The anadromous form of the rainbow trout, Oncorhynchus mykiss, is at high risk of extinction; however, studies on this taxon in southern California are limited. I examined relationships between O. mykiss populations in Santa Barbara and Ventura Counties and the characteristics of their habitats. A total of twenty-two reaches on ten creeks were surveyed for rainbow trout distributions, densities, body sizes, and habitat characteristics. Samples of fin tissue were taken for mitochondrial DNA (mtDNA) analysis. I determined habitat relationships for fry and adult age classes and total trout, as well as southern endemic versus widespread mtDNA haplotypes. Data were analyzed at three different spatial scales, including unit (individual pool, riffle, or run), reach, and stream scales.
Rainbow trout densities and genotypes varied among streams sampled. Rincon and Agua Caliente Creeks contained no rainbow trout when sampled, whereas Fox Creek had the highest density of rainbow trout. Fox and Gaviota Creeks contained only southern mtDNA haplotypes, whereas Santa Paula and Piru Creeks and the Ventura River contained only widespread mtDNA haplotypes. Santa Paula and Piru Creeks and the Ventura River contained southern and widespread mtDNA types.
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Although there were no significant relationships between trout densities and environmental variables at the unit scale for fry, adults, or total trout, significant relationships between trout densities and habitat variables emerged at the reach and stream scales. At the stream scale fry were negatively related to instream cover. Densities of adults and total trout were positively related to canopy cover at the stream scale and densities of total trout were negatively related to instream cover at the stream scale. Widespread and southern trout haplotypes showed different habitat associations. Densities of widespread trout haplotypes were positively related to maximum depth at the reach scale. At the unit and reach scales, densities of southern trout haplotypes were positively related to canopy cover. These differences in habitat affinities of southern versus widespread haplotypes probably reflect behavioral adaptations to conditions in different ancestral habitats.