When we came across the headline “How shark poo keeps coral reefs healthy” we first had a bit of a chuckle and then clicked through to read about some interesting research on vital role marine predators have in supplying nutrients to the coral reef ecology.
As hobbyists, we know all about the importance of nutrients, but in our closed systems our goal is primarily to rid our tanks of excess nutrients — particularly fish waste.
The team that fielded the study looked at the grey reef shark (Carcharhinus amblyrhynchos), one of the most common reef sharks in the Indo-Pacific, to better understand their wider ecological role. They found that the sharks may eat their fill in the open ocean, but they do transfer key nutrients to shallower reefs all from their fecal matter.
You are probably wondering how they figured this out, I know I did. They set out to track these sharks on the 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) using acoustic tags to map their movements.
Combining this with we already know about their open ocean feeding habits, they were able to correlate the sharks’ movements and then estimate the amount of nitrogen deposited on the reef. Yup, they estimated how much poo dropped on the reef and how much vital nitrogen — 208.3 pounds to be exact — were deposited on the reef each day thanks to these predators.
Now this number is the result of the estimated 8,300-plus sharks and as we know in our tanks, reefs have unique nutrient needs. The benefit is the sharks are harvesting a rich source of nutrients miles away from the reef and act as nutrient vectors — like target feeders — for the reef that likely contributes substantially to reef productivity.
Why does this matter? Scientists continue to fully understand the dynamics of coral reef ecosystems — particularly with current trends of acidification, warming waters and bleaching events. Plus there is more understanding needed of the importance of predators like grey reef sharks, especially as 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,” said senior co-author David Jacoby of the Zoological Society of London’s Institute of Zoology.
Other co-authors are Darcy Bradley of UCSB, Jessica Williams of Imperial College London and Yannis Papastamatiou of Florida International University.
The international team’s findings appear in the journal Procedings of the Royal Society B.