Of sharks and scallops and questionable science

(from the June, 2007 issue of National Fisherman)

Overfishing sharks has wiped out the bay scallops! That’s the conclusion of a recent paper published in Science magazine (Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean, 03/30/’07) and widely reported by the media. Lead author Ransom Myers and four other researchers wrote that, because large sharks have been “functionally” wiped out by overfishing (which is in keeping with his controversial “all the big fish are gone” thesis from several years back), rays and other species among these sharks’ natural fare have proliferated at the expense of the bay scallops that the rays love to munch on. As proof they offer the paucity of sharks, the abundance of rays and their fellow scallop munchers, and a decline in bay scallop landings.

It seems pretty convincing; convincing enough to get past the supposedly rigorous peer review process in place at Science and other prestigious scientific journals.

If everything else affecting bay scallops has remained constant, then the decreased landings might be the result of an ominous sounding “trophic cascade” caused once again by those dastardly commercial fishermen. That’s not the case.

But before I get into that, consider the biology of bay scallops. Their range is from Cape Cod south and through the Gulf of Mexico. They’re short-lived, spend their entire life cycle in estuaries, spawn in the warm weather months and undergo a two-week planktonic stage before attaching to submerged aquatic vegetation (SAV). Obviously, they’re highly dependent on both estuarine water quality and the availability of adequate – in terms of both quality and quantity – SAV.

How to best describe what’s been happening with the estuaries these scallops, and the other shellfish the article implies have fallen victim to the demise of the large sharks, are dependent upon? Can you say “development?” A really lot of people have moved to the coasts in the last several decades. Since 1980 the population density has increased by 22%, 70% and 55% respectively in the coastal counties of the Northeast, Southeast and Gulf states. That’s a lot of additional flushing, a lot of additional non-point source pollution, a lot of additional silt, a lot less wetlands and a lot less SAV in those estuaries that bay scallops call home.

In addition, since 1980 the number of recreational boats in use in the U.S. has increased from twelve million to eighteen million. That’s a big increase, one inflicted proportionally on our estuaries.

Consider the impact of all that propeller-generated turbulence on the fragile scallop larvae bobbing around in the water column (coincidentally, they do all of their bobbing during the summer months when recreational boating is at its peak). Think of millions of huge, powerful horizontal blenders, each injecting noxious fumes into the water. And how about SAV impacts? You’ve probably seen those denuded tracks caused by boaters blasting across eel or turtle grass flats, and even when they’re not digging up the bottom, propellers are really good at increasing turbidity and obliterating SAV.

Predictably, Dr. Myers and crew discounted impacts on bay scallop landings other than shark fishing. And the people responsible for the culture of crisis permeating our print and broadcast media did their customary Chicken Little bit as well.

There’s a type of reasoning termed “after the fact.” In Latin, it’s described by “post hoc, ergo propter hoc," translated as "after this, therefore because of this." It seems to me that Myers’ et al’s major conclusion was based almost entirely on the idea that correlation means causation – and the only other support it had was a limited study of bay scallop predation in North Carolina involving six sites observed over three years (and with the confounding factor of a bay scallop fishing season coincident with the observations). Without a lot more supporting data than that presented, it’s hard to look at this research as anything more than a conclusion in search of a cause. But the media bought it hook, line and sinker.

Could cow nose ray predation impact bay scallop stocks? Could fewer large sharks mean more cow nose rays? Could more fishing pressure result in fewer large sharks? Maybe. But to assume no other factors at play, or that other factors aren’t as or more important to landings than shark fishing, is a far leap. In this instance, much too far.

(Dr. Ransom Myers died on March 27. Though I disagreed with much of his recent research, he was a pioneering fisheries scientist with many valuable contributions to his credit. His passing will affect us all.)  

Nils E. Stolpe