With the recent shark bites occurring around Hawaiʻi, Carl Meyer, assistant researcher at the University of Hawaiʻi at Mānoa’s Hawaiʻi Institute of Marine Biology, shared some interesting shark facts. Meyer is part of a research team using tracking devices to gain new insights into tiger shark movements in coastal waters around Maui and Oʻahu. The ongoing study provides insights into how these ocean predators swim, eat and live.
Insights from Carl Meyer:
Hawaiian oral traditions clearly link the fall months to a risk of shark bites. This traditional knowledge is reflected in our current shark bite statistics. In recent decades, almost one third of all shark bite incidents in Hawaiʻi have occurred during the months of October and November alone.
Shark bites may be linked to tiger shark pupping season
Hawaiians have also long known that fall is pupping season for tiger sharks, and the ‘fall spike’ in shark bites may well be linked to this natural, annual phenomenon.
By using modern scientific tools, we’ve been able to add some more details to the existing body of traditional knowledge of shark ecology in Hawaiʻi. For example, we have shown via electronic tracking, that adult female tiger sharks captured at remote atolls in the Hawaiian chain are most likely to migrate to the main Hawaiian Islands (MHI) during the fall months when pupping occurs. However, we don’t yet know how much this increases the overall number of large tiger sharks in MHI waters (there are also adult females living in MHI waters year round).
“Female tiger sharks migrate during fall pupping season,” September 6, 2013
“Study reveals tiger shark movements around Maui and Oʻahu,” November 26, 2014
It takes a tremendous amount of energy to nourish each litter of shark pups during pregnancy. Previous scientific studies have shown that pregnant females of other species of shark are in poor condition in the run-up to giving birth and during the postpartum period. Sharks store energy in their oil-filled livers and the livers of females shrink significantly during pregnancy, indicating that the energetic cost of nourishing their pups outpaces the replacement of energy via feeding. These pregnant females are effectively starving themselves to nourish their pups.
We do not yet know for sure whether this loss of condition also occurs in pregnant female tiger sharks, but we have no reason to believe that it does not. Tiger sharks have relatively large numbers of pups compared to their closest relatives, so it is likely that liver shrinkage does occur during pregnancy in tiger sharks. It is possible that pregnant and postpartum female tiger sharks are feeding more frequently than other individuals, as these hungry females try to replenish their diminished energy reserves. However, this is just an unproven hypothesis at this point.
Thus the fall spike in shark bites seen in the main Hawaiian Islands may be driven by a combination of increased numbers of large female tiger sharks in our coastal waters at this time of year, and perhaps also by changes in behavior, specifically greater feeding activity by pregnant and postpartum female sharks during the fall months.
Be mindful of ocean safety
However, it is also very important to remember that shark bites occur in all months of the year in Hawaiʻi, and that the number of bites at any time of the year is extremely low compared to the number of people in the water. The number of people living in Hawaiʻi and using the ocean for recreation has increased over time and this is the single most likely reason for a higher number of shark bites in recent years.
Ocean safety professionals provide sound advice on how to reduce the already very low risk of shark bite, and how to improve the outcome when shark bites occur. Surfing and swimming with other people and staying out of murky water are simple actions we can take to avoid being injured by sharks.
Read the following UH News stories to learn more about Meyer’s research.
- “Deep sea sharks are buoyant,” June 19, 2015
- “Researchers reveal insights into how sharks swim, eat and live,” February 27, 2014
- “Researchers tag more tiger sharks to track online,” February 5, 2014
- “Research team tags tiger sharks off Maui,” November 19, 2013