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MARKING CYPRINID LARVAE WITH TETRACYCLINE A r ,~ ~ 50}~n~ B S, A ?*~ , i y ' I ~r ~~ 1001,m C , .-- _ _ 100 ~m 93 FIGURE 2.-Microphotographs of unground otoliths under white (left) and ultraviolet (UV) light (right). Otoliths from Colorado squawfish larvae immersed in a 350.mg tetracycline hydrochloride/L solution for 4 h. A. Sagitta from a 9.1-mm total length (TL) larva, 7 d after treatment. m =fluorescent mark; e = edge. of otolith under UV light. B. Lapillus and C. Sagitta from a 18.2-mm TL larva, 56 d after treatment. m =fluorescent mark; edge of otoliths under UV light indicated by dashed line. light, and its absorption capacity increases with increased amounts of dissolved solids and organic matter (Koller 1965). Under natural conditions, the small amount of solar UV radiation that pen- etrates the atmosphere and strikes the earth is attenuated in the surface layer of natural waters (Nicol 1974; Golterman 1975; Reid and Wood 1976; Ali and Klyne 1985; Kirk 1986). Also, the skull and skin of fish should directly protect internal structures from UV radiation (Koller 1965). Skin pigments, particularly melanin on the dorsal surface (including the head and nape), afford protection from light. Melanin helps protect underlying tissues from UV radiation by scatter- ing rays, absorbing damaging radiation (less than 400 nm), and dissipating absorbed energy as heat (Pathak and Stratton 1969). Finally, the successful use of tetracycline-marked fish has been reported in field mark-recapture studies. Blackler (1974) indicated that tetracycline marks may persist for at least several years in internal structures such as bones and otoliths. Pacific salmon Oncorhynchus spp., fed pelleted food that contained tetracycline and then released into the wild, retained fluores- cent marks in internal bone for at least 3.5 years after treatment (Weber and Ridgway 1967). Ko- kanee Oncorhynchus nerka, fed OTC-bearing food as larvae and early juveniles and then re- leased into Lake Granby, Colorado, retained flu- orescent marks in caudal vertebrae for 5-6 years (W. J. Wiltzius, Colorado Division of Wildlife, personal communication).