There is increasing realization that diversity of life-histories within and among populations can influence their persistence and stability. However, for species that exhibit complex life-histories, quantifying this life-history can be challenging. Thus, understanding life-history diversity of migratory salmon is a critical challenge for their conservation and management. Anadromous salmon forage in the ocean as adults prior to migrating to natal freshwater streams to spawn. Juveniles spend days to years within the freshwater system before entering the ocean. Degradation and removal of freshwater habitats in the southern extent of the Chinook salmon range has led to a drastic decline in stock abundance and diversity. In particular, Sacramento River spring-run populations have been either extirpated or are federally listed as threatened under the ESA. These populations historically exhibited both a short (3-6 months; ocean-type) and long (10-15 months; stream-type) juvenile freshwater rearing strategy. Here we investigate the prevalence of these two juvenile rearing strategies within the remaining spring-run populations, fish earbone (otolith) microchemical methods are applied to adult spawners to retrospectively measure spatial and temporal patterns of freshwater residence during the juvenile life stage.

Results/Conclusions

We analyzed over 75 otoliths from 2007 spawning adults collected from the three remaining independent Sacramento River spring-run populations. Thus, these individuals represent the small fraction of juveniles with a life history strategy that allowed them to survive to spawning. Preliminary results suggest successful spawners spent very little time as juveniles (<45 days) in the Sacramento Delta, even though this is often assumed to be an important rearing habitat. This region is characterized by high native and introduced predator abundance, habitat degradation, and water diversion. Residence time in natal streams after first feeding ranged from <2 to 8 months, with most leaving either before 100 days or after 180. Total residence time ranged from 2 to 11 months, however >70% of individuals spent 6 months or less in the freshwater system. This bimodality in residence times is evidence of ocean- and stream-type life-histories. The current dominant extant populations in the southern range of Chinook salmon are fall-run which only rarely exhibit stream-type behavior. This represents a shift from historical conditions; stream-type runs (i.e., spring-run) were prevalent in the Sacramento River system before dams removed much of the headwaters spawning and rearing habitat associated with stream-type behavior. Habitat degradation of freshwaters likely disproportionately impacts stream-type salmon, decreasing life-history diversity with consequences for evolutionary trajectories and population persistence.