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I q q 7 'Osmu rLd $61'1 I Transactions of the American Fisheries Society 126:687-698, 1997 @ Copyright by the American Fisheries Society 1997 7/~1 Growth and Survival of Colorado Squawfish in the Upper Colorado River D. B. OSMUNDSON . Colorado River Fishery Project, U.S, Fish and Wildlife Service 764 Horizon Drive, South Annex A, Grand Junction, Colorado 81506-3946, USA R. J. RYEL Department of Rangeland Resources, Utah State University Logan, Utah 84322-5230, USA T. E. MOURNING Colorado River Fishery Project, U.S. Fish and Wildlife Service Abstract.-Growth and adult survival rates were estimated for the endangered Colorado squaw- fish Ptychocheilus lucius inhabiting the upper Colorado River by using data from fish captured during 1990-1995. Mean annual growth rates of fish aged 3-6 years ranged from 32.2 (age 6) to 82.0 (age 3) mm/year. Growth rates for older fish were highest for fish 400-449 mm total length, TL, (42.7 mm/year) and declined to 19.8 mm/year for fish 500-549 mm TL. Fish 550 mm and longer grew an average 9.5 mm/year. Survival rates for fish 550 mm and longer were estimated by comparing measured size distributions with simulated stable age and size distributions; these ranged from 0.83-0.87, with the best fit at 0.85. Though lack of historical data precludes com- parisons with past growth and survival rates, our data serve as a baseline for future population monitoring efforts. Growth and survival rates, fundamental com- ponents of most demographic studies, are essential to understanding population dynamics and requi- site to formulating recovery and management strategies for endangered species. The Colorado squawfish Ptychocheilus lucius, an endangered, long-lived, cyprinid fish endemic to the Colorado River system of North America, has been the focus of much life-history research. To date, age and growth studies (e.g., Vanicek and Kramer 1969; Seethaler 1978; Hawkins 1992) have relied on standardized scale analysis techniques (e.g., Tesch 1968; Carlander 1969). However, scales have proven unreliable for some species (Beamish and McFarlane 1987; Mc- Carthy and Minckley 1987) and have considerably underestimated age in long-lived individuals of many western North American catostomids and cyprinids (Scoppettone 1988). Also, critical vali- dation of aging techniques has been lacking due to a scarcity of known-age individuals. Although aging with other bony structures (e.g., vertebrae, otoliths, opercles) may corroborate results from scales, this constitutt:;s a partial validation at best (Beamish and McFarlane 1983) and requires sac- rifice of these rare animals. Problems specific to aging Colorado squawfish with scales include lack of annulus formation for most individuals in the first year and compression and loss of outer annuli by older fish (Hawkins 1992). Although the first issue can be adjusted for, problems in distinguishing and counting outer an- nuli can result in many fish being assigned to ages near the age point where the method fails (Beamish and McFarlane 1983). Hawkins (1992) suggested that discrepancies in average growth increments for Colorado squawfish greater than total length 500 mm (TL) calculated from scales (30 mm/year) and those from recaptured tagged fish (10-15 mm/year) were due to the negative effects of Car- lin dangler tags. Indeed, Floy tags, another exter- nal tag, reduce growth in salmonids (Carline and Brynildson 1972, Mourning et al. 1994), though not in largemouth bass Micropterus salmoides (Tranquilli and Childers 1982). The recently developed small, internally im- planted passive integrated transponder (PIT) tag (Biomark, Inc., Boise, Idaho) enhances the per- manent marking of individual fish and reduces or eliminates biases in growth previously ascribed to dangler tags. Burdick and Hamman (1993) de- tected no difference in growth between PIT-tagged fish and non-PIT-tagged controls. Further, 98- 100% tag verification was realized 20-24 months after PIT tags were implanted in Colorado squaw- fish. 687