Status and Trends of the Endangered Colorado Squawfish in the Upper Colorado River

Abstract

Status of the Colorado River population of the endangered Colorado squawfish Ptychocheilus lucius (recently renamed the Colorado pikeminnow) was investigated by (1) 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-0 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-0 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.