Abstract
The mortality of salmon smolts during their migration out of freshwater and into the ocean has been difficult to measure. In the Columbia River, which has an extensive network of hydroelectric dams, the decline in abundance of adult salmon returning from the ocean since the late 1970s has been ascribed in large measure to the presence of the dams, although 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. We measured the survival of salmon smolts during their migration to sea using elements of the large-scale acoustic telemetry system, the Pacific Ocean Shelf Tracking (POST) array. Survival measurements using acoustic tags were comparable to those obtained independently using the Passive Integrated Transponder (PIT) tag system, which is operational at Columbia and Snake River dams. Because the technology underlying the POST array works in both freshwater and the ocean, it is therefore possible to extend the measurement of survival to large rivers lacking dams, such as the Fraser, and to also extend the measurement of survival to the lower Columbia River and estuary, where there are no dams. Of particular note, survival during the downstream migration of at least some endangered Columbia and Snake River Chinook and steelhead stocks appears to be as high or higher than that of the same species migrating out of the Fraser River in Canada, which lacks dams. Equally surprising, smolt survival during migration through the hydrosystem, when scaled by either the time or distance migrated, is higher than in the lower Columbia River and estuary where dams are absent. Our results raise important questions regarding the factors that are preventing the recovery of salmon stocks in the Columbia and the future health of stocks in the Fraser River.
Highlights
Many Columbia River salmon stocks are listed as threatened or endangered [1,2], a result often attributed to the construction and operation of the Columbia River dams [3,4,5]
We examine one phase of the lifecycle of Columbia River and Fraser River salmon stocks by comparing the freshwater survival of freely migrating salmon smolts down the extensively dammed Snake-Columbia River system with that of the same species migrating down the ThompsonFraser River system, which lacks dams, using components of a large-scale acoustic telemetry system, the Pacific Ocean Shelf Tracking (POST) array
Salmon smolt survival to the Pacific Ocean was comparable in both the dammed Columbia and undammed Fraser rivers, it was higher in the Columbia once distance or travel time was taken into account—and higher within the hydropower system than below the dammed section
Summary
Many Columbia River salmon stocks are listed as threatened or endangered [1,2], a result often attributed to the construction and operation of the Columbia River dams [3,4,5]. Concurrent with the start of construction of the Federal Columbia River Power System in 1938, and especially following the completion of the last dam in the Snake River in 1975, major declines in abundance of adult salmon returning to the Columbia have occurred [2,7]. Much of the salmon decline from historic abundance occurred as a result of overfishing and habitat loss before 1938, when Bonneville, the first federal dam, became operational. The poor adult return to the Snake River is variously ascribed to mortality on salmon smolts migrating to sea caused by the eight hydropower dams [1,2,9], habitat disruption [2,10], interactions with hatchery fish [11,12,13], and changes in ocean climate affecting salmon survival after the smolts leave the river [14,15,16]
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