US Forest Service

Mount St. Helens

Mount St. Helens National Monument

Mount St. Helens
Pacific Northwest Research Station
1220 SW 3rd Ave.
Portland, OR 97204

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(FAQ 10) How were fish affected by the eruption?

It took fish over a decade after the eruption to return to Spirit Lake, but fish are now thriving in the lake (photo by Charlie Crisafulli).


Before the 1980 eruption, the lakes and streams of the Mount St. Helens area supported about 26 fish species, including anadromous species that spawnedin freshwater and migrated to the Pacific Ocean. Many of the rivers were well known for their spectacular runs of coastal rainbow trout (also known as steelhead when migratory) (Oncorhynchus mykiss irideus), coho salmon (O. kisutch), and chinook salmon (O. tshawytscha), all of which had important recreational and aesthetic value. The high mountain lakes were historically fishless because fish could not reach them, but beginning in the early 1900s, these lakes were stocked regularly with fish.


The 1980 eruption devastated some water bodies with fish and hardly changed others. Thus fish survival and recovery had very different patterns in the various bodies of water across the disturbance zones.

Eruption’s effects on fish in small lakes

The lakes received differing amounts of volcanic debris and organic matter such as fine material from the shattered and burned forest. Lake water was turbid, and fish food sources such as tiny animals called zooplankton and aquatic insect larvae declined. Even so, just weeks after the 1980 eruption, biologists found that fish had survived in most lakes in the blowdown zone where fish had been previously stocked, owing to ice on the lakes at the time of the eruption. Brook trout (Salvelinus fontinalis) was the most frequently found species in the summer of 1980.

Water transparency improved greatly in the blowdown-zone lakes after a few months, and zooplankton and aquatic insect larvae began to recover. Brook trout as well as other trout species were successfully spawning within a few years after the 1980 eruption and continued to survive through 2005 without any stocking.

Eruption’s effects on fish in Spirit Lake

So much volcanic debris slid into Spirit Lake that the lake bottom was raised 200 feet, and trees dragged into the lake covered the surface. All fish in Spirit Lake perished. After about 6 years, the lake’s grossly changed conditions had improved substantially and it appeared that the lake could once again support fish. In the early 1990s, scientists detected the first fish in Spirit Lake, a rainbow trout. Since then the lake has been sampled many times and results have shown a burgeoning trout population. Scientists have found that most of the fish are less than 4 years old and that the fish grow rapidly, an indication of abundant food. In fact, they found most 3-year-old fish to be about 23 inches long and weigh nearly 5 pounds. Spirit Lake continues to be highly productive, largely owing to the nutrients and minerals deposited during the eruption, and also the formation of shoals during the 1980 eruption. These shoal habitats support dense and complex aquatic vegetation that in turn support a diverse assemblage of plankton and insects.

Eruption’s effects on fish in Coldwater and Castle Lakes

The 1980 eruption created two new large lakes, Coldwater and Castle, which had no fish for several years. In the early 1990s, the Washington Department of Fish and Wildlife stocked rainbow trout in Coldwater Lake. Two years later, fish appeared in Castle Lake, presumably originating from fish that swam from Coldwater Lake through the river connecting the two lakes. Once established, fish populations in both Coldwater and Castle Lakes have grown well. The size of individual fish decreased from 1990 to 2004, suggesting that either the lake’s production of prey sources for fish decreased, or that the very abundant fish are competing for available food. Biologists have confirmed that rainbow trout have successfully spawned in streams associated with each lake and the populations appear to be self-perpetuating. Thus, no additional stocking has occurred.

Eruption’s effects on fish in rivers and streams

Most river fish were killed during or shortly after the 1980 eruption from suffocation in ash-choked waters or indirectly from the loss of habitat. Eventually, the flowing water flushed the fine sediment from the larger, less movable gravels and boulders in the streambeds, improving water quality and other habitat conditions. Fish prey began to recover. Once habitat conditions had improved in blowdown-zone streams, fish that had survived in headwater lakes swam downstream, and fish from tributary streams in the less disturbed tephra-fall zone also colonized the recovering stream reaches.

With streamside forests knocked down by the 1980 eruption, blowdown-zone streams were completely open to sunlight, dramatically increasing algae growth which, in turn, fueled a highly productive food web. Abundant food created very good conditions for fish growth. However, within 10 to 15 years after the 1980 eruption, deciduous trees had taken root along streamsides and were growing tall enough to shade the streams, presumably reducing food for fish, which caused fish populations to decline.

In the pyroclastic flow and debris avalanche zones, most of the streams had chronic high levels of fine sediment and shifting channels. By 2005, these streams had not developed conditions suitable for fish, and decades will likely pass before these streams can support fish.

Hazard management in streams affected natural stream processes

After the 1980 eruption, the Army Corps of Engineers built sediment dams on the Toutle River to block mudflows, a serious hazard to human life and property downstream. The sediment dams block the natural migrations of fish, including salmon migrating to the ocean and returning to freshwater to spawn, and also block resident fish which spend their entire lives in freshwater but normally move up and down the river. The sediment dams protect downstream areas but hamper natural processes that would improve the habitat; the impounded water drops fine sediments, which clog the river and its tributary channels. As mitigation, the Corps of Engineers built a fish collection facility and trucks captured salmon and steelhead upstream above the dams where the fish are released to spawn.

In some areas outside the national volcanic monument, salvage logging has affected streams and lakes and thus the natural responses of native fish. Hatchery fish and nonnative fish species have been stocked in some places, influencing the natural recovery, but the exact effects of stocking are unknown.

Overall, fish populations have rebounded remarkably since the 1980 eruption. Habitat conditions were often less than ideal, with water warmer than usual for native fish species or fine sediment levels high. But the food supply was often very rich, owing to the amount of sunlight reaching streams and fueling the food web, and fish are thriving in many bodies of water.

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US Forest Service - Pacific Northwest Research Station
Last Modified: Thursday,14May2015 at13:56:25CDT

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