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Microtags contain physical marks on their surface that can be <br />interpreted under a microscope. Because individual microtags are not <br />unique and all tags from a given group contain the same information, <br />such microscopic analysis allows the researcher to determine only of <br />which group a tagged fish is a member. Decoding requires that the fish <br />be killed and the tag removed. About 30,000 Colorado squawfish received <br />microtags. Fish were separated into two groups of 10,000 each and 4 <br />groups of 2,500 each. Each group received microtags unique to that <br />particular group. <br />The smaller groups of Colorado squawfish were stocked into each of <br />the four river backwaters (Figures 3-6), whereas the two larger groups <br />were separately stocked into two gravel pits (Figures 7 & 8). Blocking <br />nets were placed in the entrance to each river backwater at time of <br />stocking; the next day the backwaters were sampled to obtain baseline <br />catch per unit effort values after which block nets were removed. <br />Before stocking the gravel pits as many predatory fishes as could be <br />collected from one pit were removed using mechanical means (seining, <br />trammel netting, trawling, and electrofishing). <br />Experimental backwaters were regularly examined as described <br />above. Gravel pits and river backwaters were sampled once or twice a <br />week. All predatory fishes collected were passed through the tag detector <br />to determine the presence of tagged Colorado squawfish within the stomach <br />of the predator. Six predators of each species were killed initially to <br />validate the accuracy of this dectection technique. Collected fish were <br />measured and marked with a pelvic fin clip. Recaptured fish were noted; <br />data were used in cumulative census techniques of population estimation <br />(Ricker 1975). <br />-7-