Net Production of Juvenile Coho Salmon in Three Oregon Streams

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Abstract
Net production of juvenile coho salmon was estimated in three small streams in Oregon for 4 consecutive years. Annual net production of coho was greatly different in the 4 years, but production per unit area was similar among streams, averaging about 9 g/m2 per year. No significant differences were found among streams in production per unit area for 14 months from emergence of fry one spring through seaward migration the next spring. For 4 years biomass averaged 5‐12 g/m2 shortly after emergence of fry, declining to 2‐3 g/m2 by July and remaining at about 2‐4 g/m2 until emigration of smolts in the following spring. In all years, mean production declined from 1.9‐2.8 g/m2 per month after emergence to 0.2‐0.3 g/m2 per month in winter, then increased to 0.5‐0.6 g/m2 per month prior to emigration. Monthly instantaneous growth rates were highest shortly after emergence of fry, declining until late winter, then increasing just before smolt emigration. The mean monthly instantaneous growth rate was about 0.19 for all streams and years. Yield of smolts as seaward emigrants ranged from 18 to 67 per 100 m2. Net production was 1.5 to 3.0 times greater than yield as biomass of smolts. Net production of all fish in one stream containing coho, steelhead and cutthroat trout, and cottids was estimated to be 16 g/m2 per year and compared with data from other waters. Relatively large freshets appeared to cause large downstream movements of juvenile coho. Downstream drift of postemergence fry and emigration of yearlings tended to bias estimates of growth and net production in the residual populations.

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