Maternal Origin and Migratory History of Steelhead and Rainbow Trout Captured in Rivers of the Central Valley, California

摘要

Steelhead Oncorhynchus mykiss (anadromous rainbow trout) in the Central Valley of California were listed as threatened under the Endangered Species Act in 1998. Unfortunately, little is known about the distribution of steelhead in the tributaries of the Sacramento-San Joaquin River system in the Central Valley or the relationship between sympatric anadromous and nonanadromous life history types. We used analysis of otolith strontium : calcium (Sr:Ca) ratios to determine maternal origin (anadromous versus nonanadromous) and migratory history (anadromous versus nonanadromous) of rainbow trout collected in Central Valley rivers between 2001 and 2007. Of the 964 otoliths examined, 224 were determined to be from the progeny of steelhead females and 740 from the progeny of nonanadromous rainbow trout females. Progeny of steelhead maternal origin were present at all of the sites sampled, but the proportion varied among sites (0.04-0.74). Based on transects of otolith Sr:Ca ratios, only five fish were confirmed to be adult steelhead. This is a conservative estimate of the distribution of adult steelhead since, due to conservation concerns, our sampling only collected a limited number of adults. The remaining 214 fish older than age 4 were not anadromous, and 16 of them were determined to be the progeny of steelhead females. Overall, these results refine our understanding of the distribution of steelhead in Central Valley rivers and confirm reports of steelhead occurrence in those rivers.