On September 15, 1979, seven 2-year-old coho salmon return to the Seattle Aquarium fish ladder to spawn. The salmon were released in May 1978 in the hope of establishing a continuing salmon run at the aquarium, making it possible for viewers to study the spawning, incubation, and release of young salmon. One chinook salmon from an earlier release has also made it up the ladder.
Scientists say salmon use their sense of smell to find their way back to their home streams when its time to spawn. Organic material from trees and other plants and trace minerals give each stream its own scent "fingerprint," and this information is imprinted on young salmon during the time they spend there.
At the aquarium, young salmon were imprinted with the smell of the chemical morpholine, which had been used successfully with fish in Lake Michigan.
Although salmon in nature and in conventional hatcheries return to fresh water, the aquarium salmon climbed the ladder in salt water and were being kept in salt water tanks. They were later transferred to fresh water to trigger the final maturing process and to allow for spawning to be observed.