Recovery of Fish Populations in Lakes Affected by the May 18, 1980 Eruption of Mount St. Helens

The May 18, 1980, eruption of Mount St. Helens severely impacted a number of lakes in the vicinity of the mountain. Intense heat, tephra deposits, and a pressure wave that blew trees into these lakes appeared to doom fish populations; however, trout survived in most waters. Many lakes were covered with ice on May 18, which reduced thermal warming and protected resident fish. This report documents trout population changes in the 22 years since the eruption.

Nineteen of the 24 lakes studied had self-reproducing trout populations in 2001. Four lakes were barren of fish, but current Mount St. Helens Monument policies preclude stocking of these lakes. Two new lakes, Coldwater and Castle, were created when material from the debris avalanche blocked stream drainages. These relatively large lakes have provided new recreational opportunities.

Although eruption effects were often devastating to fish populations, inorganic volcanic material and organic debris from surrounding forests added nutrients to these lakes. Since many spawning streams were buried in ash, presumably fish recruitment was limited. With fewer fish to compete for food, substantial fish growth occurred in many lakes. Likely, increased productivity from nutrient enhancement also accelerated fish growth. As spawning areas recovered, populations expanded and growth rates generally declined. After the initial influx of nutrients, productivity in many lakes decreased.

Debris avalanche material in Coldwater, Castle, and Spirit lakes greatly enhanced productivity. Anaerobic conditions in Spirit Lake were due to bacterial decomposition of organic material. By 1986, phytoplankton photosynthesis in Spirit Lake had nearly replaced bacterial chemosynthesis.

Zooplankton diversity and abundance declined in most lakes immediately after the eruption. Turbid conditions from ashfall and erosion curtailed phytoplankton production. As water clarity improved, primary production increased, enhancing zooplankton communities.