After spending three to four years at sea, they travel under the Golden Gate Bridge, up through the Delta and back into the Sacramento River. Around November, the chinook salmon make an upstream migration in order to spawn in the same stretch of river where they originally hatched. They stay in the river for a couple of months before spawning peaks between May and August. The primary threats to this population are the drastic changes to the Sacramento River basin. These fish are threatened by blockage of migration due to dams and other water diversions, increasing water temperatures, agricultural and industrial pollution, and drought conditions.
Chinook salmon are in danger on their way upstream as well as on their way back to the ocean.
Approximately two-thirds of the fish will never make it. The young ocean-bound fish have a difficult time navigating through diversion channels and are likely to be eaten by predatory fish and birds in the estuaries or to get sucked up into the water diversion pumps of the California Water Project. If these fish do indeed make it to the ocean, they are not yet free of danger. Davis Bodega Marine Laboratory and the Steinhart Aquarium of California Academy of Sciences. When the captive fish are mature, they are released back into the wild at the Coleman Fish Hatchery where they were first collected.