Chris Fallow, a white shark expert discovered this in December of 2012 while he was located around 20 nautical miles off the shore.
The seal not only eat on sharks but the food chain in the open sea is not accurate as they thought it be before the discovery.
Fallow discovered that there was a group of ten blue sharks in the regions and a young cape fur seal about the size of the shark. The seal launched an attack on the group and five of the sharks in the group succumbed to the seals attack.
Fallow documented the entire episode on camera and every one was surprised to know it. Seal usually feeds on crab, small fish, squid.
The discovery enlightened that seal also feeds on infant shark and they also scavenge food from deceased ones.
But they are never seen attacking adult sharks.
Fallow says that they attack shark periodically to reduce the completion for food.
It is not clear whether this is the periodical event or a new behavior in the seals due to some outside source.
Research is needed to clear all these doubts and to come to a conclusion.