Just days after state fish and wildlife officials warned that six of the Delta’s imperiled fish species suffered declines last year, a report released Jan. 17 found that 93 percent of juvenile salmon on the Tuolumne River perish due to predation.

The alarming finding was revealed in a report prepared as part of the Federal Energy Regulatory Commission’s relicensing of Don Pedro Dam, located along the Tuolumne River. Researchers estimated the abundance of predatory fish, analyzed their stomach contents, and tracked the progress of juvenile salmon making their way along the Tuolumne River toward the ocean.

While the discovery echoes earlier predation estimates, the study marks one of the most comprehensive investigations yet of predation on the Tuolumne River and indisputably demonstrates that predation is placing a significant strain on fragile salmon populations.

The evidence also underscores the serious flaws plaguing a controversial proposal by the State Water Resources Control Board that would require 35 percent of unimpaired flow on the Merced, Stanislaus and Tuolumne Rivers to boost ailing salmon populations. The proposal provides no proof that such a requirement would improve salmon survival and fails to even mention predation, which the science clearly shows is a critical factor that must be addressed.

“There is a disconnect between the Water Board’s theory about what might help the Delta and what trained scientists are finding in the field,” said Allen Short, executive director of the San Joaquin Tributaries Authority. “No matter how much water we send down the river, if nothing is done to prevent the striped bass and other non-native predators from eating young salmon, the population will never recover. If the Water Board is serious about restoring the salmon population, predation must be addressed in an aggressive and comprehensive way.”

While California has yet to respond to the significant damage caused to salmon populations by non-native predators, predation suppression programs have been successful in other regions. In the Columbia River Basin, for example, a program providing incentives to anglers who catch predators has resulted in a 30 percent increase in juvenile salmon survival.

“It makes no sense to permit the destruction of 93 percent of the salmon smolt population on a key Delta tributary without taking the slightest measures to mitigate the problem,” said Short. “Unlike the continual increases in flow, which has failed to reverse salmon population declines, predator suppression works and should be a top priority for the Water Board.”