Pupping season is February through April. In their early weeks of life, mother Harbor seals may carry their pups on their backs while swimming and diving. The average lifespan of a Harbor seal is 20 - 25 years. After mating there is a natural two month period of delayed embryo implantation as the mother regains her strength. She then carries her young for nine months.
Seals can rest intermittently underwater.
A Harbor seals' lungs partially collapse when diving, their heart rate slows dramatically, and blood flow to some organs is restricted. Seals dive repeatedly for short periods but can dive up to 600 feet and hold their breath under water for as long as 30 minutes. Harbor seals spend their entire lives along the same stretch of coastline. Seals maintain a thick layer of fat beneath their skin to guard against cold water temperatures, giving them a thick sausage shape and considerable buoyancy in the water. They spend many hours during the day hauled out on a sandbar or rocky island resting and warming up.