Do Endangered Razorback Suckers Have Poor Larval Escape Performance Relative to Introduced Rainbow Trout?

—Poor recruitment has generated the hypothesis that the endangered razorback sucker Xyrauchen texanus is particularly vulnerable to predation early in its life history. We compared the escape responses of razorback suckers, which are adapted to the historically warm waters of the Colorado River, with those of rainbow trout Oncorhynchus mykiss, an introduced coldwater species, throughout early development at water temperatures of 128C and 188C. We quantified escape performance using maximum velocity, acceleration, and time to maximum velocity and acceleration. Both species showed complete temperature compensation for escape performance; individuals reared at 128C performed as well as those reared at 188C. Performance was also similar between species, although two variables exhibited a species 3 size interaction. Small razorback suckers were faster (greater maximum velocity and acceleration) than small rainbow trout, while large larvae performed similarly. We also determined that razorback sucker larval escape performance falls within the range reported for other fishes. Therefore, we conclude that razorback suckers do not have ‘‘poor’’ escape performance and that temperature does not directly cause decreased performance. However, a cold temperature reduces growth rates and delays razorback suckers’ attainment of a ‘‘predator-proof’’ size. Small larvae are also more likely to perform uncoordinated, ineffective escape responses. Hence, razorback sucker performance is indirectly diminished by temperature. The endangered razorback sucker Xyrauchen texanus is endemic to the Colorado River system (Tyus et al. 1982; Minckley 1983). Once abundant and widely distributed, the razorback sucker is now limited to scattered populations in the upper and lower Colorado River basin (Minckley et al. 1991). The remaining wild populations of razorback suckers appear to be relict subpopulations of very old individuals, ones that are 30 years old or older (Minckley 1983; McCarthy 1986; McCarthy and Minckley 1987). These populations are reproducing in the wild (Bozek 1984; Marsh and Langhorst 1988; Mueller 1989), but larval recruitment appears to be severely limited (Minckley 1983; Marsh and Minckley 1989). The proximate cause of limited recruitment remains unknown, but predation on razorback sucker early life history stages by introduced fishes has been proposed as the single most important factor (Minckley et al. 1991). In a series of experiments in the Lake Mohave portion of the Colorado River, razorback sucker larvae showed high survival rates (80%) when predators were excluded (Minckley et al. 1991). The ability of larvae to persist and grow when predators are absent supports the hypothesis that predation is the major * Corresponding author: [email protected] Received April 19, 2002; accepted April 29, 2003 factor responsible for the limited recruitment of the razorback sucker, although other factors (habitat loss, physical changes to the environment, etc.) certainly will also affect survival. Historically, the Colorado River supported a limited number of piscivorous fish (Johnson et al. 1993), only one fish (the endemic Colorado pikeminnow Ptychocheilus lucius) being known to prey on adult razorback suckers (Minckley et al. 1991). As such, selection for escape performance may have been weak. Currently, at least five introduced fish predators are known to prey on the razorback sucker: striped bass Morone saxatilis, largemouth bass Micropterus salmoides, channel catfish Ictalurus punctatus, flathead catfish Pylodictis olivaris, and green sunfish Lepomis cyanellus (Marsh and Brooks 1989; Minckley et al. 1991; Johnson et al. 1993). The introduction of these and other exotic fish species has drastically changed the predation rates experienced by the razorback sucker in the