I was under the impression that unclipped hatchery fish with a CWT are part of what is called the "double index tag group" and that most hatchery stocks, or maybe management areas had this going on with a static 5% of the smolt release group. It is done for the reasons as said above, to better the odds of getting data back. I wager that 5% is more than the missclip rate for smolts.
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Fish gills are like diesel engines, don't run them out of fuel!