Comparative Performance of Genetically Similar Hatchery and Naturally Reared Juvenile Coho Salmon in Streams

—Hatchery-reared salmon have been reported to be inferior to wild fish in some studies and competitively superior in others. We examined the influence of early rearing environment (hatchery versus natural) on the summer survival, movement, and growth of genetically similar juvenile coho salmon Oncorhynchus kisutch in streams. In each of 2 years, 5,000–10,000 fry from a hatchery cohort were placed above barrier falls in each of two streams to rear naturally at low density. The rest were reared at high density in hatchery raceways. After 3 months (late spring), we electrofished the streams, marked the naturally reared salmon caught, and then added equal numbers of marked hatchery-reared salmon to the streams. The streams were electrofished again in the summer to monitor survival, movement, and growth. Hatchery-reared and naturally reared juveniles survived equally well (about 90% survived each summer), and few fish of either rearing type emigrated from the study streams. Hatchery fish were about 10% larger than naturally reared fish at the time of introduction, but there was no evidence for size-related survival in the streams. When adjusted for size, hatchery fish grew at faster rates than naturally reared fish. Our results suggest that hatchery-reared coho salmon perform similarly to naturally reared salmon when introduced into streams in low numbers and with a relatively small size advantage. Wild salmon populations are declining in many areas of the world, and there is increasing pressure to develop management strategies to increase recruitment (Jonsson and Fleming 1993). There are mixed opinions in the scientific community about the efficacy of using hatchery fish to enhance (‘‘supplement’’) wild production (Lichatowich and McIntyre 1987; Meffe 1992; Cuenco et al. 1993). Many studies have concluded that hatchery fish perform poorly in streams (Miller 1954; Bachman 1984; Maynard et al. 1996). However, other studies indicate that hatchery-produced salmon outcompete and displace wild fish (Nickelson et al. 1986). Nickelson et al. (1986) examined the effect of hatchery supplementation on the production of wild coho salmon Oncorhynchus kisutch in 15 Oregon coastal streams and concluded that hatchery fish displaced wild fish. Densities of wild coho salmon were 44% lower in streams stocked with hatchery-produced salmon than in unstocked streams; however, the relative frequencies of hatchery and wild fish displaced from the study streams were unknown. The streams were stocked at high densities and if levels exceeded carrying * Corresponding author: [email protected] 1 Present address: Department of Zoology, 430 Lincoln Drive, University of Wisconsin, Madison, Wisconsin 53703, USA. Received March 16, 1998; accepted December 29, 1998 capacity, displacement of wild fish may have resulted primarily from large numbers of fish in the streams rather than from displacement by competitively superior hatchery fish (Berg and Jor-