Blue Shark
The blue shark has a long slender body, long nose and long pointed fins. Blue sharks are around 6 to 12 feet in length and live to be more than 20 years old. They have dark blue backs and lighter blue sides. Blue sharks prefer to hunt squids, herring and cuttlefish, as the hunts are often relatively easy for them. Larger specimens, however, will actively and aggressively hunt rockhoppers as well. They will also scavenge dead penguins and marine mammals.
Leopard Seal
Leopard seals are large, aggressive seals found mainly in the Antarctic. They grow to up to 12 feet (3.7m) long and commonly weigh more than 1,000 pounds (455kgs). Penguins are the leopard seal's main source of food. The leopard seal will often wait in shallow water surrounding the area where rockhoppers breed. When the newly bred rockhopper penguins go into the water for the first time, the leopard seal will quickly snatch them up. The leopard seal's speed and the rockhoppers' inexperience make it an easy catch.
Fur Seal
The fur seal is a species of seal with fine fur, sharp eyesight and extraordinary hearing. Fur seals are typically 4 to 10 feet (1.2 to 3.1m) long and weigh up to 700 pounds (718kgs). Fur seals prefer to stay in the water, sometimes for weeks. They, like rockhoppers, eat krill and smaller fish, and thus often run into rockhoppers in the water. Larger fur seals will commonly forget the krill or small fish they're hunting and go after the rockhopper penguins instead.
Other Predators
Birds, namely Striated Caracaras which is also known as Johnny Rook, prey on eggs and newly hatched chicks. They typically stay in the air and swoop down, grabbing a chick. Kelp gulls and dolphin gulls are noted to attack eggs and hatchlings as well. In some regions, these penguins are also preyed upon by non-native animals. Cats and pigs will attack both adults and hatchlings, and rats have been noted to attack hatchlings.