The Dynamics and Ecological Effects of Salmon Colonization: Lessons Learned from the Cedar River, WA

A variety of riverine species require access to multiple linked habitats to grow, survive and reproduce; therefore, maintaining habitat linkages is essential for maintaining stream ecosystem structure and function. Thus, anthropogenic barriers, such as culverts, road crossings and dams lead to habitat loss and alter important ecological processes contributing to population declines and local species extirpation. Anthropogenic barriers have been especially problematic for anadromous fish. To address the ecological effects created by these barriers, there are widespread efforts to provide fish passage, replace culverts or remove dams. We initiated an intensive study in 2000 on the Cedar River, WA, and USA to quantify the effectiveness of reconnecting a migratory corridor in restoring anadromous fish populations above Landsburg Dam. This dam was constructed in 1903 blocking access to about 43 km of relatively high quality habitat for migratory fish species, with a fish ladder installed in 2003.  Adult Chinook and coho migrated above the dam soon after provision of fish passage. Coho abundance has increased over time; Chinook abundance is higher relative to 2003 but it is also more variable than coho. Both adults and juveniles have contributed to spatial expansion of salmon above Landsburg. After five years, juvenile coho density during summer was similar to rivers with continual anadromy. Salmon reintroduction has increased total fish density about 5-fold since 2003, with no measureable effects on resident trout. Using stable isotopes, there has been no change in levels of salmon-derived C or N in stream biota or riparian plants despite increases in total adult salmon flux (kg wet mass/year). Thus, after seven years of intensive monitoring, salmon populations are re-established above Landsburg Dam after being absent for over 100 years; however, a key ecological feedback – adult salmon nourishing future generations through release of nutrients and gametes – remains absent.