The Abundance And Distribution Of Hatchery And Naturally Produced Chinook Salmon In Columbia River Estuary Nearshore Habitat

Columbia River hatcheries release millions of juvenile Chinook Salmon Oncorhynchus tshawytscha to support fisheries and mitigate for historical productivity declines from dam construction and upriver habitat losses. Recent federal biological opinions for Federal Columbia River Power System operations also have mandated restoration of estuarine rearing habitats to mitigate for mortality of naturally produced (NP) salmon at main-stem dams. We compared genetic stock compositions, distributions, abundances, and length frequencies of hatchery-marked and unmarked Chinook Salmon to evaluate the extent of hatchery influence on nearshore estuarine habitat use and the potential for interactions between hatchery-origin (HO) and NP salmon. Cumulative Chinook Salmon releases from all hatcheries in 2010 and 2011 dictated estuarine stock composition, abundance, and life histories during the spring-summer migration peak. Naturally produced salmon from the lower Columbia River populations, including many fry migrants, were most abundant, but larger HO juveniles also frequented nearshore sites, where their total biomass often equaled or exceeded that of NP fish. Large releases of a single stock of origin (Spring Creek Group fall run) in April and May created sharp pulses in the estuary entry timing of fingerling-sized salmon and dominated abundance and stock composition at some sites. Successive releases from scores of hatcheries during spring and early summer ensured a consistent presence of HO salmon near shore, regardless of the habitat-specific residence times of individual fish. Overall, the historical replacement of diverse wild populations with fewer hatchery stocks of a narrow size range and migration timing has intensified nearshore habitat use during the spring-summer migration peak and reduced life history variation of Columbia River Chinook Salmon. Such changes could undermine the fish conservation goals of both hatchery mitigation and estuary restoration programs.