Sharktober: Study links October shark bite spike to tiger shark reproduction
Female tiger shark. (Photo credit: Cory Fults)
New research from the Hawai‘i Institute of Marine Biology (HIMB) Shark Lab has uncovered a strong, three-decade-long seasonal pattern in shark bite incidents in Hawaiian waters, confirming that the month of October experiences a statistically significant spike in incidents. The study, published in Frontiers in Marine Science, analyzed 30 years of data (1995–2024) and found that approximately 20% of all recorded bites occurred in October, a frequency far exceeding any other month of the year.
The research, led by HIMB Professor Carl G. Meyer, determined that this pronounced pattern, informally known as “Sharktober,” is primarily driven by the seasonal movements and biological needs of tiger sharks (Galeocerdo cuvier), which account for at least 63% of the incidents during this month. Meyer’s statistical analysis revealed that the elevated risk is not correlated with increased numbers of people in the water but rather reflects seasonal changes in the sharks themselves.
“The October spike is real and statistically significant, but the overall risk remains very low,” explained Meyer. “This pattern appears to be driven by tiger shark biology, not by more people being in the water.”
The findings suggest a strong link between the October spike and the tiger shark’s reproductive cycle, specifically parturition (giving birth), which occurs during the September–October window. The evidence indicates two primary mechanisms acting together. First, there is a temporary increase in the abundance of large adult females in nearshore habitats, including a partial migration of mature females from the Northwestern Hawaiian Islands to the Main Hawaiian Islands. Second, giving birth to a large litter is energetically taxing, likely leaving postpartum females in a state of poor nutritional condition and increasing their motivation to forage actively to recover energy reserves. This conclusion is supported by multiple independent lines of evidence, including the peak in tiger shark sightings at ecotourism sites that aligns precisely with the pupping season.
The study uses Hawai‘i-specific data to address a Hawai‘i-specific concern, providing science-based information that allows residents and visitors to make informed decisions about ocean use during this time of year, supporting awareness and coexistence without sensationalism.
“Understanding when risk is slightly elevated helps people make informed choices, not fearful ones,” noted Meyer.
Ocean users are advised to be aware that large tiger sharks are more likely to be present in the nearshore waters of the Main Hawaiian Islands during October, and extra caution is advisable during this month, particularly for high-risk, solo activities such as surfing or swimming in coastal areas.
Future research will focus on directly studying adult female tiger sharks during the pupping season, including tracking their movements and using non-invasive tools to assess their reproductive status and body condition. The work was conducted as part of the HIMB Shark Lab program at the University of Hawai‘i at Mānoa, utilizing publicly available shark incident data compiled by the Hawai‘i Department of Land and Natural Resources.
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