Sharks and Trophic Cascades: Cut and Dry?

ResearchBlogging.orgA recent post over at Chronicles of Zostera referenced a paper that has become a monster in the world of marine ecology and shark conservation.  That paper: Myers et al. (2007).  It’s actually a relatively unassuming paper kind of tucked into an issue of Science, but it lit the elasmobranch world on fire very quickly.  Whenever you hear anyone, be they scientist, fisherman, manager, or Joe Schmo making conversation at a grad student party (yes, we are known to combine nerdy conversation with drinking) talking about how the cownose rays are gobbling up all the scallops, whether they know it or not, they are most likely talking about this paper.

Myers et al. (2007) has had such an impact, less than three years after its publication it’s considered by many to be a classic and has been affecting management policy and at least one movement in the fishing industry.  In the process, cownose rays may soon become more reviled than spiny dogfish (have fun with that, batoid fans).  That said, remember, this is science, so it’s always complicated.

First off, here’s a quick primer on marine trophic cascades.

Your basic marine trophic cascade, simplified. From Ferretti et al. (2010).

Apologies for the blurriness above, but it should still be legible.  This diagram shows a highly simplified trophic cascade in the marine environment.  The basic idea is that fishing pressure removes the top predators (the large sharks).  This reduces the natural morality of their prey, and the mid-level or meso-predators enjoy a period of increased abundance and prosperity freed from the fear of being eaten.  However, the prey of these meso-predators now has even more of a cause to look over their shoulders, since their predators are now the ones running the place.  As a result, the forage species crash due to being devoured by increased numbers of things like sharpnose sharks, spiny dogfish, and of course cownose rays.

That’s the theory.  According to Myers et al. (2007), this is the theory in practice:

From Myers et al. (2007).

Using data from several different shark surveys (mostly longline), you can see marked declines in what the authors have termed the “great sharks” (the top row), occurring simultaneously with increases in meso-predator abundance (middle row), and finally leading down to a sharp downward slope for the humble bay scallop (bottom row).  The take-home message: not enough sharks means too many rays and not enough scallops.

So imagine my surprise when I attended AES over the summer and learned that many prominent shark and ray experts are actually pretty annoyed with this paper.  Enter Burgess et al. (2005).  Though this is actually a rebuttal to some older papers by Myers and frequent collaborator Baum, a lot of what is brought up is potentially relevant to the little monster they wrote up in 2007, and was referenced in most of the conversations I had about it in Providence.  The first and probably most striking thing about this rebuttal is the list of authors, people like George Burgess, Gregor Cailliet, Enric Cortes, Dean Grubbs, Jack Musick, Colin Simpfendorfer… these aren’t shills for the shark-finning industry here.  And they make a point several times in the paper to mention that they agree with Myers and Baum that sharks are in decline and that there is a need for concern.  What they are afraid of is that these papers that have had a disproportionate amount of influence over the shark conservation conversation (say that ten times fast) are based on flawed analysis of very small data sets, as well as an apparent lack of interest in testing any other potential hypotheses for the decline of sharks in the data.

One important thing to keep in mind is that the data used in Myers et al. (2007) was not personally collected by Myers or any of his co-authors.  Whenever you’re running an analysis on data that you didn’t have a hand in collecting, always make sure it’s reliable.  This is especially true in fishery-dependent data, where declines can be a result of changes in gear, lowered market price decreasing demand, and straight-up miss-identification.  Burgess et al. (2005) argue that the methods used in the type of analysis used by Myers et al. (2007) are wide open for exactly these issues, and that the authors seem to have done little or nothing to validate the data.

(I realize it’s pretty lame referencing a rebuttal that’s only really indirectly a rebuttal to the paper I’m actually writing about, but this was the closest I could find.  If a direct rebuttal to Myers et al. (2007) exists, please let me know, I’d be interested to read it.)

For a more even-handed synthesis of what is currently known, I recommend Ferretti et al. (2010) (tip o’ the hat to lab-mate Andrea for sending me a copy).  Aside from going over the concept of trophic cascades and even the debate over shark declines, it also offers some interesting food for thought on the meso-predators.  One thing that’s piqued my interest is that we hear all this noise about cownose rays when there are plenty of other small and mid-sized elasmobranchs out there eating commercially-important species.  The reason posited by Ferretti et al. (2010) is that other meso-predators may be regulated by fishing pressure themselves.  While cownose rays don’t really have much in the way of fishing pressure aside from the “Save the Bay, Eat a Ray” movement (which has yet to explode into a large-scale fishery), sharpnose sharks and smooth dogfish are taken as part of the coastal shark fishery, and spiny dogfish have their own directed fishery (though they’re blamed for plenty of ecological damage; see this post for my own personal rebuttal of most of that).  Also, the coastal ecosystem of the southeast U.S. may not have other marine predators capable of filling the gap in cownose ray coverage (in other systems other large predators such as tunas, billfish, and orcas may be able to regulate some of the same species fed upon by large sharks).  Due to these factors cownose rays may have been given a “get out of jail free card” that hasn’t been available to many other elasmobranch meso-predators, which is why they seem to stand out as such a model problem species.

Possibly lost among all this is the fact that cownose rays have effectively been villainized.  Due to this, it may be possible that any fishery that does open up for them will be a thinly-disguised eradication fishery.  This fishery will be on a species in which migration behavior, reproduction, and even true feeding habits are currently poorly understood.  I got a chill up my spine when towards the end of this article, a supposed fisheries policy expert makes the following quote: “There’s not a big fishery for them, so by default, it’s sustainable.”

Did the Myers et al. (2007) paper make the public aware of a shark conservation and bring the problem directly to their dinner plates and wallets?  Yes, and that’s probably a good thing.  However, is there still cause to proceed with caution?  Hell, yes.

Burgess, G., Beerkircher, L., Cailliet, G., Carlson, J., Cortés, E., Goldman, K., Grubbs, R., Musick, J., Musyl, M., & Simpfendorfer, C. (2005). Is the collapse of shark populations in the Northwest Atlantic Ocean and Gulf of Mexico real? Fisheries, 30 (10), 19-26 DOI: 10.1577/1548-8446(2005)30[19:ITCOSP]2.0.CO;2

Ferretti F, Worm B, Britten GL, Heithaus MR, & Lotze HK (2010). Patterns and ecosystem consequences of shark declines in the ocean. Ecology letters, 13 (8), 1055-71 PMID: 20528897

Myers RA, Baum JK, Shepherd TD, Powers SP, & Peterson CH (2007). Cascading effects of the loss of apex predatory sharks from a coastal ocean. Science (New York, N.Y.), 315 (5820), 1846-50 PMID: 17395829


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  2. John Carroll · October 6, 2010

    Hey man, good post. Thanks for putting it out there. I wasn’t aware there was so much debate. But I did see Demian Chapman give a shark lecture last week and mentioned it to him, and he raised some of the same issues you talked about here. His lab is looking at genetic diversity to try and get an idea on how much sharks (I think white sharks in particular) have declined. Again, good post and thanks for the shout out.

  3. WhySharksMatter · October 6, 2010

    I reference the Myers paper often in my public education talks, but I try to mention that it’s controversial among the scientific community. I also advise against the “Save the Bay, eat a ray” movement.

    Many of the criticisms of Worm, Baum and Myers as really criticisms of how some in the media or conservation community overhype their work (i.e. collapse of all seafood by 2048). Some are undoubtedly valid. There isn’t always good enough data to make the kind of ecosystem-wide analyses that these scientists are interested in, but they attempt to make do with what exists. An imperfect model is likely better than no model at all, as long as you acknowledge that it is imperfect. If they are guilty of anything, it is overselling their conclusions.

    • Chuck · October 6, 2010

      I think that’s the point Burgess et al. are trying to make. They state over and over again in their paper that they agree that sharks are in decline and that there are probably ecological consequences, but they think the problem has been overstated. In any case Myers et al. (2007) may ultimately be a good thing because as flawed as it may be, it definitely brought shark conservation a lot of press.

  4. Southern Fried Scientist · October 6, 2010

    Nothing is ever as simple as “X eats Y eats Z therefore less X = more Y = less Z” also missing from the paper was any look at why, if this really was the trend, do we not see it in other shark -> ray -> shellfish fisheries undergoing the same pressure. European as well as Asian fisheries have seen huge declines in sharks, rays, and scallops, yet none of these see the same cascade effect that is apparently so clear here.

    • Chuck · October 6, 2010

      I thought the Ferretti paper kind of got at that by mentioning that some of the mesopredators have fishing pressures of their own. Cownose rays have been off the hook (yay puns!) compared to other small sharks, skates, and rays when it comes to fishing pressure, so that could be why you see such an exaggerated effect involving the cownose rays. In other parts of the world those smaller sharks and rays may be fished just as hard as the large sharks (think Europe where they’ve not only fished out the big sharks, but also the dogfish and skates).
      And let’s not forget that predation is not the only thing that causes declines in prey species.

  5. Hannah Waters · October 6, 2010

    Great post! I’m actually a little too familiar with this paper, as my undergrad thesis advisor chose to grill me on this one during my defense. Oy. I was unaware of how controversial it is: I’m going to read through this post a few more times. Thanks!

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  7. Simon · October 6, 2010

    I’m just glad there is a conversation going on. Keep it up! It does everyone a great service.

  8. Ralph Thompson · October 6, 2010

    great post thanks

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  12. John Chamberlin · October 6, 2010

    In 2005 on the southeast portion of Kent Island, I took an associate on a walk in mid June, along the shore line for about 3/4 ths of a mile. There was a full SAV for about 300 feet from shore. Coming back in mid August I found the same area extremely muddy, most of the SAV gone, and in walking in the water saw or nearly stepped on at least 9 rays, The next year I visited Claiborne harbor, on the other side of Eastern Bay. Previously this was one of the remaining SAV beds. The area was basically stripped of all SAV. I saw rays in the shallows.

    Having walked and clammed at low tide, the clam population is too small to sustain a ray population. The question asked is what do they eat. I recommended to VIMS that during fisheries research that they catch and see what the rays are actually eating. I think that we have the thought that they only eat mollusks. I think that is in error and that they also eat SAV. If so this is probably one of the reasons that the SAV beds have not come back and actually decline.

    Objective research should be conducted and decisions made.


    John Chamberlin

    • Chuck · October 6, 2010

      Thanks for the comment John. There is actually a hypothesis that cownose rays rip up SAV as they feed and I’ve personally seen the pits dug by them in the middle of SAV beds. While I doubt that the rays actually eat the seagrass, they might be incidentally destroying it.
      I agree that diet studies need to be done, both on the rays and the sharks. Actually finding cownose rays in the stomach contents of large coastal sharks would be a lot more convincing than comparing graphs, and similar work needs to be done to determine just how big an impact the rays have on clams and scallops.

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