Turf wars: Sharks vs. seabirds cause habitat shifts

Contact:

Seasonal seabird nesting in the Northwestern Hawaiian Islands triggers shark turf wars and habitat shifts among different shark species, according to a new study led by scientists from the University of Hawaiʻi at Mānoa Hawaiʻi Institute of Marine Biology (HIMB) Shark Lab. The research, published in Ecosphere, found that the annual summer arrival of fledgling seabirds at French Frigate Shoals (FFS) 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 Chloé Blandino, lead author of the study and shark husbandry research specialist at HIMB Shark Lab. "It's a clear example of how a seasonal food source can influence habitat use by an entire predator community."

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.

Avoiding predators, competition

The researchers found that when tiger sharks are present, the 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. 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," said 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."

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. This research was funded by NOAA's National Marine Sanctuary Program and the National Marine Fisheries Service.

Photo caption and credit

Image 1: Tiger shark captures albatross fledgling off of East Island, French Frigate Shoals atoll. Credit: Joe Spring, NOAA

Image 2: Hawaiian monk seal pups cautiously watch a Galapagos shark patrolling in the shallow waters of Trig Island, French Frigate Shoal atoll. Credit: Carl Meyer