If you had to pick one species in the ocean that best meets the criteria for being an apex predator, it would probably be the white shark. Also known as the great white shark, this godfather of sharks is a marine celebrity. Commanding worldwide attention from the public and the media, these sharks land starring roles in blockbuster movies and annual television programs and are highly valued in the scientific community for what they can teach us about ecology and conservation.
It’s surprising that a species with so many eyes fixed on it still presents us with more questions than answers. Just when we think we know something about these sharks’ behavior or ecology, they surprise us and force us to rethink our theories. A recent study by our team — Chris Fallows of Apex Shark Expeditions; Neil Hammerschlag, Ph.D., director of the R.J. Dunlap Marine Conservation Program at the University of Miami; and me, a doctoral candidate at the University of Miami’s Abess Center for Ecosystem Science and Policy — proved just that.
Our study took place off a small rocky islet called Seal Island in False Bay, South Africa. During the winter months (May through September), white sharks can be seen patrolling these cold waters looking for Cape fur seals leaving and returning to the island on foraging excursions. Since 1997 we have been studying the ecology of these predator-prey interactions, and during this time we also observed four instances of white sharks scavenging on large whale carcasses. At each scavenging event, we recorded information about the sharks (including size, abundance and identity), environmental data (wind, water temperature, visibility and more) and details about the sharks’ behavior as they scavenged. The two whale species involved in this study were the southern right whale and the Bryde’s whale.
On each occasion it didn’t take long for the sharks to show up and begin scavenging. We observed multiple sharks feeding on the carcass each day (an average of five to 15 sharks per day and a maximum of 40 per day), with the same individuals returning on successive days. The scavenging events progressed in a mechanical and almost elegant fashion; it was as if the sharks were programmed to know how to best exploit the resource. Sharks swam to the carcass slowly and showed an initial preference for feeding first on the whale’s fluke before moving to the rest of the body. They also showed a clear preference for the high-blubber areas and would mouth several parts of the body before settling on a preferred area. Sharks would generally bite and perform lateral headshakes to tear off flesh and then move laterally to another area (like eating corn on the cob).
In many cases we observed sharks tearing off blubber only to regurgitate it and go back in for a different piece, which suggested that the sharks seek areas with higher caloric yields. We also watched as one white shark went into a carcass and pulled out and consumed a near-term fetus. A clear size-based pecking order among the sharks emerged, with the largest animals getting the first and longest scavenging opportunities, although the sharks did not show any signs of aggression toward one another. Taken together, these behaviors suggest that the scavenging is strategic and efficient.
Our results suggested that wind speed had a positive and significant effect on white shark abundance at the carcasses, which means that high winds likely play a role in dispersing the whale scent and attracting large sharks from offshore. The white sharks commonly seen feeding on seals at Seal Island average around 12.5 feet long, but when the whale carcasses showed up we saw animals up to 18 feet long. This could mean that as sharks mature and approach 15 feet, they lose the ability to successfully hunt the agile seals, and they might then switch to whales. Large congregations of these otherwise rare and solitary predators are not normally seen, so these scavenging events could also be important in promoting social interactions such as mating.
The impact of the dead whales on the ecosystem is also notable. The early arrival of the apex predators on the scene determines the availability of the resource to other species. In our study, the average number of shark predation events on seals dropped significantly when a whale carcass was present at Seal Island, but seal-hunting activity returned to normal rates two weeks afterward. The white sharks clearly change their behavior when a carcass is around, and this in turn changes the pressures on and behavior of their normal prey, the seals.
Scavenging is very important in food-web studies, but studying it in ocean ecosystems is challenging because of observational limitations. Scavenging, whether by vultures, wolves or insects, plays an important role in many ecosystems. Spotted hyenas are regarded as one of the most efficient predators in sub-Saharan Africa because they are top predators as well as efficient scavengers.
While our work is just one study, it suggests that scavenging on whales is probably more important to white sharks than previously thought. In fact, white sharks actually earned their name during the whaling era because fishermen saw flashes of their white undersides as they ate whale carcasses. As a scientist, it’s cool when your observations and research relate to something as vital to our concept of a species as its name.
The seasonal presence of white sharks at seal colonies and rookeries worldwide may create a biased perspective of their diet and foraging behavior. There is nothing quite like seeing a white shark breach on a seal (I think it is one of the most raw and spectacular behaviors in the animal kingdom), but the behaviors and patterns that emerged from our data on whale-carcass scavenging gave me a new appreciation for the flexibility and vigilance of these apex predators.
As these threatened animals mature and grow, so does their arsenal of predatory tactics — an amazing set of adaptations that contributes to two of the greatest stories ever told: biodiversity and evolution.
Reference
Fallows C, Gallagher AJ, Hammerschlag N. “White Sharks (Carcharodon carcharias) Scavenging on Whales and Its Potential Role in Further Shaping the Ecology of an Apex Predator.” PLOS ONE 8(4): e60797. doi:10.1371/journal.pone.0060797 (2013).
© Alert Diver — Q3 Summer 2013