COS 1-9
Patterns of spring Chinook salmon prespawn mortality within the Columbia River Basin
Efforts to recover threatened and endangered salmon populations can be hindered by unexpectedly high rates of prespawn mortality, when salmon die after migration to natal habitat but prior to successful reproduction. Prespawn mortality in coho salmon (Oncorhynchus kisutch) populations has been associated with urban stormwater runoff, whereas the phenomenon has been linked to high spawner densities, water temperature, and premature run timing in populations of sockeye salmon (O. nerka). High rates of prespawn mortality have also been observed in some spring-run Chinook salmon (O. tshawytscha) populations, but the causes of mortality are poorly understood. Adult spring Chinook salmon spend several months in spawning streams prior to spawning and are thus susceptible to numerous different stressors during this long holding period. To assess potential environmental and demographic factors affecting prespawn mortality rates in spring Chinook salmon, we synthesized data from numerous populations throughout the Columbia River Basin. We evaluated the relationship between annual prespawn mortality rates and environmental variables, including stream temperature and stream discharge. Where available, we also assessed the effect of demographic variables on prespawn mortality, including spawner density, migration timing, and natural versus hatchery origin.
Results/Conclusions
Rates of spring Chinook prespawn mortality varied dramatically among years and populations, and ranged from less than 1% to more than 90%. In many populations, prespawn mortality was consistently less than 10%, suggesting that this may serve as a baseline rate of “natural” mortality for low-density spring Chinook salmon populations in relatively undisturbed spawning streams. In several watersheds, annual prespawn mortality rates were positively correlated with stream water temperatures. Additionally, periodic high mortality events occurred in a few locations and were associated with unusually high summer water temperatures, suggesting the potential for threshold temperature effects. Preliminary results also indicate that populations of hatchery-origin fish may experience higher rates of prespawn mortality than wild-origin spring Chinook salmon. In light of projected climate-related stream temperature increases, recovery of imperiled Chinook salmon populations will benefit from a better understanding of additional factors that influence prespawn mortality rates, and the interactions among these factors. Additionally, results from this research suggest that spring Chinook salmon populations that spawn in inland locations with warmer summers and in streams without groundwater inputs may be more susceptible to prespawn mortality related to elevated water temperatures.