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<br />Muth <br /> <br />Efforts to recover the endangered razorback sucker (Xvrauchen texanus) include <br />hatchery propagation of fish for reintroduction and research projects, documentation of <br />reproduction and recruitment in the wild, and evaluation of reasons for low recruitment and <br />ways to improve survival of young fish. Minckley et al. (1991) recommended marking <br />hatchery-produced razorback sucker to distinguish them from fish produced naturally. <br />Because razorback sucker are sometimes stocked in large numbers as larvae or small <br />juveniles, a mass-marking technique applicable to early life stages is required. The marking <br />technique also could be used in field mark-recapture studies to facilitate investigation of the <br />early biology of razorback sucker and fate of reintroduced fish. <br />Marking otoliths of fish has great potential for use in stock identification, assessment <br />of stocking success, and life-history studies (Brothers, 1990; Buckley and Blankenship, <br />1990). One technique for rapid mass marking is incorporation of fluorescent chemicals in <br />otoliths by immersion of fish embryos or larvae in solutions of these chemicals. Two <br />compounds proven effective for this technique are alizarin complexone (ALC) and <br />tetracycline hydrochloride (TC). Once deposited in bone, ALC fluoresces red and TC <br />fluoresces yellow when illuminated by ultraviolet (UV) light. Single or multiple treatments <br />with either or both of these chemicals can be used to produce potentially long-term marks <br />unique to different batches of fish with virtually 100% initial marking success (Tsukamoto, <br />1985, 1988; Dabrowski and Tsukamoto, 1986; Muth et aI., 1988; Brothers, 1990). Our <br />objective was to determine optimum treatments for marking otoliths in late embryos and <br />newly hatched larvae of razorback sucker. We evaluated fish survival and mark quality in <br />different ALC or TC concentrations and immersion durations for 1) single marking otoliths in <br />embryos with either chemical and 2) double marking otoliths in larvae with both chemicals. <br /> <br />2 <br />