Back in 2007, a landmark paper in Science changed how everyone thought about cownose rays. These smiley aquarium ambassadors suddenly became the most hated fish in the Atlantic. As the press release for that paper stated:
A team of Canadian and American ecologists, led by world-renowned fisheries biologist Ransom Myers at Dalhousie University, has found that overfishing the largest predatory sharks, such as the bull, great white, dusky, and hammerhead sharks, along the Atlantic Coast of the United States has led to an explosion of their ray, skate, and small shark prey species.
“With fewer sharks around, the species they prey upon — like cownose rays — have increased in numbers, and in turn, hordes of cownose rays dining on bay scallops, have wiped the scallops out,” says co-author Julia Baum of Dalhousie.
The study, which described the evidence for a shark-ray-shellfish trophic cascade leading to a collapse of the Chesapeake Bay scallop fishery, became an instant classic. “This is the first published field experiment to demonstrate that the loss of sharks is cascading through ocean ecosystems and inflicting collateral damage on food fisheries such as scallops,” said Ellen Pikitch, then a professor at the University of Miami Rosenstiel School of Marine and Atmospheric Science and executive director of the Pew Institute for Ocean Science, in the original press release.
The study had all the ideal components of blockbuster research: it was led by one of the world’s most preeminent fisheries biologists. It played to both sides of an age-old rivalry; it had a strong shark conservation message, providing much needed data to support the claim that sharks are vital ecosystem components. Yet at the same time, the study hit home with locals and fishermen, explaining why a once lucrative fishery was reduced to a mere shell of its former glory. The phenomenon it described—a top-down trophic cascade, with sharks as the key species—had been hypothesized for years but never demonstrated. And, whether intended or not, the paper provided an easy and achievable solution for the area’s woes: fish the rays instead of the sharks, and everyone wins. Frankly, it just made sense.
“People thought ‘Finally! Some evidence for this top down control by sharks,’ and accepted it without critically reading and reviewing the paper,” said Dean Grubbs, an elasmobranch ecologist with Florida State University. Though Grubbs and others had issues with the paper’s methods and conclusions, especially regarding the reproductive biology of rays, their initial worries were drowned out by the loud trumpeting the paper received. “We were concerned that this could quickly get out of hand,” Grubbs said. And it did. The paper became one of the most well known studies ever conducted in marine ecology, garnering almost 900 citations in the last 9 years.
“It just seems like virtually everyone who wants to talk about shark conservation knows this story, and most of them believe it,” said Sonja Fordham, founder and president of the non-profit Shark Advocates International. But when she read the press release almost a decade ago, she remembers being “troubled” by the reference to “hordes” of cownose rays. Rays, after all, are just flattened sharks, and share many of the same life history characteristics that make sharks so vulnerable to overfishing in the first place. “The paper was very pro shark conservation, so I found it very surprising that it would not see the potential danger of suggesting that a different type of elasmobranch had run amok.”
Her worst fears were soon realized, as cownose rays became touted as sustainable seafood. The states where the rays are native, including Maryland and Virginia, pushed to put ray fillets on everyone’s plates. “As this fishery developed and this paper became more and more widely cited, we decided we had to put a rebuttal together,” said Grubbs. That rebuttal was published last week in Scientific Reports, and it tears the notion of a shark-ray-shellfish trophic cascade to shreds.
Sharks, Flat Sharks, and Shellfish
Grubbs was immediately skeptical of the the 2007 paper. One of the first red flags was that the species involved in the food web domino effect don’t interact that much in the oceans. Yes, they occur in the same areas, but if large sharks control cownose ray populations through predation pressure, then scientists should find lots of rays in the stomachs of those sharks all the time. Instead, all elasmobranchs combined (sharks and rays) are only a fraction of large sharks’ diets (less than 13%), and for most species, cownose rays are just a small portion of that. “Upon review of 39 published diet studies for the large coastal shark species considered, we determined that cownose rays have been identified only in the stomachs of blacktip and sandbar sharks in the northwest Atlantic, but at low frequencies of occurrence,” the authors explain. In blacktips, cownose rays were only 3% of the diet, while in sandbar sharks, they were a mere 0.3%. The lack of predation on cownose rays actually makes sense, given that a recent study off Australia found that reef sharks aren’t the top predators in their ecosystem, as previously believed; instead, sharks are akin to groupers or similar mid-size predatory fish. So right from the get-go, the idea that sharks control ray populations because they’re apex predators is tenuous at best.
The support for the cascade gets even shakier when the stomachs of the rays are factored in. Scientists have found that scallops and oysters aren’t among the main prey items of cownose rays. In Chesapeake Bay, for example, scientists identified oysters and hard clams in less than 3% of cownose ray stomachs, and only 5% of rays harvested from commercial oyster grounds actually contained oysters. In fact, cownose rays prefer clams over oysters to begin with, and softer shelled mollusks in general, as large oysters are too big and tough for the rays to eat.
So not only do the sharks rarely eat the rays, the rays rarely eat the shellfish that they are supposedly wiping out. Of course, that’s not nearly as compelling a story.
“I have heard tales of how a school of cownose rays can decimate the young shellfish in a clam or oyster bed,” wrote Mary Reid Barrow in 2010, opining on why it is so important to eat cownose rays. “Gangs of bad-boy rays are free to roam the bay wreaking havoc on clam and oyster beds for their favorite meals, with no sharks to police them.” Opinion pieces like Barrow’s began to appear in papers and magazines all along the east coast beginning in 2007. Even non-profit organizations were convinced by the compelling narrative, helping spread the notion that the rays were an environmental disaster. Though sharks barely eat rays, and rays barely eat scallops and oysters, the tale of a trophic cascade and the renegade rays sold hook, line, and sinker. Suddenly, it was open season on cownose rays, and people began killing as many as they could.
'Cownose ray bow hunts, in particular, have become a popular sport. Photo by aon168
Scientists hope Grubbs et al.’s paper gives this guy something to smile about. Photo by aon168
It was a mistake to interpret the 2007 paper as an indictment of cownose rays. And when you think about it, the ‘kill the rays’ reaction to Myers paper makes no sense: the overwhelming response to a paper about how overfishing elasmobranchs can have negative consequences was to heavily fish an elasmobranch without any regard for the consequences.
“We’re most hopeful that this paper leads to more discussion and movement among the fisheries managers,” said Grubbs. “If they’re going to allow these bow fishing tournaments as well as other recreational fisheries and commercial fisheries to exist for cownose rays, then we hope that they try to assess what the stock status is and implement at least some precautionary catch limits.”
“These rays are very similar to (and perhaps even more vulnerable than) those large sharks that everyone is worried about,” he continued. “Allowing an unregulated fishery for them is as foolish as it was to let the fisheries for the large sharks to develop unregulated.”
Citation: Grubbs, R. Dean, et al. “Critical assessment and ramifications of a purported marine trophic cascade.” Scientific Reports 6 (2016): 20970. doi:10.1038/srep20970
