VARIABILITY IN BIOLOGICAL CHARACTERISTICS OF NORTHERN SQUAWFISH IN THE LOWER COLUMBIA AND SNAKE RIVERS

We described and compared population structure, growth, mortality, and reproduction of northern squawfish Ptychocheilus oregonensis among fo ur reservoirs of the lower Columbia River and among four reservoirs of the lower Snake River. We also pooled data for comparisons among thre e larger areas: Columbia River reservoirs, Snake River reservoirs, and the unimpounded Columbia River downstream from Bonneville Dam. Female s made up 76-100% of northern squawfish 380 mm fork length or greater in individual reservoirs, and 87-99% of fish 380 mm or greater in the three combined areas. Females lived longer and grew faster than males in all reservoirs and areas and had lower annual mortality rates in al l but two reservoirs. Proportional stock density was highest and annua l mortality rates were generally lowest in Columbia River reservoirs. Northern squawfish downstream from Bonneville Dam had a lower mean rel ative weight and higher relative fecundity (eggs per unit fish weight) than populations in impoundments. Snake River populations had the hig hest mortality rates but exhibited intermediate values for other popul ation parameters. None of the biological characteristics were strongly correlated with density of northern squawfish except relative fecundi ty. Differences in life history traits of northern squawfish among res ervoirs and between free-flowing and impounded reaches underscore the ability of northern squawfish to adapt to altered conditions. Spatial heterogeneity in population characteristics creates uncertainty regard ing the effects and relative benefits of a northern squawfish manageme nt program to reduce predation on juvenile salmonids.