Adult anadromous fish returning from the ocean and migrating upstream to spawning grounds face a variety of hazards and stressors. Those adults attaining the reproductive phase are the fewest in number among all prior life stages. Significant changes in the numbers of these adult fish can have resulting profound impacts on subsequent generations.
- Barriers – In the Delta there are a number of barriers that are believed to block or delay upstream migration of adult fish. There are additional barriers upstream and in the tributaries as well as attractant flows that cause adults to stray off of the main channel. In addition, Shasta Dam operations in the spring causing a decrease in downstream flows can increase the barrier to upstream migration into tributaries such as Antelope Creek that provide spawning habitat for spring-run Chinook.
- Harvest – In the Delta, Sacramento River and in tributaries sport fishing, while heavily regulated, does impact adult salmon escapement. In addition, poaching (illegal harvest) can also impact adult survival.