The death rate has climbed in the last five years among the heavily studied three resident pods that frequent the San Juan Islands. Transient whales are more highly contaminated but also more mobile, making precise population counts difficult. The current population estimate is around 220. What is killing the resident whales remains unproved. Usually the animals just disappear.
Some females give birth successfully every few years while others produce no calves at all.
The southern population, around the San Juan Islands, is much smaller than that of northern residents, which live in remote Canadian waters and are less contaminated. The southern population also produces fewer calves. PCBs block formation of vitamin A, a hormone. Calves are hit with a large dose in their mothers' milk just at the time they need vitamin A to develop normally. Whale populations also may not have fully recovered from the effects of marine parks and aquariums, which removed 48 of the region's young animals in the 1960s and '70s, leaving a 15-year gap in calving.