Seasonal Patterns of Abundance, Growth, and Site Fidelity of Juvenile Steelhead in a Small Coastal California Stream

Abstract

Populations of steelhead Oncorhynchus mykiss throughout the central California coast are currently listed as threatened under the federal Endangered Species Act. To better understand demographic processes during the juvenile freshwater stage, we quantified abundance, movement, and growth of juvenile steelhead among five reaches in a primary spawning area of Soquel Creek. We used mark-recapture methods to examine seasonal, density-dependent, and size-dependent patterns in these population metrics. Many trends were stronger in age-0 fish than in age-1 and older fish. Estimated survival and retention within a reach were generally highest in summer and lowest in winter-spring. Density dependence was evident only in age-0 fish, with lower survival occurring at higher densities. Survival was typically size-dependent, with lower survival of smaller fish in both age-classes. Immigration, based on movement of untagged fish into our sites between sampling periods, was higher in winter-spring compared with summer and fall. Immigration was generally independent of density but was size-dependent, with smaller fish more likely to move than larger fish. Average growth rates were low, rarely exceeding 0.3 mm/d. Growth was fastest in winter-spring, potentially a function of higher flow rates and increased food availability. Growth was generally size-dependent, with smaller fish growing faster than larger fish. A negative effect of fish density on growth was apparent only for age-0 fish in summer. High site fidelity was evident in high recapture rates of tagged fish. Recaptures over periods of up to 4 years suggested likely expression of a nonanadromous life history by some individuals. The growth rates in this system suggest that fish rearing in upstream habitats cannot undergo smoltification and emigrate until they are at least age 2. Our results provide a valuable baseline of demographic patterns for juvenile steelhead in representative freshwater habitat of the Central California Coast Steelhead Distinct Population Segment.