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l~i4s~ Repiirited from THE SOUTHWESTERN NATURALIST Vol. 34, No. 3, 1989 Made in United States ofAmerica -r~ ' ~;:f~ l ~~W a.,C) 'Tl MARKING COLORADO SQUAWFISH EMBRYOS AND NEWLY HATCHED LARVAE WITH TETRACYCLINE Studies on reproduction and early life history of the endangered Colorado squawfish (Ptychocheilus Lucius) have been identified as research needs for the rare Colorado River fishes. A series of mark- recapture studies on Colorado squawfish larvae in the Yampa and Green rivers, Colorado and Utah, were proposed in 1986 by the Colorado Division of Wildlife (CDOW). These studies would investigate dispersal patterns of larvae, environmental requisites of age-0 fish, relationships between larval fish abundance and recruitment, and causes of early-life mortality. As an initial phase of the proposed work, Muth et al. (1988) developed techniques for mass marking 5-day-old (posthatching) larvae of Colorado squawfish by incorporation of tetracycline hydrochloride (TC) in otoliths through whole- body immersion in TC solutions. Tetracycline compounds are well-known markers for calcified struc- tures and Ruoresce yellow under ultraviolet (UV) light. They have been used to mark embryos or larvae of several non-cypriniform fishes (e.g., Hettle~, 1984; Tsukamoto, 1985). Muth et al. (1988) reported that once deposited in otoliths, TC was retained in detectable amounts for at least 77 days after treatment, and they recommended that 5-day-old Colorado squawfish larvae be immersed 4 to 12 h in a solution of 350 mg TC/1 for best fish survival and mark intensity. Under natural conditions, Colorado squawfish larvae begin to "drift" downstream a few days after hatching (Nesler et al., 1988). For this reason, we believe that objectives of the mark-recapture studies proposed by the CDOW would be better served if larvae were marked and released before or immediately after hatching. Marking techniques used successfully on some fish species or life stages might require modification or be unsuitable for use on other fish species or life stages. Therefore, the purpose of this work was to adapt the tetracycline marking technique described by Muth et al. (1988) to late embryos and newly hatched larvae of Colorado squawfish. ~,'