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H-Cl\ ~"e,sr /V\odd c: tQCf0 North American Journal of Fisheries Management 16:905-912, 1996 American Fisheries Society 1996 O'7~07 ------ Evaluation of Marking Techniques to Estimate Population Size and First-Year Survival of Colorado Squawfish ~ G. BRUCE HAINES AND TIMOTHY MODDE u.s. Fish and Wildlife Service, Colorado River Fish Project 266 West JOO North, Suite 2, Vernal, Utah 84078, USA Abstract.-Three marking methods-tattoo ink injected with a dental inoculator, an elastic poly- mer injected by syringe, and fin clipping-were tested to determine a suitable technique for estimating population size, sur\;'ival, and movement of age-O Colorado squawfish Ptychocheilus lucius. Laboratory tests indicated that all three marks were retained at sufficient rates (>97%) to make population estimates over a 2I-d period. However, fish marked with tattoo ink had higher mortality (10%) than fish marked with elastic polymer (< 1%) or with a fin clip (0%). Fish marked with the elastic polymer had higher retention (85%) than those marked with fin clips (34%) or tattoo ink (26%) after 142 d. No differences in vulnerability to predation were observed among fish with the three marks or between marked and unmarked fish. A field test of the elastic polymer showed that it was easy to use, caused low mortality (5% for fish held overnight in live-cages), and produced marks that were readily visible. Mark-recapture methods were used to estimate population size of Colorado squaw fish (5,595 in the autumn and 2,523 the following spring) and winter survival (45%). Survival estimates from mark-recapture data were more accurate and precise than those from catch-per-unit-effort data, at least for short-term studies in river reaches less than 50 km long. Simulations showed that doubling the size of the study section and increasing the probability of capture by 33% would reduce possible bias from 14.5% to 1.4% and the coefficient of variation (SE/mean) from 0.25 to 0.06. .' Survival during the first year of life is a primary factor. determining year-class strength for most temperate-zone fishes (Oliver et al. 1979; Shuter and Post 1990) and may be an important factor limiting recruitment for Colorado squawfish Ptychocheilus lucius in the upper Colorado River basin (Kaeding and Osmundson 1988; Thompson et al. 1991). However, identifying the magnitude of winter mortality of age-O fish has been difficult, in part because of the lack of an adequate marking technique for small fishes (<60 mm total length, TL). In the absence of a suitable marking method, catch per unit effort (CPUE) in backwaters, the primary habitat occupied in autumn and spring (Tyus 1991), has been used as a population size index for estimating relative year-class strength of Colorado squawfish (Tyus and Haines 1991). Sur- vival estimates based on CPUE have required the assumption of equal vulnerability of capture in au- 'I tumn and spring, whereas those based on mark- recapture do not and may provide more accurate and precise estimates. Dyes have been used successfully for marking several fish species (David 1955; Arnold 1966; Nielson 1992). Kelly (1967) reported that injected 1 Mention of trade names or commercia] products does not constitute endorsement by the U.S. Fish and Wildlife Service. dyes were useful in some instances, but were im- practical for marking small fish. Hart and Pitcher (1969) and Laufle et al. (1990) successfully marked small fish by using air-injected tattoo ink. Northwest Marine Technology (Shaw Island, Washington 1), recently developed a fluorescent elastic polymer that is applied by syringe injection and shows promise for marking small fish. Al- though methods exist to mark small fish, the du- rability of the marks and the individual size and species on which these methods are effective vary. No information is available on the susceptibility to predation or delayed mortality from handling and injection for fish marked with these techniques (Moring 1990). The objectives of this study were to (1) compare fish survival and mark retention rates among three marking procedures, (2) compare the vulnerability of marked fish to predation, and (3) determine the applicability of mark-recapture methods to esti- mate population size and winter survival of Col- orado squawfish. Methods Marking procedures.-Three marking meth- ods-tattoo ink injected with a dental inoculator, an elastic polymer injected by syringe, and fin clip- ping-were tested for persistence and percent re- tention at a U.S. Fish and Wildlife Service fish hatchery at Ouray, Utah. The fish used for labo- 905