Identification and Enumeration of Steelhead Kelts at a Snake River Hydroelectric Dam

Abstract

Improvement of iteroparity rates in U.S. Endangered Species Act (ESA)-listed Snake River populations of steelhead Oncorhynchus mykiss requires a means of distinguishing prespawn (mature) steelhead from postspawners (kelts) and sufficient kelt abundance to aid recovery efforts. We used ultrasound imaging of gonads to identify and enumerate prespawn steelhead and kelts incidentally collected in the juvenile bypass facility at Lower Granite Dam (LGR) on the Snake River, Washington. We also evaluated the accuracy of visual identifications based on external coloration and condition in relation to the known maturational status determined by ultrasound examinations. Steelhead (n = 1,353) were sampled during 10 weeks between April and June 2000, a period that spanned the peak of adult steelhead occurrence in the juvenile bypass system. Based on ultrasound appraisals, we estimated that kelts composed 94.6% of the 3,968 adult steelhead encountered in the LGR juvenile bypass system during the study period. Of these kelts, 2,050 were wild or naturally produced ESA-listed steelhead. This number represents 17% of the entire protected Snake River population that passed the LGR fishladder as prespawners during 1999. Visual methods grossly underestimated the number of kelts; only 18% of the ultrasound-sampled adults were classified as kelts by visual identification. Most kelts were in good external morphological condition (69.5%) and were either bright or intermediate in coloration (84.7%). Most kelts also were female (77.0%). Clearly, the potential for iteroparity persists in Snake River steelhead, but we do not yet know what proportion of these kelts survive to spawn again.