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fgQl ~ u~ 666 N in efficiencies of thrs~ sampling techniques for es- timating fish populations in small streams. Pro- gressive Fish-~tilturist 46:180-.184. Northcote, T. Cy ;"and D. W. Wilkie. 1963. Underwater census yf stream fish populations. Transactions of the Pl.lnerican Fisheries"Society 92:(46-151. Watke~;" C. R., R. E. Lennon, and B. L. Berger. 1964. reliminary observations on the toxicity of anti- mycin A to~ish and other aquatic animals. U.S. Trnnsactions ojthe Amerfcnn Fisheries Society 120:666-668, 1991 ® Copyright by the American Fisheries Society 1991 OTES g1H~ Bureau of Sport Fisheries and Wildlife, Investiga- tions in Fish Control 2, Washington, D.C. Werner, E. E., D. J. Hall, D. R. Laughlin, D. J. Wagner, L. A. Wilsmann, and E C. Funk. 1977. Habitat partitioning in a freshwater fish community. Jour- nal of the Fisheries Research Board of Canada 34: 360-370. Received October 1, 1990 ~. h Accepted March I, 1991 A Effect of Sunlight on Tetracycline Marks in Otoliths of Colorado squawfish Larvae ROBERT T. MUTH ANa KEVIN R. BESTGEN Department ojFishery and Wildlife Biology and Larval Fish Laboratory Colorado State University Fort Collins, Colorado 80523, USA Abstract. -Embryos and newly hatched larvae of Col- orado squawfish Ptychocheilus Lucius were immersed in tris-buffered solutions of 350 mg tetracycline hydro- chloride (TC)/L of distilled water for 24 h (embryos) or 12 h (larvae). Larvae from both treatments were held indoors prior to swim-up and indoors or outdoors for 7 d afer swim-up. Exposure to sunlight under simulated natural conditions had no detectable effect on quality or retention of fluorescent TC marks in otoliths. Marks in all otoliths examined were distinct and lucid to bright. Mark-recapture studies on age-0 Colorado squawfish could facilitate research needed for applied management and population recovery. Tetracycline marks in otoliths appear to be suitable for this purpose. Research on early life stages is a principal focus of the recovery program for the Colorado squaw- fish Ptychocheilus Lucius, which has been listed federally as an endangered species. Areas of in- vestigation include dispersal patterns of larvae, environmental requirements of age-0 fish, rela- tionships between abundance of larvae and re- cruitment to juvenile and adult stocks, and causes of early mortality (e.g., Kaeding and Osmundson 1988; Nesler et al. 1988; Tyus and Karp 1989; Haines and Tyus ] 990). Mark-recapture studies could be used in many of these investigations if adequate methods were adapted or developed for marking early life stages (Wydoski and Emery 1983; Tsukamoto 1985). Tetracycline antibiotics are in vivo bone tissue markers that bind to calcium ions at sites of bone proliferation. These compounds, once deposited in bone, fluoresce yellow when illuminated by ul- traviolet (UV) light, particularly at 360 rim (We- ber and Ridgway 1967). Incorporation of tetra- cycline marks in otoliths by immersion of fish embryos or early larvae in a tetracycline solution is one technique for mass-marking young fish (Hettler 1984; Tsukamoto 1985, 1988; Tsuka- moto and Kajihara 1987). In preparation for fu- ture field experiments, Muth et al. (1988) and Muth and Nesler (1989) established procedures for suc- cessful application of this marking technique to early life stages of Colorado squawfish. The possibility that exposure to the UV portion of sunlight will reduce the quality or retention of tetracycline marks in otoliths remains a concern (Hettler 1984; Brothers 1985). Once excited to a higher energy level by absorption of UV light, tet- racyclinemolecules are unstable and decay through fluorescence almost instantaneously (Lehninger 1975). I,orson and Mudrak (1987) examined ef- fects of sunlight on fluorescent marks in otoliths of young American shad Alosa sapidissima that had been treated with tetracycline 15 to 18 d after hatching. However, UV degradation of tetracy- cline marks in otoliths could be most severe in embryos or newly hatched larvae released into the wild soon after marking. Although the UV spec- trum is rapidly attenuated in water (Koller 1965), recently hatched larvae of many fishes, including Colorado squawfish, are nearly transparent, have little protective melanophore pigmentation, and often swim near the water surface. Within the first few days of marking, tetracycline deposited in oto- liths would be covered by only a few new layers of calcium carbonate. This study addressed the question of tetracycline mark degradation due to