Forms of Production Loss During the Early Life History of Fall Chinook Salmon

The Hanford Reach of the Columbia River supports one of the largest populations of naturally-spawning fall Chinook salmon in the Pacific Northwest.  Efforts are currently underway to model the productivity of this population and gain a better understanding of factors that benefit, or limit the population.  A key knowledge gap in productivity modeling exists from the time that eggs are discharged from females until fry emerge from redds.  The objective of this study was to examine multiple sources of loss during the early life history stages of fall Chinook salmon.  Four primary factors were examined: egg predation, loss of eggs from redds by superimposition or other means, proportion of viable (fertilized and living) eggs in redds, and egg to emergence survival.  Twelve overnight drift net sets were used to assess the number of eggs that were not buried or were dislodged from redds.  Mean bottom velocity at drift net locations (at the time of deployment) was 0.57 m/s and 3,165 eggs were sampled (10.9 eggs/hour).  Examination under the microscope determined that 8% of eggs were fertilized and ranged in developmental stage from the 2-cell stage to the choroid of the embryo being fully pigmented (eye-up).  Based on stage of development and thermal sums, it is estimated that eggs may have been dislodged from redds up to 30 d post-fertilization.  Eighty-five percent of the eggs showed no evidence of fertilization; however, many may have been collected shortly after fertilization prior to the first cell division occurring.  Evidence regarding fertilization status was not obtainable from the remaining 7% of eggs.  Largescale sucker and mountain whitefish were lethally sampled and gastric contents examined for evidence of Chinook salmon egg predation.  Mean number of eggs consumed by largescale sucker was 0.4 eggs/fish; whereas, mountain whitefish averaged 14.0 eggs/fish.  Underwater observation was conducted for 66.5 hours during the fall Chinook salmon spawning period.  White sturgeon were observed on 81 occasions with the majority of observations on or near Chinook salmon redds in water < 2 m in depth.  On three occasions, white sturgeon were observed actively feeding (i.e., pumping substrate) in fall Chinook salmon redds.  These results indicate that a biologically meaningful amount of loss may occur during the early life history stages of fall Chinook salmon.  These losses are currently not accounted for in productivity monitoring and future research is needed to further quantify the sources and magnitudes of loss.