging to gain energy after long pregnancies. Both of these factors may put them in closer proximity to, and in more frequent contact with, humans—potentially leading to more bites.

But even if bites do increase in Hawaiian waters in October, they’re still not a common occurrence. “The key implication is awareness, not alarm,” Meyer told Live Science. “Extra caution is advised during this month, particularly for high-risk, solo activities such as surfing or swimming in coastal areas.”

Which Shark Species Are Responsible for the Most Attacks?

Shark attacks are incredibly rare—according to the Florida Museum of Natural History’s International Shark Attack File, there were just 88 incidents in 2024, 47 of which were unprovoked. But great white sharks, bull sharks, and tiger sharks are generally considered the “Big Three” of attacks because, as the Florida Museum of Natural History points out, “they are large species that are capable of inflicting serious injuries to a victim, are commonly found in areas where humans enter the water, and have teeth designed to shear rather than hold.” 

Between 1580 and today, great whites have been involved in 351 unprovoked incidents; tigers in 142; and bulls in 119. Notably, bull sharks are thought to be behind a recent spate of encounters in Australia.