Earlier today WhySharksMatter and I had a little light-hearted smack talk about new-school (stable isotopes) vs. old-school (gastric lavage or straight-up dissection) methods of measuring the diet of sharks. These are the things you could be privy to if you were following me on that newfangled Twitter thing (at “press time” I only have 113 followers so there’s still room to get in on the ground floor). This coincided with/provided procrastination during one of my occasional literature searches to make sure I’m not missing some brilliant paper on shark feeding habits that I should be citing (grad school life is thrilling), and got me to thinking about what else we can learn from shark puke.
Generally we’re looking for relatively direct information when we look at feeding habits, and most of this information tells us about the predator. Look at enough samples and you can tell what the most common prey is, estimate roughly how much is eaten in a certain period of time, figure out what prey is most nutritious for the predator, etc. By looking at the damage to the prey items you can find out how fast digestion occurs, or even how the unfortunate critter was attacked (a lot of my dogfish like to grab large menhaden head-first, though plenty also took out the tail). If you have information on the abundance of the prey species in the environment, you can also fairly easily find out whether you’ve got a generalist predator or one that is picking out favorite foods, or where the predator is doing its hunting. But again, all of this is telling you about the predator.
Diet data can also tell us quite a bit about the prey, and not just how many fish we can expect to lose to the jaws of death in a year. Even in the post Census of Marine Life era, there are still species out there that are known mostly or entirely from showing up in the stomachs of their predators. Depending on where the predator was found, we can learn new things about the distribution of the prey species. Finally, though it comes with a lot of caveats, we may be able to find out whether some prey species are increasing or decreasing in abundance.
One of the papers I stumbled across today dealt with the feeding habits of blue sharks off Baja California, using data from longliners operating out of Ensenada, Mexico. What initially attracted me was that this study (Markaida and Sosa-Nishizaki 2010) involved blue sharks, one of the coolest-looking sharks ever and a species I hope to work with someday. Like spiny dogfish, blue sharks are a wide-ranging generalist predator whose diet tends to change depending on their geographic location (as a large shark, their diet actually includes spiny dogfish). In general, blue sharks tend to eat more fish inshore and more cephalopods offshore. This study made a couple interesting squid-related discoveries by looking at what the local blue sharks are eating offshore.
The first was that a few heavyweights of the cephalopod world showed up in the diet, one of which had not been found in the area before. While it’s cool that Humboldt squid and vampire squid (which has one of the coolest scientific names ever) are being fed upon by blue sharks, it’s also not terribly surprising. What is surprising and a little enlightening is that a pair of giant squid beaks were found. Because the only hard part of a squid’s body is the beak, that’s often all that remains for identification in the bellies of their predators. The presence of giant squid beaks, aside from raising the question of whether blue sharks are a regular predator of Architeuthis dux, represents the southernmost appearance by giant squid in the California Current ecosystem and is also the first record of one in Mexican waters. And this discovery was only made because blue sharks decided to eat a couple juveniles (back calculation showed a mantle length of about 74 cm or a little less than 30 inches, which, once you stretched out the tentacles, would still be a pretty big squid).
Possibly the most interesting find is that a poorly-known squid known as Gonatus californiensis made up the highest percentage of the blue shark diet by weight. Apparently adults of this squid have never been seen and there is some debate as to whether they are a separate species or the juveniles of something already known. What can be told from blue shark gut contents is that whatever this species is, it’s very common on the shelf break off of Ensenada and is apparently very tasty.
But then this is a dogfish blog, isn’t it? Don’t worry, those infamous piranhas of the sea have been pressed into service to tell us about their prey too.
Ctenophores are big concern for marine biologists and fisheries managers because their numbers seem to be exploding and they are voracious predators of the planktonic eggs and larvae of many economically-important fish and shellfish. Among their predators is the mighty spiny dogfish, which gulps them down like candy despite the fact that they’re over 90% water (then again, it’s hard to think of anything in the ocean that would be easier to catch). The opportunistic feeding habits of spiny dogfish have been well-documented, and the general rule is that they prey upon whatever is most common in the environment at the time and location. Link and Ford (2006) used these feeding habits to provide evidence that yes, ctenophore abundance is exploding, because their presence in the diet of spiny dogfish has also exploded since the 1980s. It should be noted that feeding habits are influenced by much more than just prey abundance, and using dogfish as a kind of “living trawl survey” is a bit of stretch. That said, gelatinous critters tend to be tough to quantify by traditional means, and an increase in the amount of snacking on ctenophores just may imply that there are more ctenophores out there. This type of analysis is best suited to supporting data taken by other means, but it’s better than nothing in a pinch.
That may not be a whole 101 uses for shark puke, but it does show the diversity of information that can be gathered by figuring out what the predators are eating. In the right hands with the right data, diet studies can be a restaurant guide to the ocean.
Link, J., & Ford, M. (2006). Widespread and persistent increase of Ctenophora in the continental shelf ecosystem off NE USA Marine Ecology Progress Series, 320, 153-159 DOI: 10.3354/meps320153
Markaida, U., & Sosa-Nishizaki, O. (2010). Food and feeding habits of the blue shark Prionace glauca caught off Ensenada, Baja California, Mexico, with a review on its feeding Journal of the Marine Biological Association of the United Kingdom, 90 (05), 977-994 DOI: 10.1017/S0025315409991597