I've seen this tail cutting phenomena before, way back in the early 90's when I worked for WDFW out of Montesano on the coast. I was on a crew that walked on the many streams counting redds and live & dead fish. There was another crew however, that worked along side us and they gathered all the biological & genetic info (scales, weight, and length of fish). The fish were netted and bonked and then sampled. The fish then had their tales cut completely off, so that the same fish wasn't sampled twice. This may be the answer to your question.