Declivity in steelhead (Oncorhynchus mykiss) recruitment at the Keogh River over the past decade

Abstract

Survival and return of unharvested winter-run steelhead (Oncorhynchus mykiss) at the Keogh River, British Columbia, declined abruptly and remained persistently low after 1990. Adult returns averaged 1168 fish from 1976 to 1990 but were significantly lower from 1991 to 1998 (mean 223). Forty wild females returned to the 35-km river in 1995-1996, 20 in 1996-1997, and <10 in 1997-1998. The positive linear relationship between smolts and returns was significantly lower after 1990 and no longer correlated with smolt size. Smolt-to-adult survival averaged 15% (1976 to 1989) but recently averaged 3.5% (1990 to 1995). Smolt number steadily declined to <1000 by 1998 from an average annual count of 7000. Smolts per spawner from 1991 to 1994 were, on average, 70% lower than previous estimates based on the same spawner abundance. Recruitment scenarios based on survival histories during freshwater and marine life stages indicated that adult recruits are currently below replacement and unsustainable if conditions continue or worsen. Factors influencing steelhead in the ocean and freshwater are likely similar for other salmonids; harvest impacts must be reduced and appropriate stock rebuilding measures implemented.