Juvenile Life History of Wild Fall Chinook Salmon in the Snake and Clearwater Rivers

Abstract

Dam construction in the 1950s and 1960s blocked passage to the historical spawning area of Snake River fall chinook salmon Oncorhynchus tshawytscha. We obtained water temperature data and collected juvenile fall chinook salmon in three present-day spawning areas from 1992 to 2000 to investigate the relation between water temperature and juvenile life history events. We used historical water temperatures and the literature to depict juvenile life history in the historical spawning area. Water temperatures in the three present-day spawning areas differed significantly from winter to spring, when eggs were incubating (P ≤ 0.0001), as well as during spring, when juveniles were rearing and starting seaward migration (P ≤ 0.0001). When water temperatures were warmer, the timing of most life stages was generally earlier. The life stages included fry emergence (r ² = 0.85, N = 14, P < 0.0001), growth to parr size (r ² = 0.94, N = 15, P < 0.0001), and smolt emigration (r ² = 0.93, N = 14, P < 0.0001). The percentage of parr that overwintered in freshwater and migrated seaward the next spring increased when spring water temperatures decreased (r ² = 0.40, N = 12, P = 0.02). As the historical spawning area was warmer than present-day spawning areas, fall chinook salmon juvenile life history progressed on an earlier time schedule. We conclude that dam construction changed juvenile fall chinook salmon life history in the Snake River basin by shifting production to areas with relatively cool water temperatures and comparatively lower growth opportunity.