NOAA Fisheries Service listed a population of killer whales, known as the Southern Residents, as "endangered" under the ESA in December 2005. These orcas spend several months of the summer and fall each year in Washington State's Puget Sound. The population is composed of three family groups of whales that have been named J, K and L pods. Individual animals are identified by a number based on pod membership and birth order.

The Southern Resident population has fluctuated considerably over the 30 years that it's been studied. All three Southern Resident pods were reduced during 1965-75 because of captures for marine parks. In 1974 the group comprised 71 whales, peaked at 97 animals in 1996, and then declined to 79 in 2001. The population now numbers in the 80s.

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 Service 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.