EFFECTS OF CLIMATE CHANGE ON THE BIOENERGETICS OF JUVENILE SALMONIDS AND THE AQUATIC ECOSYSTEM OF THE YAKIMA RIVER BASIN

We summarized data from water temperature models for the Yakima River Basin to explore the possible consequences of climate change on the growth of stream-rearing juvenile Chinook salmon Oncorhynchus tshawytscha and steelhead O. mykiss. Climate change scenarios “down-scaled” for the Pacific Northwest were modeled based on current temperatures, and air temperature increased by an average of 1°C or an average of 2°C. We used bioenergetics models, the results of water temperature models using the “down-scaled” air temperature increases and information on fish size and diet in the Yakima River to model the effects of climate change on fish growth. We established a baseline scenario using existing information on temperature, diet, and fish size to evaluate the effects of temperature increases. With a constant diet and consumption rate, our simulations showed that fish living in the predicted warmer stream temperatures weighed substantially less than fish living in cooler temperatures by mid-summer, even though the warmer temperatures were within their tolerance range. We surmise that higher metabolic costs for fish inhabiting warmer waters will minimize the energy available for somatic growth. Thus, based on temperature effects alone, fish living in warmer waters will need to consume more to achieve historical sizes at time of out-migration, or perhaps they will need to modify life history characteristics (e. g. out-migration timing). Further, earlier seasonal increases in river flows and the timing of freshets could affect activity levels of juvenile salmonids resulting in potentially higher activity costs and lower growth. Habitat model outputs, such as increased river velocities and warmer temperatures, along with bioenergetics modeling suggest a habitat bottleneck could occur at the same time that fish growth is being affected. Collectively, negative growth and dwindling habitat will have a much more powerful, perhaps synergistic, impact on population viability than either variable alone. Expansion of favorable habitat conditions (i.e. warmer water temperatures) for non-native predators such as largemouth Micropterus salmoides and smallmouth bass M. dolomieu may further compound the negative effects of climate change on juvenile salmon populations.