Day 3 of the AES conference was the big feeding symposium, and there was plenty to sink my teeth into (all puns always intended). Today is mostly made up of the stress symposium, which is a little technical for my tastes but still interesting and an important topic. As you saw yesterday, I decided to choose my social life over blogging, something the Hot Girlfriend and my friends who came into town to visit probably appreciate. On to the good parts…
-One of the big trends in shark research has been the use of stable isotopes to determine diet and trophic level. This is viewed as a viable, minimally invasive way of getting at the overall place in the food web, and is especially useful for the larger, harder to lavage sharks. Marcus Drymon’s talk was noteworthy in that it directly addressed my main questions about the method: how specific can it get? Is it possible to use this method to determine prey species? It turns out that when you directly observe the stomach contents, you can use the isotopic signatures of both the prey species and the sharks themselves to verify what you’re finding in the diet. However, this still requires you to validate your isotope findings with gut contents, and the results really only line up well for species that have highly specialized diets (a generalist feeder like the dogfish will apparently end up vaguely in the middle of the signals of its prey, and won’t give very good results). The irrepressible David Shiffman ran into exactly this problem using stable isotopes to determine the diet of sandbar sharks. He did attempt to determine the isotopic signals of his prey species, but sandbar sharks are a generalist enough feeder that no one prey species ever quite lined up.
In my editorial opinion, the isotope method shows promise but still needs to be validated by directly observing the stomach contents. Unless you have a very good idea of the shark’s diet you won’t be able to check for every common prey species, and this can vary widely by size, environment, and geographical area. Shark puke analysis is anything but dated; if anything, it’s become more important.
-Also of note were two talks on two of my favorite sharks, the blue and the mako. Kathleen Duffy compared the diets of blue and mako sharks in the Northwest Atlantic and found that blues are a very generalist feeder, while makos are almost absurdly favoring bluefish. The diet of blue sharks has also shifted recently to include more marine mammals (always a good thing) and as a result the trophic level of blues has actually increased, making them almost even with makos. What I found especially noteworthy was that both species have recently added more spiny dogfish to their diets, coinciding with the recent resurgence of the species in the Northwest Atlantic. Strangely, spiny dogfish are more important in the diet now than they were at their peak abundance before their crash. On the Pacific coast Dr. Preti compared the diets of blue, mako, and thresher sharks, and actually got the complete opposite result from the Atlantic makos observed by Duffy. In the Pacific it appears that makos actually have the most generalized diet, though they favor a particular cephalopod. The coolest part of this talk? Both blue and mako sharks are apparently major predators of the Humboldt squid. Suck on that, squid fans.
-On the smooth side of the dogfish equation, Derek Perry studied the feeding habits of smooth dogs in Buzzards Bay, Massachusetts. What he found was that smooths are major predators of both cancer crabs (which was expected) and American lobster (which was kind of surprising to me). These aggressive little Triakids apparently exert a heavy influence on their prey: Mass DMF ventless trap surveys show lowered catch of cancer crabs during the summer, when smooth dogfish predation is highest.
-There were some good review talks as well. David McElroy put together a very comprehensive review of stomach content analyses, Joe Bizzarro presented a massive review of everything currently known about ray feeding habits (including the poorly understood sawfish and torpedo rays), and this morning Greg Skomal reviewed the general concepts behind studying stress in sharks. Today is mostly made up of stress talks, which are a little beyond my scope of expertise. That effectively concludes my review of the talks at this year’s AES, but feel free to discuss anything I overlooked in the comments section.