In the Columbia River, the presence of an extensive network of hydroelectric dams has been widely blamed for the decline in abundance of adult salmon returning from the ocean since the late 1970s. However, the completion of the hydropower system occurred at the same time as large-scale shifts in ocean climate, as measured by climate indices such as the Pacific Decadal Oscillation.
The undammed Fraser River lies directly north of the Columbia River (the Fraser and The Columbia are the two largest rivers on the west coast of North America, and have, or formerly had, some of the world’s major salmon runs).Within these two watersheds, the Thompson (Fraser) and Snake (Columbia) rivers form major tributaries and are located in similar climatic zones. At the end of the last ice age, salmon colonized the upper Fraser watershed (including the Thompson River) from the upper Columbia River, thus providing a relatively recent genetic linkage. There are thus broad similarities between the two river systems, making for an interesting comparison of salmon survival during the freshwater phase of the juvenile outmigration between 2 rivers, one with dams and one without.
Our findings indicate that survival during the downstream migration of at least some endangered Columbia and Snake River stocks appears to be as high, or higher, than that for the same species migrating out of the un-dammed Fraser River in Canada. Equally surprising, smolt survival through the hydrosystem, when scaled by either the time or distance migrated, is higher than those seen in the lower Columbia River and estuary where dams are absent. These results raise important questions regarding the factors preventing the recovery of salmon stocks in the Columbia and the future health of stocks in the Fraser River.
The Columbia hydrosystem dams clearly had large impacts on the mortality of migrating salmon smolts in the 1960s and 1970s. However, subsequent modifications to the dams have improved survival substantially. Our results suggest that survival through the hydropower system – at least for the size range of smolts Kintama tagged – has now increased to levels similar to those experienced in both the undammed lower Columbia River and in the Fraser River, an important finding that was not technically possible before the development of large scale application of acoustic telemetry. However, our data do not address whether the possible delayed effects of hydropower system passage affects mortality after the fish leave the river for the ocean, nor is it clear whether salmon survival in the Fraser River has substantially changed during the last 100 years, as prior baseline measurements of survival are absent.
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