Estuary provides refuge after dam removal for bull trout

Bull trout use a bewildering array of strategies to aid in their survival, from remaining in streams their whole lives, like rainbow trout, to spending part of their lives in the ocean before returning to streams to spawn, just as salmon do. Bull trout are present in only one of two neighboring rivers in the Olympic peninsula, Washington state, and in this one (the Elwha River), two large dams were removed during the period 2011-2014. Dam removal resulted in massive outflow of sediments, reducing the clarity of the water and also building up a large delta and expanding the size of the estuary at the mouth of the Elwha River. Standardized sampling for bull trout before, during, and after dam removal was used to detect whether bull trout changed their use of the Elwha River estuary, or moved into the adjacent Salt Creek stream where they were formerly absent. Sampling revealed no movement into Salt Creek, but numbers of bull trout in the Elwha River estuary increased greatly during and immediately after dam removal, coinciding with large sediment outflow, before returning to their original low levels. Thus bull trout appear to have used the enlarged estuary as a refuge from the effects of dam removal, then returning to the river when the river water cleared up from the sediment, although confirmation is needed from tagging studies from other rivers experiencing dam removal. Of additional interest is the long-term response of bull trout to the additional habitat opened up above the former dams. The new research by SAFS graduate student Alexandra Lincoln, Anne Shaffer of the Coastal Watershed Institute, and SAFS professor Thomas Quinn, appears in the journal Environment Biology of Fishes.

(a) Map showing the Salt Creek watershed and Elwha River watershed, (b) aerial photo showing Elwha River estuary sampling sites, (c) aerial photo of Salt Creek estuary sampling sites.

 

Increase in bull trout sampled during surveys of the Elwha River estuary before (white), during (gray) and after (white) dam removal.
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