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~Y t ~ :}3 -~ ~~~ American Pisheries Society Symposium 5:89-95, 1988 ® Copyright by the American Fisheries Society 1988 : r ~;~ `~" ~f L Marking Cyprinid Larvae with Tetracycline 1 ROBERT T. MUTH Larval Fish Laboratory, Department of Fishery and Wildlife Biology Colorado State University, Fort Collins, Colorado 80523, USA THOMAS P. NESLER Wildlife Research Unit, Colorado Division of Wildlife Fort Collins, Colorado 80526, USA ANTHONY F. WASOWICZ2 Department of Fishery and Wildlife Biology, Colorado State University Fort Collins, Colorado 80523, USA Abstract.-Laboratory experiments were conducted on the use of tetracycline hydrochloride CfC) for mass marking recently hatched Colorado squawfish Ptychocheilus lucius. Protolarvae were immersed in tris-buffered solutions of 200, 350, or 500 mg TC/L of distilled water for 4, 12, or 36 h and then reared for up to 77 d. Overall survival was lowest (0-81%) for the 36-h and 500-mg/ L treatments. However, fish growth and development were not affected by treatment. Larvae preserved within 15 d of treatment exhibited externally visible fluorescence when scanned with ultraviolet (UV) light. Otoliths extracted from larvae that were preserved in 95% ethanol were in good condition, but those from larvae preserved in formalin solutions buffered to near neutral pH with phosphate degraded during storage. The otoliths of treated larvae exhibited fluorescent marks when examined by UV-light microscopy, and these marks were retained throughout the experi- mentwith no apparent decrease in intensity. Whole-body immersion in 350 mg TC/L for 4-12 h was found to be optimal for best mark intensity and fish survival. In another experiment, protolarvae of fathead minnows Pimephales promelas were immersed in a tris-buffered solution of 350 mg TC/L for 4 h and then exposed to various intensities of artificial white light in the 340-650-nm range (includes near-UV spectrum) for 12 h/d for 7 d. Exposure to light had no effect on the presence or intensity of fluorescent marks in the otoliths. Our results suggest that the TC-marking technique can be used infield mark-recapture studies of Colorado squawfish larvae. Marking individual fish is an effective means for obtaining various kinds of information on fish species and populations (Wydoski and Emery 1983). Data from mark-recapture studies often provide solutions to questions left unanswered by basic survey sampling. Tsukamoto (1985) ob- served that information on the early-life behavior and environmental requisites of fish is critical to understanding the dynamics of fish populations, and he noted that effective methods of marking fish early-life stages for mark-recapture studies need to be developed. Marking techniques used successfully for the early-life stages of some fish species might require modification or be unsuit- able for use on other fish species. The objective of this work was to develop or adapt amass-marking technique for larvae of the 'Contribution 35, Colorado State University Larval Fish Laboratory. ZPresent address: Utah Cooperative Fish and Wildlife Unit, Utah State University, Logan, Utah 84321, USA. federally endangered Colorado squawfish Ptycho- cheilus lucius for proposed mark-recapture stud- ies in the Yampa and Green rivers in Colorado and Utah. This technique would facilitate investi- gation of dispersal patterns of larvae, habitat utilization by age-0 fish, relationships between larval fish abundance and recruitment, and causes of early-life mortality. For such studies, an opti- mal mark must be (1) applicable to late embryos or recently hatched, yolk-bearing larvae or both, (2) suitable for mass marking in the field, (3) innocuous, that is, must not affect the normal life of the fish, (4) detectable for at least 8 weeks, and (5) relatively easy and inexpensive to apply and detect. Techniques for marking very small fish were reviewed by Laird and Scott (1978), Hettler (1984), Brothers (1985), and Tsukamoto (1985). These techniques included the use of dyes or stains, fluorescent antibiotics, trace element or nonradioactive isotopes, and growth inhibitors or environmental manipulations (e.g., of tempera- 89