WESP, H. M.; GIBB, A. C.*: Do endangered razorback suckers have poor larval escape performance relative to introduced cold-water fishes?

The escape responses of the razorback sucker, Xyrauchen texanus, and the rainbow trout, Oncorhynchus mykiss, were studied from the post-hatching larval stage to the early juvenile stage. Both species were reared and tested at 18 and 12�C to emulate historic and current Colorado River temperatures. Escape performance was quantified using twenty-four variables, including velocity, acceleration, and length-specific curvature. Temperature did not significantly affect escape performance in either species; both species showed complete compensation when acclimated to the colder temperature. There were surprisingly few differences in escape response performance between species. Only two variables, maximum size-specific velocity and maximum length-specific curvature at 50% body length (during stage 1), were statistically different. In both instances, razorback sucker performance was slightly better. However, there were significant interactions between performance and body size when the two species were compared. Small razorback suckers have higher swimming velocities and accelerations than small rainbow trout, but large razorback suckers and large rainbow trout have similar velocities and accelerations. Both razorback suckers and rainbow trout achieve maximum velocities that are similar to those reported for larvae of other species. Therefore, escape performance of the razorback sucker is not poor relative to other fishes. However, cold temperatures delay developmental processes and cause larvae to grow slowly. Smaller larvae achieve lower velocities and are more likely to perform uncoordinated responses. Thus, the current temperature of the Colorado River may indirectly diminish razorback sucker escape performance and contribute to poor survivorship.