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An interesting point that is not discussed is similar tag effect studies have been conducted by the army corp of engineers research program. This research which compared Pit Tag survival to acoustic tags, the acoustic tags where smaller than the ones used in this study, revealed that there was significant survival difference between the two tags, it was not based so much on distance, but on time. This would indicate that the effects from the tags take a period to time to affect the juveniles. This effect would be somewhat muted since only larger fish were used. Smaller fish would show the impacts of tagging sooner than larger, so distance is not the best metric, rather time. Furthermore, larger fish tend to have better survival through the dams which would tend inflate the survivals.
This study is a proof of concept of the acoustic technology, which has advantages over Pit Tags which is the current gold standard, used in reach survival studies. Pit Tag are ideal for Smolt to Adult studies since they have minimal longer term effects and do not require batteries, they are also relatively inexpensive and can be used to tag large numbers of juveniles that incorporate the entire population size and not only available for larger fish. However, it is difficult to determine how and where the fish passed the dams and where they migrate. Therefore it is difficult to determine which management strategies are best for juveniles, ie does spill provide the best means of passage when compared to other routes of passage and transportation. These studies take many years to conduct. Where acoustic tags may prove invaluable is near dam passage, such that it will be know exactly when fish pass the dam, what route they took, how long they took to move through the forebays and tailraces of the dams. However, what is most needed for either technique is adult returns. Regardless of survival of the juveniles to the mouth of the river, if no adults return you still have not met success. Therefore you may have similar survivals between the two systems but have vastly different ocean survivals that may or may not have something to do with how they got through the rivers systems with and without dams. Lastly, just food for thought, the current Biological Opinion allow for 96% survival at each dam, when multiplied by 8 dams equates to survival rate of 72.14%, thus over of a quarter of the smolt population can be lost at just the dams, this does not include any mortality in the pools between the dams and mortality post Bonneville dam and to the estuary. So yes the dams do have an effect, the better question is, is it a limiting factor in the overall survival of salmon.

posted 4 years, 6 months ago
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