Sex Biased Survival and Differences in Migration of Wild Steelhead Smolts from Two Coastal Oregon Rivers

Female salmonid smolts are more likely to migrate to the ocean than males, but it is not known whether sex affects survival during migration.  Fin tissue was taken for genetic sex determination from wild steelhead (Oncorhynchus mykiss) smolt populations captured with rotary screw traps. We implanted acoustic tags to monitor downstream migration and survival to the Pacific Ocean.  We then tested for differences in survival based on sex during smolt migration.  We have 2 years of data in the Alsea River and 1 year in the Nehalem River.  There was no effect of sex on survival in the Nehalem River, or in the Alsea River during 2010.  In 2009 males had substantially lower survival than females in the Alsea River.  Differences in migration duration or timing do not explain the sex-biased survival.  Larger males had higher odds of survival than smaller males in 2009, but the body size of females did not affect survival.  The difference in survival between years in the Alsea River could be due to differences in environmental conditions.  Our data suggest that sex may affect smolt survival, but only during stressful migration conditions.