A new study led by scientists from the University of Hawaiʻi (UH) Mānoa Hawaiʻi Institute of Marine Biology (HIMB) Shark Lab reveals a critical link between seasonal seabird nesting and the movements of top marine predators in the remote Northwestern Hawaiian Islands. The research, published this month in the journal Ecosphere, found that the annual summer arrival of fledgling seabirds at French Frigate Shoals (FFS) concentrates tiger sharks in specific areas, forcing other species, including gray reef and Galapagos sharks, to drastically shift their own habitat use to avoid predation and competition.
The discovery highlights a profound, indirect connection between terrestrial and marine ecosystems, demonstrating that the presence of a seasonal food source—fledgling seabirds—influences the behavior of an entire community of apex predators.
Using acoustic transmitters, the research team tagged 128 sharks and tracked their movements around FFS over two years. They compared shark habitat use during the seabird season and the off-season, observing clear behavioral shifts.
"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," explains Chloé Blandino, lead author of the study and Shark Husbandry Research Specialist at HIMB Shark Lab. "It was exciting to see our predictions line up so closely with reality; it's a clear example of how a seasonal food source can influence habitat use by an entire predator community."
The researchers found that when tiger sharks are present, the smaller gray reef sharks avoid these areas completely to reduce the risk of being eaten. Meanwhile, Galapagos sharks shift to different times or zones within the atoll to minimize competition. Once the seabirds disperse, the tiger sharks move on 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," notes Carl Meyer, co-author on the study and principal investigator of the HIMB Shark Lab. "Understanding these predator-prey links is crucial for managing Hawai'i's marine ecosystems, especially in fragile places like French Frigate Shoals, and helps protect Hawai'i's native wildlife and fragile ecosystems."
The study underscores the sensitivity of these ecosystems, noting 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. By studying these shark-seabird dynamics in Papahānaumokuākea, UH scientists are generating knowledge that supports conservation and stewardship across the islands.
