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Steelhead Oncorhynchus mykiss display a dizzying array of life history variation (including the purely resident form, rainbow trout). We developed a model for female steelhead in coastal California (close to the southern boundary of their range) in small coastal streams. We combined proximate (physiological) and ultimate (expected reproductive success) considerations to generalize the notion of a threshold size for emigration or maturity through the development of a state-dependent life history theory. The model involves strategies that depend on age, size or condition, and recent rates of change in size or condition during specific periods (decision windows) in advance of the actual smolting or spawning event. This is the first study in which such a model is fully parameterized based on data collected entirely from California steelhead populations, the majority of data coming from two watersheds the mouths of whose rivers are separated by less than 8 km along the coast of Santa Cruz County. We predicted the occurrence of resident life histories and the distribution of sizes and ages at smolting for steelhead rearing in the upstream habitats of these streams. We compared these predictions with empirical results and show that the theory can explain the observed pattern and variation.[Article] FIGURE 1.-Timeline of the model of steelhead life history. The intervals are designated according to their corresponding survival rates (s p ), as described in the appendix. 534 SATTERTHWAITE ET AL.

We use a state dependent life history model to predict the life history strategies of female steelhead trout (Oncorhynchus mykiss) in altered environments. As a case study of a broadly applicable approach, we applied this model to the American and Mokelumne Rivers in central California, where steelhead are listed as threatened. Both rivers have been drastically altered, with highly regulated flows and translocations that may have diluted local adaptation. Nevertheless, evolutionary optimization models could successfully predict the life history displayed by fish on the American River (all anadromous, with young smolts) and on the Mokelumne River (a mix of anadromy and residency). The similar fitness of the two strategies for the Mokelumne suggested that a mixed strategy could be favored in a variable environment. We advance the management utility of this framework by explicitly modeling growth as a function of environmental conditions and using sensitivity analyses to predict likely evolutionary endpoints under changed environments. We conclude that the greatest management concern with respect to preserving anadromy is reduced survival of emigrating smolts, although large changes in freshwater survival or growth rates are potentially also important. We also demonstrate the importance of considering asymptotic size along with maximum growth rate.

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While it is widely recognized that financial stock portfolios can be stabilized through diverse investments, it is also possible that certain habitats can function as natural portfolios that stabilize ecosystem processes. Here we propose and examine the hypothesis that free-flowing river networks act as such portfolios and confer stability through their integration of upstream geological, hydrological, and biological diversity. We compiled a spatially (142 sites) and temporally (1980-present) extensive data set on fisheries, water flows, and temperatures, from sites within one of the largest watersheds in the world that remains without dams on its mainstem, the Fraser River, British Columbia, Canada. We found that larger catchments had more stable fisheries catches, water flows, and water temperatures than smaller catchments. These data provide evidence that free-flowing river networks function as hierarchically nested portfolios with stability as an emergent property. Thus, free-flowing river networks can represent a natural system for buffering variation and extreme events.

Oncorhynchus mykiss exhibit high plasticity in their life history patterns. Individual life history decisions are hypothesized to result from genetic thresholds shaped by local adaptation, with variation in environmental factors influencing the trajectories of growth and condition (e.g., Fulton's K, lipid content). We compared growth rates and life history patterns in two coastal creeks (Scott and Soquel) and two Central Valley (CV) rivers (American and Mokelumne) in California. The two regions differed markedly in habitat and physical factors, including hydrograph timing and amplitude, temperature regime, and food availability (measured as drift). Growth rates of coastal age‐0 fish averaged 0.1 mm/d in summer–fall and 0.2 mm/d in winter–spring. Growth rates of CV fish were up to 10 times faster than those of fish on the coast and had the opposite seasonal pattern, in which growth in summer–fall was faster than that in winter–spring. Fish growth also differed between CV rivers; the mean growth rates were 1.0 mm/d in summer–fall and 0.7 mm/d in winter–spring among American River fish and 0.7 mm/d in summer–fall and 0.5 mm/d in winter–spring among Mokelumne River fish. The life history expression and age structures of O. mykiss in the coastal creeks were similar, with populations being dominated by age‐0 fish but including mature residents up to age 6. The two CV populations were strikingly different in life history expression. In the American River, a single cohort was present and nearly all fish emigrated in the spring following their birth year. In the Mokelumne River, a broad diversity of ages (up to 4 years) was present, with a large proportion of presumed residents. The observed variation in life histories aligned with predictions based on state‐dependent life history models developed for the four streams, further demonstrating the adaptability of O. mykiss to contrasting rearing environments.

We tested the effect of temporal patterns in food supply on life history decisions in coastal steelhead Oncorhynchus mykiss irideus from a Central California coastal (CCC) population (Scott Creek) and a Northern California Central Valley (NCCV) population (upper Sacramento River basin). We manipulated growth through feeding experiments conducted from May to the following March using warm (2006 cohort) and cool (2007 cohort) temperature regimes. Survival in seawater challenges just before the time of typical juvenile emigration provided an index of steelhead smolt versus nonsmolt life history pathways. Survival varied significantly with fish size (with larger fish being more likely to survive than smaller fish) and by source population (with CCC steelhead being more likely to survive than NCCV steelhead of the same size). The timing of increased food supply (treatment group) did not significantly affect seawater survival rates in either NCCV or CCC steelhead. For both strains, the eventual survivors of seawater challenges (putative smolts) diverged from the eventual mortalities (putative nonsmolts) in both size and growth rate by June in both years, suggesting that the initial growth advantages were maintained throughout the experiments. A significant divergence in condition factor between smolts and nonsmolts by December matched the expected morphological transition of smolts, which showed faster growth in length than weight compared with nonsmolts. The apparent timing of the decision window, several months before the typical period of smolt emigration, matches the patterns observed for other salmonids. In coastal California, this decision must occur before fish have had the opportunity to take advantage of improved winter–early spring feeding conditions. These results support the role of early growth opportunity in life history decisions and provide insight into the applicability of life history models for managing California steelhead.