Events & Media

February 21, 2017

Counting Sharks to Solve a Trophic Mystery
A new study finds fewer of the predators than expected — and that may be a good thing


Santa Barbara, Calif.
― A new study led by Bren School alumna Darcy Bradley (PhD 2016) has found that a shark population on a remote coral reef is much smaller than had been estimated from previous population assessments, suggesting that the same is true on other coral reefs. And that, it turns out, is not necessarily bad news.


Grey reef sharks cruise the pristine waters around Palmyra atoll. This healthy population of the predators has far fewer individuals than previously thought. Photo: Palmyra Atoll Research Consortium

In “Resetting predator baselines in coral reef ecosystems,” published today in the journal Scientific Reports, Bradley and her co-authors shed new light on shark populations, finding that previous estimates of shark populations on a remote Pacific reef were about 100 times too high. That is important in the context of shark conservation. Joining her in the study were Bren School dean, Steve Gaines, Bren professor Bruce Kendall, UCSB assistant professor Doug McCauley, and UCSB research biologist Jenn Caselle, as well as researchers from The Nature Conservancy, the U.S. Fish and Wildlife Service, and Florida International University.

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It is widely known that shark species around the world are under assault on many fronts. Scientists and ocean managers who think about how to protect those that still swim in the sea or how to restore populations of species that have been hit hardest need a baseline to establish goals and decide how best to direct their efforts. For that, they need to know what a healthy, relatively undisturbed shark population looks like. While humans’ ubiquitous impacts make finding such a baseline ecology on land nearly impossible, in the oceans, relatively wild ecosystems remain. Palmyra Atoll, a remote, historically uninhabited U.S. Fish and Wildlife Refuge located roughly a thousand miles south of Hawaii, is one such place. With a no-fishing marine refuge extending fifty miles from shore, Palmyra’s marine community is largely intact.


Darcy Bradley and a fellow researcher tag a grey reef shark off Palmyra atoll.
Photo: Palmyra Atoll Research Consortium

In early studies at Palmyra conducted nearly twenty years ago, researchers examining the relationship between predators and prey counted fish while SCUBA diving. They counted a lot of fish, and a lot of big fish. In fact, to their surprise, they found a higher biomass of predators, like sharks, than of smaller fish that were their prey. The idea that a reef could support more predators than prey was so novel that it flipped some conventional views on food webs, giving rise to the idea that healthy coral reefs might have “inverted trophic biomass pyramids.” In a land-based parallel, one could imagine an African Serengeti where lions outnumbered the wildebeest they hunt.

"The results of those studies suggested a fundamental shift in the way we thought about healthy coral-reef ecosystems," said Jenn Caselle.

The inverted trophic biomass pyramid meant that healthy reefs were not like the reefs seen in most of the world, where small fish are abundant and large, patrolling predators are few. It meant, rather, that healthy reefs are loaded with predators. And if there are supposed to be more predators than prey, then reef conservation efforts that have not resulted in gangs of roving predators have failed, leaving a lot of work ahead to return reef ecosystems to health.

But can a system really support a permanent inverted trophic biomass, as early Palmyra studies suggested? Because inverted biomass pyramids had been observed previously only as temporary phenomena, many scientists doubted that a natural, stable inverted biomass pyramid could exist. A number of theoretical, math-based papers supported those doubts and motivated the researches in this study to return to where the phenomenon had been identified.

“We aren’t the first to question the validity of the inverted trophic biomass pyramid in relatively pristine reefs, but we wanted to actually resolve the debate by going back to Palmyra and using assessment tools that account for animal movement directly,” said Bradley, who is currently a postdoctoral researcher at the Bren School. “It’s hard to count things in the ocean, and even harder to accurately count things that move and move fast, like sharks. We suspected that over-counting highly mobile animals had led to inaccurate estimates of Palmyra’s predator population, so we focused our reassessment on the most-mobile, most-abundant predator at Palmyra, the grey reef shark.”

Over an eight-year period, the team tagged more than 1,300 grey reef sharks across all of Palmyra’s reef habitat and used acoustic telemetry tags to track the movements of some of them. Their findings show that there are far fewer — nearly one hundred times fewer — sharks at Palmyra than the previous study had estimated, effectively shattering the idea of a stable inverted trophic biomass pyramid in a healthy coral reef.

There is an irony here, which is that in a world of depleted shark populations, “The fact that there are fewer sharks on a healthy reef is not all bad news,” according to Steve Gaines.

Because the previous shark population estimates had been too high, recovery targets for shark conservation had also been set too high — impossibly high, as it turns out. A healthy reef simply can't support that many sharks, which means that many damaged reefs may not have as far to go as previously thought to restore shark populations to a healthy, near-natural state.

The down side is that, generally, there aren't as many sharks, even on the healthiest reefs, as had been thought. “A quarter of shark and ray species are known to be at an elevated risk of extinction,” Bradley said. “If there are fewer sharks in a healthy population, then some shark populations are even more precarious and far more prone to overfishing than we thought, making efforts to protect them more important than ever.”

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