Hatchery Chinook Smolt Mortality in Pahsimeroi River

Hatchery personnel were scrambling throughout the weekend to determine the reason for a hatchery summer chinook smolt (juvenile) kill on the Pahsimeroi River. An estimated 30,000 smolts died in the event, representing about 3 percent of the 1.1million smolts released from the hatchery. On the morning of April 16, Pahsimeroi Hatchery personnel found that the 12 inch smolt bypass at the hatchery intake canal was plugged with dead hatchery smolts, which are released from upstream rearing ponds. No other obstructions were found at the bypass. A smolt bypass is necessary to move fish around large roller screens in the Pahsimeroi River to ensure that naturally migrating salmon and steelhead smolts are not diverted into the hatchery. The bypass and screens are checked multiple times daily. The crew scrambled to unplug the bypass. Department personnel worked to collect the dead smolts to get a good estimate of the number and composition of the mortalities. It appeared from the sampling that all of the mortalities were from the hatchery release and that no natural smolts had been affected. Crews worked throughout the weekend to assess the mortality and determine its extent. Aside from the initial mortality, no further problems at the intake canal were encountered. Last year, a similar number of smolts were released without any problems. It is not known why so many smolts tried to enter the bypass, rather than staying in the river. There may have been a fluctuation in river flows during the hatchery release that may have affected the normal flow in the river channel and caused an unusually large number of smolts to seek the intake canal, in excess of its normal capacity. The smolts were progeny of adults collected at the hatchery in 2002 and were considered ESA-listed because of previous broodstock history, due to a supplementation experiment conducted at Pahsimeroi Hatchery. IDFG rears summer chinook salmon at the Pahsimeroi Hatchery as mitigation for the Idaho Power Company's Hells Canyon complex of dams; the hatchery is funded by Idaho Power Company.