Scientists have discovered an important link between seasonal seabird nesting sites and the movements of several shark species in the French Frigate Shoals, a large atoll of the Northwestern Hawaiian Islands in the Pacific Ocean.
Spanning 50 kilometres, the atoll’s crescent-shaped barrier reef encloses a shallow lagoon, which attracts a range of juvenile animals in the summer months, including newly hatched turtles, seal pups and fledgling birds such as albatrosses.
The arrival of so many young animals – particularly the seabirds – doesn’t go unnoticed by marine predators. Tiger sharks appear to know when this increase in vulnerable prey will occur, concentrating in specific areas in time with their arrival.
Now, a new study reveals that the sudden increase in tiger sharks in turn forces other smaller shark species to drastically shift their own habitat use to avoid predation and competition.
The research, published in the journal Ecosphere, highlights a critical, indirect connection between terrestrial and marine ecosystems, and shows that the presence of a seasonal food source influences the behaviour of an entire community of apex predators.
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To carry out the study, researchers from the University of Hawaiʻi (UH) Mānoa Hawai'i Institute of Marine Biology (HIMB) Shark Lab attached acoustic transmitters to 128 sharks, belonging to three species: the tiger shark, Galapagos shark and grey reef shark. They then tracked their movements around the French Frigate Shoals atoll over two years.
The team looked at the behaviour of the sharks, specifically the habitat they used both when the fledgling seabirds were present and when they were absent.
“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.”
Blandino and her team found that when tiger sharks were present, grey reef sharks, which are less than half the size of tigers, avoided these areas completely to reduce the risk of being preyed upon.
Galapagos sharks also changed their behaviour, appearing at different times or zones within the atoll to minimise competition with the tiger sharks.
As soon as the seabirds moved on, the tiger sharks dispersed and both the grey reef and Galapagos sharks returned to their usual habitat.
“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,” says Carl Meyer, co-author on the study and principal investigator of the HIMB Shark Lab.
The research shows just how finely balanced many of the world’s ecosystems are. In the case of the French Frigate Shoals, if the seabirds were to change where they fledged, or if vital nesting habitats are lost, the impact would be felt by various shark species, as well as other animals in the food chain.
“Understanding these predator-prey links is crucial for managing Hawai'i’s marine ecosystems,” says Meyer, with improved knowledge supporting decisions surrounding conservation and stewardship across the islands.
Top image: tiger shark captures albatross fledgling off East Island, French Frigate Shoals atoll. Credit: Joe Spring, NOAA
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