Migrating salmonids often return to their spawning habitats in overlapping timing patterns of multiple stocks (populations) collectively called a run that varies in its genetic makeup across and within years. Managers, tasked with developing harvest strategies on these runs, may have preseason estimates of total run size but little information on run timing. Without both it is difficult to assess a run's status in real time. Consequently, to avoid overharvest, managers tend to control the timing of harvest. However, this strategy may inadvertently affect the component stocks disproportionately and therefore the run's diversity. Thus, accurate estimates of run timing are needed to improve management. We developed a model that includes genetic and environmental factors to predict the mean run timing of chinook salmon (Oncorhynchus tshawytscha) at Bonneville Dam on the Columbia River, Oregon, USA. The model predicted mean runtiming (P < 0.00001, r2 = 0.78) by characterizing genetic run timing components from the arrival timing of precocious males returning one year prior to the remainder of the adults and environmental influences of oceanic and riverine flows that impede or advance the run timing. Variations in the relative abundances of the populations in the run explain 62% of the interannual variation in mean run timing while the oceanic and riverine factors combined account for 15.5%. We suggest that when genetic run timing characteristics are preserved in species with multiple maturation strategies the information can be used to improve run time predictions and maintain genetic diversity of harvested species.

A mechanistic temperature-dependent model for preemergent growth coupled with spawning and river temperature data is used to evaluate early life history strategies for chinook salmon (Oncorhynchus tshawytscha) in the Methow River, Washington, U.S.A. Since the model provides a predictable coupling between time of spawning and fry emergence, it helps explain the spatial and temporal patterns observed for the sympatric stocks. The model suggests that progeny of August-spawning chinook in upper river habitats emerge at button-up (no visible yolk sac) over a wide range of days in the early spring. The eggs grow efficiently, which maximizes fry mass. The progeny of October-spawning downstream chinook can exploit a wide range of habitats in the river because their emergence mass is not sensitive to location in the river, but the adults must spawn later in the season to avoid summer high temperatures. Late spawning forces fry to emerge prior to button-up to avoid scouring flows but synchronizes their emergence times. The trade-offs between the spawning strategies of these two salmon runs are complex and the seasonal and spatial distribution of temperatures plays a critical role in these trade-offs.

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Seasonal temperature and bioenergetic models were coupled to explore the impacts on juvenile salmonid growth of possible climateinduced changes to mean annual water temperature and snowpack in four characteristic ecoregions. Increasing mean temperature increases juvenile growth in streams that currently experience cool spring temperatures. In streams with currently warm spring temperatures, an increase shortens the duration of optimal conditions and truncates growth. A loss of snow enhances growth in coolsummer streams and decreases growth in warm-summer streams. The relative impacts of such climate change trends will vary significantly across ecoregions.

We projected effects of mid-21st century climate on the early life growth of Chinook salmon (Oncorhynchus tshawytscha) and steelhead (O. mykiss) in western United States streams. Air temperature and snowpack trends projected from observed 20th century trends were used to predict future seasonal stream temperatures. Fish growth from winter to summer was projected with temperature-dependent models of egg development and juvenile growth. Based on temperature data from 115 sites, by mid-21st century, the effects of climate change are projected to be mixed. Fish in warm-region streams that are currently cooled by snow melt will grow less, and fish in suboptimally cool streams will grow more. Relative to 20th century conditions, by mid-21st century juvenile salmonids' weights are expected to be lower in the Columbia Basin and California Central Valley, but unchanged or greater in coastal and mountain streams. Because fish weight affects fish survival, the predicted changes in weight could impact population fitness depending on other factors such as density effects, food quality and quantity changes, habitat alterations, etc. The level of year-to-year variability in stream temperatures is high and our analysis suggests that identifying effects of climate change over the natural variability will be difficult except in a few streams.