A population of killer whales, known as the Southern Residents, spends several months of the summer and fall each year in Washington State's Puget Sound. This population is composed of three family groups of orcas that have been named J, K and L pods. Individual animals are identified by a number based on pod membership and birth order. All three Southern Resident pods were reduced during 1965-75 because of captures for marine parks.

The Southern Resident population has fluctuated considerably over the 30 years that it's been studied. In 1974 it comprised 71 whales, peaked at 97 animals in 1996, and then declined to 79 in 2001. The population now numbers in the high 80s.

In May 2003, NOAA Fisheries Service designated the Eastern North Pacific Southern Resident stock of killer whales (Orcinus orca) as depleted under the Marine Mammal Protection Act (MMPA). The agency also announced preparation of a conservation plan to restore the stock to its optimal sustainable population.

There is a limited number of reproductive-age Southern Resident males, and several females of reproductive age are not having calves. The factors causing the decline of Southern Residents are not well known, and are likely to continue until NOAA Fisheries learns more about what needs to be done to reverse this trend. Some of the possible causes of decline are: reduced quantity and quality of prey; persistent pollutants that could cause immune or reproductive system dysfunction; oil spills; and noise and disturbance from vessels.