Neil Hammerschlag (© 2005)

Strategic hunter. A new study suggests that sharks pick their spots carefully when hunting seals.

Tracking Killers of the Sea

A criminal investigation tool used to place a suspect at the scene of a crime is now being applied to track vicious killers in the ocean--great white sharks. Typically used in serial crime cases, geographic profiling evaluates crime-scene locations to determine the most likely area of the perpetrator's residence. Now, for the first time, a research team has used the tool to study sharks hunting Cape fur seals off the coast of South Africa.

Despite their strength, sharks don't have it easy when it comes to hunting. Their main prey, seals, can turn on a dime and rapidly zigzag. "The sharks have to rely on absolute ambush," says Neil Hammerschlag, a shark researcher at the University of Miami, Florida.

But do sharks swim aimlessly until they just chance upon a seal? Or do they have a more sophisticated hunting strategy? To answer that question, Hammerschlag and colleagues documented 340 shark attacks on seals near Seal Island, where some 64,000 Cape fur seals forage. When the team spotted a great white shark chasing a seal, they determined the location using the global positioning system, estimated the shark's size and noted the environmental conditions, including water depth and temperature, and ocean-bottom topography. They also recorded whether the seal got away or was eaten.

Next, the team used a mathematical model to determine the most likely base of attacks. The results showed that the sharks they observed station themselves at specific points--but not necessarily where the chances of catching a seal were greatest. Instead, these locations appeared to offer the most advantageous balance between ability to detect prey, competition among sharks, and the chances of catching a seal, the team reports today in the Journal of Zoology. The team also found that smaller, younger sharks tend to search for prey over larger, more dispersed areas and have less success capturing seals. That could mean that sharks learn through experience to concentrate their efforts in areas that offer the best hunting conditions or that larger sharks exclude the smaller ones from the prime hunting spots.

"Geographic profiling is a novel approach that allows the analysis of predator movements with relation to their prey," says marine biologist Robert Hueter of Mote Marine Laboratory in Sarasota, Florida. Hueter cautions, however, that the study yields information only on predatory patterns of groups rather than on the hunting behavior of individual sharks.

Posted in Earth, Environment