USA - Taking Stock of Salmon Survival, Dams and Science
Federal agencies required a minimum criteria of 96 percent of spring-migrating and 93 percent of summer-migrating juvenile salmon had to survive passage over Lower Granite Dam, located in southeastern Washington.
Because it's impossible to count each fish, scientists rely on mathematical models. These models rely on tracking a sub-sample of salmon that is designed to represent the larger population. However, traditional models have been expensive and time consuming.
Now, Pacific Northwest National Laboratory (PNNL) biologists have developed a more efficient way to estimate salmon survival through dams that uses solid science but saves over 42 percent of the cost—over $500,000 in a recent study. High-quality survival estimates involve multiple steps—including tagging thousands of juvenile fish, which is labor intensive and expensive.
Ryan Harnish, a PNNL biologist, and his team developed a new model that is a cheaper, yet accurate way to estimate salmon survival. This new model is more efficient—cutting costs nearly in half and research time by nearly a quarter—compared to the previous model. With increased efficiency and lower costs, this new model could be applied to other dams, and used by entities such as Columbia River utility companies that are required to monitor survival when conducting similar projects.
What do salmon need to survive?
Salmon have a rare ability to go from freshwater to saltwater and back again—a trait referred to as anadromy. Most fish live in either freshwater or saltwater. Few live in both. Adult salmon spawn in rivers and once juveniles hatch, the young fish head downstream to the ocean. On their journey, young salmon encounter many obstacles—dams, predators, and variable river conditions, to name a few.
PNNL researchers have studied ways to make the dam passage safer, including special water slides called weirs, and dam operations, including how much water is spilled and when. Behind the scenes of research lies a lot of math. Mathematical models help answer the question: how many salmon actually survive?
On average, about 96 of 100 young salmon, called smolts, survive the downstream migration through each lower Columbia River and Snake River dam. This means, with several dams on the Columbia River System, the total survival rate drops with each passage. For example, if 96 percent of juvenile salmon survive the first dam, 96 percent of those survive the second dam, and 96 percent of those survive the third dam, the total after three dams is 88 percent survival.