The Northwest Fisheries Science Center has released preliminary 2013 estimates of juvenile Snake River chinook and steelhead through the through the seven dams and reservoirs (Lower Granite Dam tailrace to Bonneville Dam tailrace) on the lower Snake and Columbia rivers.
The survival rate for spring chinook was 62 percent. This is the third highest survival rate for spring chinook in the past 15 years.
Steelhead survival for the same reach was estimated at 50 percent, which is relatively low compared to recent years. NWFSC noted that steelhead survival between Lower Granite and McNary Dams decreased through the last few weeks of the season, possibly due to increased predation and increasing temperatures during that period.
Other findings from the study:
q Spring flow in the Snake River was the fifth lowest in the past 21 years.
q Spill percentage on the lower Snake dams was 33.5 percent of flow.
q Low flow conditions may have increased the percentage of fish that passed the dams at the spillways. “Lower water velocities allow fish more time to react to conditions and may increase the attractiveness of the surface bypass collectors,” the memo said. (See more about surface bypass in the right-hand box on this page.)
q The estimated percentage of fish transported was among the lowest known estimates over the past 20 years.