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i q q ~ Os )iVt tA /1 d SO /1 tWcti t5 IAAri1.h M11 Transactions of the American Fisheries Society 127:957-970, 1998 @ Copyright by the American Fisheries Society 1998 'tIft; ;; Status and Trends of the Endangered Colorado Squawfish in the Upper Colorado River ~ " D. B. OSMUNDSON* ~ u.s. Fish and Wildlife Service, Colorado River Fishery Project 764 Horizon Drive, South Annex A, Grand Junction, Colorado 81503-3946, USA K. P. BURNHAM Colorado State University, Colorado Cooperative Fish and Wildlife Research Unit 201 Wagar Building, Fort Collins, Colorado 80523, USA Abstract.-Status of the Colorado River population of the endangered Colorado squawfish Ptych- ocheilus lucius (recently renamed the Colorado pikeminnow) was investigated by (I) estimating adult numbers, (2) evaluating frequency of reproduction and recruitment, (3) identifying trends via changes in size structure over time, and (4) examining historical accounts for clues to former abundance. Adults and subadults were systematically captured from 278 km of river during 1991- 1994. Larvae and age-O fish were systematically sampled in two reaches during 1986-1994. Estimated number of adults in the upper 98 km averaged 253 individuals; estimated annual adult survival rate was 0.86. In the lower 181 km, estimates of subadults and adults combined averaged 344 individuals. A sizable pulse of subadults 300-400 mm long found in the lower reach in 1991 were from three year-classes, 1985-1987. By 1992, these were distributed throughout the river. Although catch rates of larger adults did not increase significantly in the upper reach during 1991- 1994, catch rates of fish less than 550 mm long increased fivefold. Size-frequency analysis of lower-reach fish indicated the 1985-1987 cohorts were the largest produced since before 1977, and no similarly strong year-classes were produced subsequently. Estimated years of origin of these recruiting fish coincided with years of higher-than-average catch rates of larvae and age-O fish in the upper reach and catch rates in subsequent years there were comparatively low. Very few individuals less than 450 mm long were found in the upper reach during the past 15 years, suggesting that recruitment there is from colonization from the lower reach. In contrast, significant numbers of fish less than 400 mm in total length occurred in the upper reach during the mid-1970s. Abundance appears much lower than suggested in historical accounts. Low adult numbers and sporadic pulses of recruitment may make this population vulnerable to extirpation. Though adult survival rate is probably fairly constant, recruitment is highly variable and may represent the most important demographic factor to population persistence. Introduction Colorado squawfish Ptychocheilus lucius (re- cently renamed Colorado pikeminnow; Nelson et al. 1998) were historically distributed throughout warmwater reaches of the Colorado River basin from Wyoming, Utah, and Colorado south to the Gulf of California (Miller 1961). By the 1970s they were extirpated from the Colorado River ba- sin below Glen Canyon Dam (entire lower basin) and from sections of the upper basin as a result of major alterations to the riverine environment (Moyle 1976). Having lost some 80% of its former range, the Colorado squawfish was federally listed as an endangered species in 1967 (U.S. Office of the Federal Register 32:43[1967]:400l). Colorado squawfish are presently restricted to the upper Colorado River basin and inhabit warm- water reaches of the Colorado, Green, and San * Corresponding author: doug_osmundson@fws.gov Juan rivers and associated tributaries. The Green River and its two large tributaries (White and Yam- pa rivers) support the largest (Tyus 1991) and per- haps most viable (Gilpin 1993) population, where- as the San Juan River contains the smallest pop- ulation (Platania et al. 1991). A third population persists in the upper Colorado River, but relatively low catch rates of adults and young (Valdez et al. 1982; Osmundson and Kaeding 1989; McAda et a1. 1994) suggest this population may have limited viability. Because of the endangered status of this fish, assessing persistence potential of these small- er populations is important in assessing overall species viability. We investigated the status of the upper Colorado River population by assessing current demograph- ics and relating this to past and future demographic trends. Analyses included (1) estimating the size of the adult population, (2) determining the extent or frequency of reproduction and recruitment, (3) 957