Every summer in the remote Northwestern Hawaiian Islands, a dramatic ecological shift takes place—one that few divers ever witness, but one that reshapes the underwater neighbourhood. As thousands of seabird fledglings take their first flights from their nests, tiger sharks approach and wait for one to alight on the sea. And the balance of power beneath the surface changes.
Tiger surveying the surface. (Credit: Ila France Porcher)
A new study led by scientists from the University of Hawaiʻi reveals how this seasonal pulse of seabirds triggers what can only be described as “shark turf wars”. The fledgling seabirds at French Frigate Shoals concentrate tiger sharks in specific areas, forcing other shark species to change where and when they roam.
The lead author, Chloé Blandino, said that the tiger sharks gather around the small islands in summer to hunt the inexperienced youngsters. This feeding opportunity forces smaller sharks to adjust their movements. It shows how one food source can influence an entire community of predators.
A seasonal feast draws in tiger sharks
French Frigate Shoals is a shallow coral atoll dotted with small islands, which provide prime nesting habitat for seabirds and an important nursery for marine life. During the season when young seabirds take wing, the researchers found that tiger sharks move in close to these islands, waiting to grab any juveniles that end up in the water.
Using acoustic transmitters, the research team tagged 128 sharks, including tiger sharks, Galapagos sharks, and grey reef sharks, and tracked their movements for more than two years. The data revealed striking behavioural shifts comparing the seabird nesting season with the rest of the year.
How smaller sharks respond
When tiger sharks were present, grey reef sharks largely abandoned their usual haunts, likely to avoid becoming prey themselves. Galapagos sharks, which overlap more closely with tiger sharks in size and diet, did not leave entirely—but instead adjusted their schedules and locations, moving through different zones or using the atoll at different times to reduce competition.
Once the seabirds dispersed at the end of summer, the pattern reversed. Tiger sharks moved on, and the smaller sharks returned to their preferred habitats.
Seabirds reshape the food web
To rule out other explanations, the researchers conducted fish surveys and confirmed that these changes were not driven by shifts in prey fish. The smaller sharks were not following the movements of the fish—they were responding to the approach of the tigers.
Carl Meyer, co-author and principal investigator, explained that the study demonstrates the prominent impact of seabirds in ocean ecology as they influence the movements of top predators like tiger sharks. Such effects ripple through the entire food web. Understanding these connections is crucial for managing Hawaiʻi’s marine ecosystems.
What happens when nesting islands disappear?
The findings also carry a warning. If seabird populations decline, or if nesting habitats are lost, the effects will not stop at the shoreline. In 2018, Hurricane Walaka wiped out several key islands at French Frigate Shoals, dramatically altering available nesting habitat. Events like this can disrupt long-standing predator-prey relationships, with consequences that cascade through the ecosystem.
For divers, the study offers a powerful reminder: What happens above the water—on land and in the air—can fundamentally shape what we see below the surface. In places like the Northwestern Hawaiian Islands, the rhythm of wings overhead helps decide which sharks rule the reef.
