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One significant factor in comparing these survival estimates is the technology used to measure survival. Different technology is used in each river. In the Columbia, PIT tags, a little larger than a grain of rice are used. The acoustic tags used in the Frazer study are at least 2-3 times larger. Inserting the tag is likely to affect survival. This is known as tag effect. Given the large difference in sizes of tag, the tag effects are likely to be different. Other factors also affect survival estimates, including how easily the tags can be detected, etc. Another factor to consider is that we have many studies that have shown when you release fish on the upstream side of the dam and then recollect them below, a certain percentage will be killed or injured depending on the route of passage. So to say the dams have no effect is very misleading.

posted 4 years, 7 months ago
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