Sharks Scare the @#$% out of Seals

ResearchBlogging.orgHow does fear drive a marine food web? That’s right I said fear, as in blood-curdling-scream-I-think-I-just-wet-my-pants fear. Sharks just swim around and intimidate the hell out of other animals. What if a utopia state existed where sharks weren’t invited and a society developed of peace, love, and understanding? What would that society look like?

In actuality we are not too far from that state as shark populations continue to decline. Frid and colleagues publish a model this week in the journal Oikos addressing the impacts of shark declines and “fear-released systems”. The model is elegant containing foraging decisions by harbour seals on herring (near surface dweller) and walleye pollock (found deeper in areas preferred by sharks). Seals make decisions on how deep to dive and for how long, time breathing oxygen at surface, time spent traveling in water column, and how they prey upon the fish. All these affect both their risk of becoming a tasty shark treat and their own ability to fill up on tasty fish snacks. The important is that in the model seals don’t actually get eaten. It is simply the fear of being eaten that drives seal decisions.

The simulations suggest that fear, i.e. predation risk, is sufficient to drive seals toward underutilizing the riskier deep and thus ignore pollock. This scenario is an asymmetric trophic cascade, so name because the influence of top trophic level (sharks) on the next trophic level (seals) cascades to another trophic level (fish) and asymmetric because it differentially affects pollock and herring. With no shark risk seals switched to pollock.

Frid, A., G. Baker, G., M. Dill, L. (2008). Do shark declines create fear-released systems?. Oikos, 117(2), 191-201. DOI: 10.1111/j.2007.0030-1299.16134.x

Dr. M (1729 Posts)

Craig McClain is the Assistant Director of Science for the National Evolutionary Synthesis Center, a National Science Foundation supported initiative. He has conducted deep-sea research for 20 years and published over 50 papers in the area. He has participated in and led dozens of oceanographic expeditions taken him to the Antarctic and the most remote regions of the Pacific and Atlantic. Craig’s research focuses on how energy drives the biology of marine invertebrates from individuals to ecosystems, specifically, seeking to uncover how organisms are adapted to different levels of carbon availability, i.e. food, and how this determines the kinds and number of species in different parts of the oceans. Craig’s research has been featured on National Public Radio, Discovery Channel, Fox News, National Geographic and ABC News. In addition to his scientific research, Craig also advocates the need for scientists to connect with the public and is the founder and chief editor of the acclaimed Deep-Sea News (, a popular ocean-themed blog that has won numerous awards. His writing has been featured in Cosmos, Science Illustrated, American Scientist, Wired, Mental Floss, and the Open Lab: The Best Science Writing on the Web. His forthcoming book, Science of the South (, connects cultural icons of South such as pecan pie with the science behind them.

2 comments on “Sharks Scare the @#$% out of Seals
  1. Interesting study – thanks for pointing it out. I sometimes think I should have become a theoretical ecologist – there’s something appealing in combining the “crispness” of mathematical models with the “messiness” of ecological systems.

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