Selective Mortality in Chinook Salmon: What is the Role of Human Disturbance?

Zabel, Richard; Williams, John

Abstract: "While many recovery programs for threatened species focus on acute sources of mortality, understanding some of the evolutionary processes of these species may lead to more effective recovery efforts, especially in cases where human-induced disturbances have resulted in artificial selection pressures. We developed a Monte Carlo test to determine whether Snake River spring/summer chinook salmon (Oncorhynchus tshawytscha) experienced selective mortality as a function of their juvenile length and timing of downstream migration. Actively migrating juvenile fish (smolts) were captured, tagged, and released in 1995 and 1996 approximately 700 km upstream from the Pacific Ocean, and returning adults were detected at the same location. We analyzed data from two groups of fish: those that migrated downstream in-river and those that were barged downstream as part of the juvenilesalmon transportation program. These groups were further separated into wild and hatchery fish. Length at release was significantly greater in returning adults than in the general population for fish that migrated downstream in-river (both wild and hatchery) or were transported (hatchery only). From the 1995 seaward migration, adult returns of both wild and hatchery fish that migrated in-river were composed of fish released significantly earlier than the general population. In contrast, the opposite trend existed for wild and hatchery transported fish. From the 1996 seaward migration, no significant difference in release date was found between returning adults and the original population for any of the groups analyzed. Fish length at migration is a result of factors encountered in early life stages but selectively determines mortality in the smolt-to-adult stage. Thus freshwater habitat improvements, such as salmon carcass supplementation, directed at increasing nutrient levels and thus fish length may result in an increase in overall survival. The development of hydroelectric dams in the migratory corridors of these fish has disrupted their arrival timing to the estuary. Mitigation efforts designed to shift arrival timing toward that experienced prior to impoundment may confer considerable survival benefits."