Contrasts in Habitat Characteristics and Life History Patterns of Oncorhynchus mykiss in California's Central Coast and Central Valley

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.