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How shark poo keeps coral reefs healthy

It’s long been known that sharks help nourish coral reefs, but exactly how—or to what extent—has never really been mapped out. Until now.

A study with grey reef sharks (Carcharhinus amblyrhynchos), a predatory species commonly associated with coral environments but whose wider ecological role has long been debated, shows that the predators use their fecal matter to transfer vital nutrients from their open ocean feeding grounds into shallower reef environments, contributing to the overall health of these fragile ecosystems.

“The role of sharks as top predators is well understood, but their role as nutrient vectors is far less studied.”

“Our study shows that large mobile predators such as sharks may be a very important source of nutrients for even the smallest reef creatures, such as corals,” says coauthor Jennifer Caselle, a research biologist at the University of California, Santa Barbara’s Marine Science Institute. “The role of sharks as top predators is well understood, but their role as nutrient vectors is far less studied.”

Working in the waters surrounding Palmyra Atoll—a national wildlife refuge situated 1,000 miles south of Hawaii that the US Fish and Wildlife Service and the Nature Conservancy manages—the scientists used acoustic tags to map the sharks’ movements across the atoll. They combined their findings with existing knowledge about the sharks’ feeding habits in open ocean (pelagic) environments where they consume much of their prey.

Then, researchers analyzed this tracking data as a spatial network of movements and could, for the first time, estimate the quantities of nitrogen deposited around the remote unfished reef of Palmyra Atoll via the sharks’ fecal material.

Results estimate that this specific population of grey reef sharks—believed to number approximately 8,300 individual animals—contributes a combined total of 94.5 kg (208.3 pounds) of nitrogen to the reef ecosystem every day, an amount that likely contributes substantially to reef primary productivity.

By foraging for prey in deep pelagic waters often miles offshore, the sharks act as vital “nutrient vectors” to shallow reefs. They bring with them precious sources of nutrients like nitrogen, which in turn effectively act as a fertilizer for the thousands of other species that call these reef environments home.

Sensor mimics shark’s ‘sixth sense’ organ

The findings have implications for the scientific understanding of fragile coral reef ecosystems as well as for the ecological significance of grey reef sharks, scientists say. The species is currently classified as “near threatened” on the International Union for the Conservation of Nature Red List.

“Coupled with their better-known role as predators, our study underlines another, less obvious role played by reef sharks in improving the resilience of these fragile habitats and underscores the vital importance of conserving these and other wide-ranging predators,” says senior coauthor David Jacoby of the Zoological Society of London’s Institute of Zoology.

Darcy Bradley of UCSB is a coauthor of the paper that appears in the Procedings of the Royal Society B. Other contributors are from Imperial College London and Florida International University.

Source: UC Santa Barbara

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