Summer Production of Coho Salmon Stocked in Mount St. Helens Streams 3–6 Years after the 1980 Eruption

We monitored habitat use and summer production of stocked underyearling coho salmon Oncorhynchus kisutch from 1983 to 1986 in three streams affected by the 1980 eruption of Mount St. Helens, Washington. Two streams were in the blast area and one was on a volcanic mudflow terrace, Midsummer water temperatures frequently exceeded presumed stressful thresholds and occasionally surpassed the incipient lethal limit. Temperatures at the study sites (up to 29.5°C) may have been the highest ever recorded in small streams in western Washington. In addition, there was relatively little submerged cover and limited pool habitat. Despite the severe conditions created by the eruption, production rates of stocked coho salmon at all sites ranged from 15.1 to 143.8 mg/m 2 ·d (2.3–21.6 g/m 2 over an average 150‐d summer period) and were equal to or greater than those measured in other streams of comparable size in the region. Coho salmon production in the streams was more strongly influenced by population biomass and density than by average individual growth rate. Production was also influenced by timing and average weight at stocking; larger fish stocked later in the summer had higher survival than smaller fish stocked earlier. Apparent summer mortality (true mortality plus emigration) may have been influenced by the presence of other salmonids. Coho salmon density at the end of summer was consistently lowest in the mudflow stream, the only site to have a large population of steelhead (anadromous rainbow trout Oncorhynchus mykiss , formerly Salmo gairdneri ). We suspected that an abundance of both terrestrial and aquatic food was partly responsible for the high summer production of stocked coho salmon in what was an otherwise hostile environment.