A quick look around this blog will tell you that I’m very interested in fish eating other fish. My Master’s research was on the feeding habits of spiny dogfish, and I’ve tried to keep up with the literature on shark-related predation ever since. It turns out that the study of fish eating other fish can tell us a lot about other aspects of the fish’s life and how many we can potentially eat ourselves. Here are some recent papers I found interesting, involving fish eating other fish, sharks eating fish, and sharks eating other sharks. Even better, all of these papers are totally Open Access, so no need to pay up or use a school’s internet to read them.
Predation affects animals both directly (by being eaten or doing the eating) and indirectly, usually through behavior intended to minimize the risk of being eaten. Frid et al. (2012) wanted to see whether the degree of risk-taking or risk-avoiding behavior in kelp forest fishes varied by species. To test this, they set up bait stations using tethered shrimp in reef areas in Howe Sound (near Vancouver), and placed a fiberglass replica of a lingcod, a large predatory fish that functions as a high-level predator in Pacific Northwest kelp ecosystems. They then set up a camera and observed the behavior of the fish checking out the bait. What they found was that antipredator behavior trumped feeding in long-lived, slow-growing species such as rockfish, while faster-growing species like kelp greenling were more likely to risk being eaten to snatch up the bait. This is a trend seen very generally in nature. Since longer-lived species have to survive longer in order to reach the point where they can reproduce, they tend to be more risk-averse. Conversely, life is “cheaper” for the more r-selected species, and some of these species may need to prioritize food over safety in order to fuel their fast life cycles. In the case of these fish, this study implies that a reduction in predator numbers (like the lingcod) may make life easier for juvenile rockfish, which are important from both a conservation and fisheries standpoint. Whether that’s worth increasing fishing effort on lingcod is another matter, but this is the kind of information managers need when trying to manage the fishery at an ecosystem level.
Now we get to sharks. Sharks, even the smaller species, tend to be upper-level predators wherever they are, and as such may have important effects on the behavior of their prey. Hammerschlag et al. (2012) used satellite tagging of both bull sharks and tarpon (a very popular gamefish) to determine whether the paths of these big fish cross. It’s well-known that large sharks prey on tarpon (particularly if the tarpon are already hooked), but does that affect the movement patterns of these big-scaled fish? It turns out that tarpon definitely show evidence of avoidance behavior, avoiding habitat overlap with the sharks whenever possible. Also, when the fish have to cross areas where large sharks occur, they move in straight lines as fast as they can until they make it to cover. There’s always a bigger fish…
Sometimes information on feeding habits is found completely by accident. In a previous post, I summarized a paper about satellite-tagged fish being eaten by sharks, which actually showed up in the data logged by those tags. Beguer-Pon et al. (2012) had this happen to them when they deployed archival tags on American eels migrating out of the St. Lawrence River. Tags from six of their eels showed a sudden increase in ambient temperature and changes in dive profile, giving away that they were eaten by a warm-bodied fish. By comparing the gut temperature and dive profiles to those recorded for porbeagle sharks and bluefin tuna (gut temperature was too cold for a mammal, and these are the two warm-bodied fish predators in the area), they were able to figure out that their eels had been eaten by porbeagles. Tagging data from porbeagles themselves show that the sharks make long-distance migrations to the Sargasso Sea, and area of the Atlantic Ocean off the southeastern U.S. that also happens to be where eels migrate to spawn. Could the sharks be following the eels?
Sharks don’t just eat fish. Reptiles, birds, and marine mammals are all on the menu, and so are other sharks. The great hammerhead is notorious for eating other elasmobranchs, and Mourier et al. (2012) documented an epic attack by one of these sharks on a school of juvenile grey reef sharks. The hammerhead swooped in and snatched up one of the smaller sharks, and in response the entire school reef sharks gave chase. The predatory behavior must have been awesome to witness (and Mourier et al. did get some good pictures) but even more interesting is the mobbing behavior shown by the reef sharks. Most schooling fishes are totally fine with their buddies getting eaten instead of them and concentrate on getting away, but these sharks took off after their own predator, putting themselves at risk and exhibiting behavior typically seen in social birds and mammals. Was this simply an attempt to drive away their predator, or was this actually a rescue attempt? It is known that some sharks form strong social bonds, and reef sharks form especially strong ones. While it’s impossible to know the true motivation behind the chase, it’s certainly interesting.
I’ve said it before and I’ll say it again. Predation is awesome.
Béguer-Pon, M., Benchetrit, J., Castonguay, M., Aarestrup, K., Campana, S., Stokesbury, M., & Dodson, J. (2012). Shark Predation on Migrating Adult American Eels (Anguilla rostrata) in the Gulf of St. Lawrence PLoS ONE, 7 (10) DOI: 10.1371/journal.pone.0046830
Frid, A., Marliave, J., & Heithaus, M. (2012). Interspecific Variation in Life History Relates to Antipredator Decisions by Marine Mesopredators on Temperate Reefs PLoS ONE, 7 (6) DOI: 10.1371/journal.pone.0040083
Hammerschlag, N., Luo, J., Irschick, D., & Ault, J. (2012). A Comparison of Spatial and Movement Patterns between Sympatric Predators: Bull Sharks (Carcharhinus leucas) and Atlantic Tarpon (Megalops atlanticus) PLoS ONE, 7 (9) DOI: 10.1371/journal.pone.0045958
Mourier, J., Planes, S., & Buray, N. (2012). Trophic interactions at the top of the coral reef food chain Coral Reefs DOI: 10.1007/s00338-012-0976-y