Many blame the nearly 200 large dams in the Columbia River Basin for the decline in Pacific salmon, whose population has dropped by more than 90 percent since the 1980s. But new research shows that dam operation can be made salmon-friendly and suggests that other significant factors are impeding the salmon’s recovery.
“Although the hundreds of thousands of dams around the globe are not good for fish, they may vary in how bad they are,” say the two researchers. “Understanding the differences between dams is critical for setting conservation priorities.”
This work is presented by Phillip Levin of the National Marine Fisheries Service’s Northwest Fisheries Science Center in Seattle, Washington, and Nicholas Tolimieri of the University of Auckland’s Leigh Marine Laboratory in Warkworth, New Zealand, in the November issue of Animal Conservation.
To determine how dams affect salmon in the Columbia River Basin, Levin and Tolimieri compared spring/summer Chinook salmon populations before and after the construction of four dams between the mid-1960s and mid-1970s. The researchers considered sets of salmon populations in three areas: the Upper Columbia River, the Snake River, and the Middle Columbia River. These rivers have 5, 4, and 4 large dams, respectively, and the first two rivers become confluent at the latter. All 13 of these dams have structures to help adult chinook migrate upstream to their spawning grounds; however, only some of them have structures to help juveniles migrate downstream to the ocean.
Based on about 40 years of spawning counts, Levin and Tolimieri found that the Middle Columbia salmon have generally remained stable, but the Snake and Upper Columbia River salmon have declined markedly. The conservation status of the three populations is: not listed, threatened, and endangered, respectively.
Whereas dams caused the salmon decline in the Snake and Upper Columbia, they may not be the primary factor limiting recovery in the Snake, say Levin and Tolimieri. The researchers based this conclusion on differences in the two populations’ productivity (number of adults returning to the spawning ground/spawner). Salmon productivity naturally varies over time, and this variation has been about the same in the Snake and Middle Columbia. However, productivity has declined far more in the Upper than in the Middle Columbia.
Levin and Tolimieri attribute this discrepancy in part to the fact that while all four of the Snake River dams have structures for diverting juveniles from the deadly turbines, only one of the five Upper Columbia dams does. Consequently, about 75 percent of the Snake’s juvenile migrants make it downstream but hardly any of the Upper Columbia’s do. The dams on the Snake are federally operated but those on the Upper Columbia are privately owned.
“Although dams contributed to the severe declines , the key unanswered question is whether dams are presently preventing the recovery,” say Levin and Tolimieri. “Our analysis points to hydropower systems on the Upper Columbia River, but not the Snake River, as a potential force preventing recovery of endangered salmon populations.” They attribute the continuing decline of the Snake River salmon to habitat degradation, poor ocean conditions, and other factors.
For more Information
Levin, P.S. and N. Tolimieri. 2001. Differences in the impacts of dams on the dynamics of salmon populations. Animal Conservation 4:291-299.