Right now, in the ocean around Olowalu, Maui, tiger sharks are gathering for breeding season. But before University of Hawaiʻi graduate student Paige Wernli and fellow researchers at the Hawaiʻi Institute of Marine Biology made this discovery, tiger shark mating was something of a mystery.
“This is the first evidence of a recurring tiger shark mating aggregation anywhere in the world,” Wernli told HPR. “So we've kind of been able to nail down, like they don't mate randomly. It's not just random chance encounters, they actually do come together predictably in space and time.”
With six years of data, Wernli and fellow researchers have identified Olowalu as the first-ever documented mating hub for the species, which are typically known to be solitary wanderers with wide habitat ranges.
“This really fundamentally changes how we understand tiger shark behavior and ecology,” Wernli said.
Wernli is nearing completion of her Ph.D. and is the lead author of a newly released study. She’s been working on the project since 2021.
More sharks on Maui
Wernli said the research originally began as an effort to discover why Maui has more shark attacks than other islands, led by Carl Meyer, Institute of Marine Biology principal investigator. Meyer is also a co-author with Wernli and other researchers on the mating study.
“What they found essentially was that the insular shelf habitat around Maui is able to support just a larger population of tiger sharks,” Wernli explained. “There's a lot more tiger sharks in that Maui Nui area than anywhere else in the main Hawaiian Islands.”
Researchers gathered the shark data from acoustic tags placed on the sharks for 10 years at a time.
“This type of acoustic monitoring that we do is a kind of passive tracking,” she said. “So we equip tiger sharks with these acoustic tags that have their own coded pings that they send out every, you know, few seconds. So as they're swimming around, we also have our receivers out that receive these coded pings from the transmitters in the shark so we know when they're around receivers.”
Researchers not only narrowed down the location but also a peak activity time — 3 p.m.
You might wonder how, exactly, researchers tag the sharks.
“We're very smooth and efficient and very respectful to the animals," Wernli said. "And we get them in, we get the job done, we get them out."
The process involves using a hook and bait to attract the sharks, securing them safely to the side of the boat with rope while they measure and secure the tag, then remove the hook and "they swim away strong," she said.
"It's a pretty quick process, down to usually less than 15 minutes in total," she explained.
What does mating involve, and why Olowalu?
For female tiger sharks, mating can be a life-or-death activity.
“We don't see females mating with multiple males,” Wernli said. “That's likely because mating is a dangerous activity for females. They could die, essentially.”
Researchers regularly noted shark injuries consistent with mating activity.
“You'll see like they have their dorsal fins are all torn up, and they have teeth rake marks on their flank, and the males have fresh abrasions on their claspers,” she told HPR.
It’s not clear why Olowalu is favored for mating, but Wernli thinks it may be linked to the area’s high humpback whale population.
“You can't count out the fact that Olowalu has the densest population of humpback whales. So that's where the timing of the whales comes in, and that mating and foraging on humpback whales may not be these mutually exclusive activities.”
Wernli said healthy, adult humpbacks aren’t shark prey targets, but sick or vulnerable calves, along with whale placentas, may be food for tiger sharks.
Instead of a mass of mating sharks, as many people might picture, their activities span miles and weeks.
“This study expands our knowledge of tiger shark mating and challenges our conventional understanding of the term ‘gathering,’” Meyer said in a press release. “Together, the results suggest that both reproduction and food availability play key roles in shaping tiger shark movements in Hawaiʻi.”
No extra bite concern
If you’re in the ocean near Olowalu during mating season in January and February? There’s no extra danger, Wernli said.
“We don't see any uptick in the number of bites during that time of year,” she confirmed. “This behavior isn't linked to any increase in aggression or anything like that. If anything, it's just a really cool area to be in during that time that could be happening around you.”
The data backs that up.
The Department of Land and Natural Resources records list three Olowalu shark bite incidents occurring between 2000 and 2016. There are about 60 listed since 2000 in other areas of Maui — and about half that on Oʻahu.
Despite the groundbreaking research, there’s still lots to learn.
Wernli placed small cameras on tiger sharks trying to capture never-before-seen video footage of mating behavior. So far, it remains elusive. But she plans to continue her research after her Ph.D. is complete and one day get that visual evidence.
“The final piece of the puzzle will be me actually explicitly documenting that mating behavior from the camera tag,” she said.
But for now, finding proof of a group mating site is already making waves.
“Being able to see that clear of a pattern in such a wide-ranging species was definitely a really special find,” Wernli said.
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