Above this level the mortality from gas bubble trauma is significant. The model produced levels of mortality similar to those observed in the cage studies below Ice Harbor dam. Monitoring of fish passing in-river has revealed few signs of gas bubble trauma and the CRiSP modeling has likewise predicted little impact. Furthermore, model analysis indicates that the small increase in dam survival resulting from the spill program was negated by a small increase in mortality from gas bubble trauma. Uncertainty exists as to the precise levels of the factors, but given the available information the result of the spring 1995 spill program was most likely small and negative.
Spill is used to pass fish over dams.
This reduces total mortality in dam passage since mortality resulting in passage by spill is less than passage resulting through turbine passage. Spill also produces gas supersaturation in the water downstream of dams. Fish exposed to the supersaturation suffer some additional mortality from gas bubble trauma. The dashed line indicates forebay and tailrace gas levels allowed in the NMFS spill plan. The solid lines indicate the range of total dissolved gas below Ice Harbor Dam in the 1995 spill program.