Evaluation of two release operations at Bonneville Dam on the smolt-to-adult survival of Spring Creek National Fish Hatchery fall Chinook salmon By

In March 2004, two groups of coded wire tagged subyearling fall Chinook were released from Spring Creek National Fish Hatchery to directly evaluate the effects of alternative operations at Bonneville Dam on smolt-to-adult survival (SAR). The first group, released on March 1st, passed Bonneville Dam during a four-day, 25 kcfs spill operation with the corner collector closed. The second group, released on March 10th, passed Bonneville Dam during a four-day, 5 kcfs corner collector operation with the spillways closed. Tagged adults were subsequently recaptured in ocean fisheries, Columbia River fisheries, and at the hatchery during 2005-2007. The overall SAR was 0.118 % for the fish released during the spill operation and 0.100 % for fish released during the corner collector operation. The overall SAR for fish released during the spill operation was 18 % higher than the SAR for fish released during the corner collector operation, however this difference was not statistically significant. Using Bayesian statistical methods, we estimated an 80 % probability that the SAR for the spill operation release was higher than the SAR for the corner collector operation release. Applying the results from the 2004 March release operations to the March releases over 2005-2007, we estimate that a foregone loss of 15,200 adults (range 2,400-38,900) may have occurred due to corner collector-only operations during 2005-2007. Additional years of comparative study would be useful to quantify the degree of evidence that these results are consistent across years or not