Factors affecting the survival and development rate of upper Columbia River Chinook salmon

Abstract

Pacific salmonids are a highly valued resource worldwide, are invaluable to tribal traditions and livelihoods immemorial, and have been a staple of both native and non-native culture and commerce in the Pacific Northwest for nearly 200 years. Concerns regarding salmon population abundance and productivity are nearly as long-standing as their commercial harvest, identifying threats such as overfishing, dam passage, and habitat degradation. The iconic status of salmon is understandable; they are remarkable animals which rear in fresh water but then migrate hundreds or thousands of km to the ocean to feed. They then return years later, often to the location where they themselves emerged from the gravel, where they spawn, bury their eggs, and protect their nest until they die. Even in death, they provide nutrients vital to ongoing ecosystem health. Salmon possess great resilience and flexibility in life-history strategies, as is necessary to survive given unpredictable differences in habitat quality between years or locations. Due to uncertainty, salmon, and perhaps especially Chinook salmon (Oncorhynchus tshawytscha), have evolved to distribute risk of mortality across life-stages. For instance, Chinook salmon populations exhibit both ocean- and stream-type life history strategies. They vary in the duration of freshwater residence, migration timing, age of maturity, return timing, and fecundity. All these traits have been studied in detail by the scientific community, which continues to untangle the complexity of life-history strategies and specific limitations to survival at each stage. Adding to the potential impacts to survival are concerns associated with climate change. Future changes in ocean temperatures may severely impact salmon survival at sea in the coming years, and current survival rates are low. Inland populations, such as Columbia River stocks have the added pressure of passing several hydropower projects as smolts, and then again as adults returning to spawn. Still earlier, survival to the smolt stage relies on the quantity and quality of rearing habitat, food availability, and low predation pressure on juveniles. However, survival in freshwater also entails the incubation stage, when most life-time mortality commonly occurs. Incubating eggs are defenseless against extreme temperatures in the late fall or mid-winter, high flow events that may scour them from the interstitial spaces between the gravel that they occupy, or to subsequent settling of fine sediment which may fill these spaces, lowering oxygen availability at critical developmental stages or entombing fry that would otherwise swim up through and emerge from the gravel. Chinook salmon generally have a higher rate of survival during this early life-stage compared to other Pacific salmonids. However, estimates suggest mortality from egg deposition to emergence to be on the order of 50 to 70 % and as such may represent the largest proportional mortality of offspring per spawning female across all life stages. Therefore, the incubation stage may provide the best opportunity for actions aimed at maintaining listed populations. In addition to surviving the months buried below the surface of the stream, it is important that the juveniles emerge at the optimal time in the spring to take advantage of hatching insects that they prey on, and avoid predatory birds and fishes. Chinook salmon are invaluable to the culture of various Pacific Northwest tribes, are perhaps the most highly valued component of sport fisheries and are essential to commercial fisheries. Therefore, understanding not only the factors that limit their survival, but also their spatial and temporal influence on specific populations is necessary to prevent the extirpation of listed populations in response to a continually and rapidly changing environments. This work directly addresses the need for greater clarity in spatial and temporal limitations to Chinook salmon and in the methods used to assess these processes. Chapter one describes field studies to improve estimates of Chinook salmon egg-to-fry survival, providing spatially explicit estimates of survival and limiting environmental factors for four upper Columbia River Chinook salmon populations, three of which are ESA listed as Endangered. The second chapter highlights variability in thermal conditions within incubation environments and management practices leading to differences in timing of juvenile emergence in the spring, which may affect subsequent growth and survival. Lastly, we highlight the need for improved estimators of the relationship between temperature and developmental rate of embryos in the absence of empirical data, to better guide life-history models, themselves necessary to identify life-stage specific environmental changes that may be made to reduce negative impacts to already threatened or endangered populations.