Age and Length of Steelhead Smolts from the Mid-Columbia River Basin, Washington

Abstract

Steelhead Onchorhynchus mykiss exhibit a wide range of life histories within and among stocks. Varying degrees of anadromy, length of freshwater life before emigration to the sea, and age at first maturity have been observed. Steelhead in the mid-Columbia River basin are at least partially descended from fish that were relocated to the major tributaries of the mid-Columbia River when Grand Coulee Dam was built. These fish do not spawn in the main-stem Columbia River and are strictly tributary spawners. Rearing conditions in these tributaries can be harsh. Researchers have observed a wide range of lengths and ages of juvenile O. mykiss (steelhead and nonanadromous rainbow trout) in these tributaries. This led to a need to document the age at migration of fish that were clearly smolts, as demonstrated by their downstream migration. Over 970 otoliths were collected from naturally produced steelhead smolts in 1988 and 1989 to determine the age at migration. Ages ranged from 1 to 7 years at Rock Island Dam and from 1 to 5 years at Rocky Reach Dam. These ages agreed with those estimated from adult otoliths. Age structure did not differ significantly between the two sampling sites, but length did, suggesting that fish emigrating from the Wenatchee River were shorter for a given age than fish produced above Rocky Reach Dam. Females made up 63.0% of the smolts collected at Rock Island Dam and were generally older than males, suggesting that males have a higher probability of remaining in freshwater. We attempt to explain the patterns observed in age and length of naturally produced steelhead smolts. We suggest that a more or less continuous downstream movement of juveniles occurs that takes them to areas with more adequate food supplies. Smoltification of individual fish appears to result as a threshold length is approached. Harsher growing conditions result in slower growth and older smolts, which may have implications for supplementation and introduction of steelhead where resident rainbow trout are present.