Harbor seals defend themselves from humans and predators by relying on their sensitive hearing to alert them and allow them to vacate the area. During the breeding season, males protect their territory from rival males by engaging in stylized fighting. While strong and fast swimmers, harbor seals lack other defense mechanisms and often fall prey to killer whales, sharks and the occasional coyote or bobcat.

Although harbor seals have natural predators, they are classified as apex predators, feeding on fish and invertebrates alike. Their highly acute hearing is 14 times more sensitive underwater than on land, allowing them to distinguish between the calls of killer whales and their harmless cousins. Their whiskers are also able to detect sensitive vibrations in the water, and they possess an excellent sense of smell. Harbor seals have been observed to abandon a favored resting area due to human disturbance, reacting to human presence from distances reaching up to 300 feet.

Pups are the most vulnerable but have evolved to swim from birth, giving them an advantage when escaping from predators. They also grow rapidly, doubling their weight in the first month, and rarely leave the presence of their mother or other pups. Adults weigh up to 280 pounds.