Figure 1.
Adult Chinook salmon population trend.
Stacked bar graph of the total number of adult fall-run Chinook salmon (Oncorhynchus tshawytscha) spawning on the Mokelumne River (black bars), and in the Mokelumne River Fish Hatchery (grey bars) from 1940–2009 (East Bay Municipal Utility District, unpublished data). Graph shows adult spawning location and not rearing origin. Note: Mokelumne River Fish Hatchery was built in 1964.
Figure 2.
The Mokelumne River and Mokelumne River Fish Hatchery in relation to the western United States, and the Sacramento-San Joaquin river system (shaded region) in California's Central Valley (insert). The in-river spawning habitat on the Mokelumne River consists of the area between its confluence with the Cosumnes River and upstream to the Camanche Dam (∼16 km).
Figure 3.
Frequency distribution of δ34S values in salmon otoliths.
Histogram of otolith δ34S for the juvenile rearing portion of otoliths from unknown origin adult Chinook salmon (Oncorhynchus tshawytscha) spawning in-river on the Mokelumne River (USA). Fish assigned to natural origin (grey bars; N = 87) had δ34S values less than 6‰ (dashed line) and did not overlap with δ34S values from those identified as originating from a hatchery (open bars; N = 10). These results indicate that 90% of in-river spawners were produced in a hatchery.
Table 1.
Estimate of hatchery and natural-origin salmon.
Figure 4.
Population growth estimates of Chinook salmon (Oncorhynchus tshawytscha) on the Mokelumne River watershed from cohort reconstruction. Apparent growth rate estimates (open circles) show several years where cohort replacement values exceed 1 (solid line). Natural population growth rates (filled circles) remove the influence of immigration from hatchery fish. These results suggest that in-river populations are being supported by hatchery sources.
Table 2.
Cohort reconstruction and population growth rates.
Table 3.
Percent natural-origin salmon and influence on population growth rate estimates.