Turf wars: New UH research finds tiger sharks vs. seabirds cause habitat shifts of other sharks

There’s a turf war happening in the Northwestern Hawaiian Islands.

Seasonal seabird nesting in the region triggers sharks of different species to adjust their habitat use, according to a new study led by scientists from the University of Hawaiʻi at Mānoa Hawaiʻi Institute of Marine Biology Shark Lab.

The research, published in Ecosphere, found the annual summer arrival of fledgling seabirds at French Frigate Shoals concentrates tiger sharks in specific areas.

“We discovered that tiger sharks gather around small islands in summer to hunt fledgling seabirds, which, in turn, forces other smaller sharks to adjust their habitat use,” said shark husbandry research specialist at Hawaiʻi Institute of Marine Biology Shark Lab and study lead author Chloé Blandino in a release about the findings. “It’s a clear example of how a seasonal food source can influence habitat use by an entire predator community.”

The research team tagged 128 sharks using acoustic transmitters and tracked their movements around French Frigate Shoals throughout 2 years.

They compared shark habitat use during the seabird season and off-season, observing clear behavioral shifts.

Researchers found that when tiger sharks are present, smaller gray reef sharks avoid these areas to reduce the risk of being eaten. Meanwhile, Galapagos sharks shift to different times or zones within the atoll to minimize competition.

Tiger sharks move on once seabirds disperse and the other shark species return to their original habitats.

The study also used fish surveys to confirm that these shifts were driven by predator avoidance and competition, not by a change in the distribution of their usual fish prey.

“This study highlights the far-reaching impact of seabirds, showing they can shape the movements of top predators like tiger sharks, which then ripple through the entire food web,” said principal investigator at Hawaiʻi Institute of Marine Biology Shark Lab and study co-author Carl Meyer in the release. “Understanding these predator-prey links is crucial for managing Hawai‘i’s marine ecosystems.”

The study noted that if bird populations shift, or if critical nesting habitats are lost — as occurred when Hurricane Walaka decimated key islands in 2018 — it can trigger cascading effects.

Their research was funded by National Marine Fisheries Service and National Oceanic and Atmospheric Administration’s National Marine Sanctuary Program.