Ocean and dam influences on salmon survival.

The Decline of Columbia River Chinook Salmon TheColumbia River in the Pacific Northwest has been the site of the world’s most expensive effort in ecological management and restoration. Primarily using revenues from the hydroelectric system, roughly $400 million have been spent annually on the fish and wildlife program in the Columbia Basin, most of it on salmon. Much of this funding goes to operate hatcheries and modify the operation of dams, but also to a research program that, over the last four decades, has funded thousands of scientists and supported a number of technical advances, including three tagging techniques that have revolutionized our ability to understand the freshwater and marine life of salmon. One of these techniques is the use of acoustic tags on juvenile salmon. In PNAS, the article by Rechisky et al. (1) reports how these acoustic tags are used to measure survival of juvenile salmon in their early ocean life. The tags are implanted in thousands of juvenile salmon and arrays of acoustic listening devices detect their passage down the Columbia River and their northward ocean migration. Before the development of acoustic tags, the ocean was essentially a black box. Using earlier tagging techniques, individual fish were marked and nothing was known of them until they returned from the ocean, when the small stainless steel tags, known as coded wire tags, were either seen by physical inspection for a missing adipose fin, or passively interrogated tags were electronically detected at a range of a meter or two as the fish pass up fish ladders. The ocean distribution of fish could be inferred when coded wire tag-marked fish were caught, but the timing and location of ocean mortality remained unknown. The decline of Columbia River salmon is a well-documented story of the conflict between industrial-scale human activity and wild resources (2). A combination of overharvesting, loss of habitat because of land-use changes and impassable dams, changes in ocean conditions, and dam construction, has led to a loss of most of the once great migrations of wild salmon on the Columbia. The Columbia River was known for the largest runs in the world of the largest of the Pacific salmon, the Chinook or “king” salmon, and the “kings” are still a primary focus of restoration efforts. The story of the Chinook salmon on the Columbia River can be told in two phases. At first, using catches as a measure, the development of industrial fishing brought about a substantial decline. Then the Grand Coulee Dam (1942) totally blocked the upper river to salmon migration, and the four mainstem dams (beginning with the Bonneville Dam in 1937) on the lower Columbia flooded considerable habitat and proved a significant barrier to both the upstream passage of adults and the downstream migration of juveniles despite the construction of fish ladders. By 1960, the Chinook salmon runs were less than 10% of what they had been a century earlier. The second phase began in 1960. Four more dams were completed on the Snake River (between 1962 and 1972), resulting in anothermajor decline in the runs of Chinook spawning in the headwaters of the Snake River. Thanks to the fish ladders that allow us to very reliably count the fish passing upstream, scientists were able to estimate the number of adult fish that return for each adult spawner (sometimes called recruits per spawner) and also the smolt-to-adult ratio that is a measure of the fraction of juveniles migrating downstream that survive both migration and life in the ocean to return. Both recruits per spawner and smolt-to-adult ratio declined dramatically in the late 1970s, coinciding with the completion of the four Snake River dams, to the point where, even though there was no harvest, stocks kept declining and appeared to be on a trajectory toward extinction (3). In 1992, the major stock of Chinook salmon spawning in the Snake River watershed was placed on the Endangered Species list.