Razorback Sucker- EP

CRE AT-RISK SPECIES VIGNETTES – Larry Stevens

• Razorback Sucker

*The razorback sucker [RBS; Xyrauchen texanus (Abbott, 1860)] was listed as a federally endangered species in 1991. It is a large, mainstream catastomid species that ranged widely through the Colorado River basin in pre-dam time. This species grows to more than 2 ft in length and may live more than 40 yr. Adult RBS bear a pronounced dorsal keel. Like humpback chub, the sucker’s dorsal keel has been believed to stabilize navigation in swift, turbulent waters, but experimental studies fail to show such a benefit and suggest the elevated dorsal keel reduced the likelihood of predation by Colorado pikeminnow (Ptychocheilus lucius Girard 1856; Portz and Tyus 2004). RBS feed on biofilm, detritus, and associated aquatic invertebrates on firm benthic surfaces. RBS life history studies in the upper Colorado River Basin revealed that spawning occurs in April and May over sand and gravel substrates in waters ranging from 50-64° F (10-18oC). Males establish breeding territories in shallow water and defend their breeding areas with downward eye rolls, which flash UV light. Females are attended by 2-12 males and deposit 24,500-76,600 eggs/yr, which adhere to pea gravels. Eggs will not hatch in water less than 50°F, (10°C). Radio-tagging studies show that RBS prefer flat-water reaches of rivers (Tyus 1987). RBS in the Colorado River were larger than those in the Yampa/Green drainage, presumably because of warmer water temperatures in the mainstream Colorado River (Joseph et al. 1977). *RBS were reported from the lower Paria River in pre-dam time, but the last RBS known from central Grand Canyon was captured in the early 1980’s near the Little Colorado River confluence. Population declines have been attributed to reproduction failure due to impoundment-related water temperature changes and predation by non-native fish species. Even though RBS is easily propagated, and is consistently nominated as the top candidate for recovery, more than 25 yr of symposia and discussion about that topic have failed to result in its reintroduction into Grand Canyon. Old, large individuals still exist in Lakes Mohave and Mead, and 2012 fish electrofishing monitoring in lowermost Grand Canyon revealed a few RBS below Mile 240 in Grand Canyon; however, natural reproduction remains low to non-existent.