Lake Mead razorback sucker recruitment: An informative anomaly regarding continued, natural, wild razorback sucker recruitment despite nonnative fish presence

Authors: Brandon Albrecht, Ronald B. Kegerries, Paul Holden

Year: 2008 (xl)

Abstract

An ongoing razorback sucker (Xyrauchen texanus) research project on Lake Mead, Arizona and Nevada, has been funded by the Southern Nevada Water Authority and the U.S. Bureau of Reclamation for 12 years. This study continues to document the continued presence of actual, wild razorback sucker recruitment in the form of young, sexually immature individuals. This recruitment denotes the Lake Mead razorback sucker population as an anomaly in terms of razorback sucker persistence throughout the Colorado River drainage, despite similar non-native fish composition and densities as other locations. Fin ray aging data and back-calculation techniques have indicated that recruitment of razorback sucker on Lake Mead has occurred nearly every year. Furthermore, data collected have indicated that high lake elevations - those typically associated with maximum amounts of inundated terrestrial vegetation- appear to be responsible for pulses in recruitment. However, beginning with the 2007 spawning period, we have captured large numbers of juvenile/subadult, and adult razorback suckers that, based on back-calculation techniques, were spawned under low and declining lake elevations. In fact, the largest number of recruits observed to date now coincides with 2002, a low-water year which has produced 24 recruits thus far and has prompted a need to revisit our hypothesis regarding factors driving recruitment in Lake Mead. We believe that cover - both vegetative and in the form of turbidity - provides protection and food resources for larval and juvenile razorback sucker, thereby enabling them to avoid predation by nonnative sportfish present in the system. Interestingly, it appears as though turbidity (another form of cover) may be even more important that we have typically considered to date. As monitoring efforts continue, we would expect to begin capturing individuals spawned during 2007, 2008, and beyond. Continued monitoring efforts on Lake Mead should help ascertain if recruitment events continue, and perhaps begin to help understand more fully how to enable this unique trend in other locations. We suggest that Lake Mead provides a look at what naturally recruiting razorback sucker populations look like in the real world of nonnative predators and we suggest potential documentation of how functioning spawning aggregates are formed.