Incidence of Phenotypic Female Chinook Salmon Positive for the Male Y-Chromosome-Specific Marker OtY1 in the Central Valley, California

Abstract Fall-run chinook salmon Oncorhynchus tshawytscha collected during 1999 carcass surveys of seven locations within the Sacramento River basin and six locations within the San Joaquin River basin, California, were screened using the Y-chromosome-specific marker OtY1 to evaluate inconsistencies between sexual genotype and phenotype. Of 287 phenotypic females screened, 46 (16% overall) tested positive for the Y-chromosome marker. Stream populations had higher frequencies of sex-reversed males (American River, 20%; Battle Creek, 35%; Feather River, 20%; Merced River, 24%; Mokelumne River, 38%; and Yuba River, 25%) than hatchery populations (Feather River Hatchery, 0%; Nimbus Hatchery, 12%; Merced River Hatchery, 14%; and Mokelumne River Hatchery, 4%), and several other streams had intermediate frequencies. All 150 male fall-run chinook salmon from the sampling locations tested positive for the Y-chromosome marker. These results present evidence that some genetic males have been sex reversed and have th...