Male-biased sex ratio of nonanadromous Oncorhynchus mykiss in a partially migratory population in California

Differential rates of anadromy between males and females are common in partially migratory salmonid populations, but this pattern is not fully clear for Oncorhynchus mykiss (rainbow trout/steelhead) from the limited but mixed data available. In particular, there are very few data on sex ratios of juvenile and nonanadromous (resident) fish to help assess sex composition of various life stages and life-history types. We used a recently developed Y-chromosome genetic marker to assess sex ratio of stream-dwelling (i.e., juvenile and nonanadromous) O. mykiss in a small coastal basin in central California, USA. We analysed 384 samples collected from three contiguous study reaches over 3 years. Sex ratio was 1:1 among juvenile-sized O. mykiss (< 150 mm) but highly male-skewed (83%) among nonanadromous-sized individuals (150 mm), and this sex ratio x size pattern did not differ among years or study reaches. Our results suggest that the rate of anadromy differs between males and females in this basin. Our study also demonstrates the application of new genetic markers to determine sex composition of immature and nonanadromous salmonids, which will help assess sex-specific life-history behaviour in partially migratory populations of O. mykiss and other species.