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In reading this study, I searched for confounding variables that might make the survival percentages of the Fraser River smolts appear to be lower than they really are. First, the researchers used tags that required large smolts to be used in the study. They could not use smolts smaller than 120mm and the V9 tags required fish to be greater than 140mm. These are fairly large for salmon smolts, and for reference an average spring Chinook smolt on the Warm Springs River in only 100mm long. There are plenty of published papers in existance supporting the idea that large smolts are more likely to become precocial than smaller smolts, and precocial smolts have lower survival than their non-precocial counterparts. If the researchers wanted to compare apples to apples in this study, they would have only used PIT tag data on smolts greater than 120 mm from the Columbia, but they made no mention of doing so.

Perhaps the most siginificant problem with the study is the placement of the final audio recording stations on the Columbia and the Fraser. In the Columbia they placed there equipment on the Astoria bridge which is just a few miles into the brackish estuary. However, the audio recievers in the Fraser were placed several hundred miles into the saltwater. I would guess there was a significant amount of mortality occuring considering the amount of saltwater the Fraser River smolts had to navigate before arriving at the location of their final detection array. I would have to argue that if the recievers were placed the same distance into saltwater on both systems, you would see drastically different results.

posted 4 years, 6 months ago
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