Comparison of water quality, zooplankton density, and cover in razorback sucker (Xyrauchen texanus [Abbott]) spawning areas of Lake Mead and Lake Mohave

Abstract

Las Vegas Bay and Echo Bay in Lake Mead have small, self-sustaining populations of razorback sucker (Xyrauchen texanus [Abbot]). Increased productivity and cover have been hypothesized as reasons for successful recruitment of razorback sucker in Lake Mead. Conversely, reproduction has been documented on Lake Mohave, another lower Colorado River reservoir, but no recruitment has been observed. In 2000, BIO-WEST, Inc. was contracted by the Southern Nevada Water Authority to design and implement a study to examine nutrient levels, zooplankton density, and cover in areas with and without razorback sucker recruitment success. We sampled Echo Bay, Las Vegas Bay, and Trail Rapids Bay on Lake Mead, along with the Arizona Bay and Tequila Cove areas on Lake Mohave. The Lake Mohave locations were chosen because they are known to have razorback sucker reproduction, but no recruitment. During the first year of the study, we found that Las Vegas Bay had higher nutrient levels than all other locations. However, the amount of cover seemed to be the only factor distinguishing Las Vegas Bay and Echo Bay from the other three locations. We hypothesized that increased cover may provide larval and juvenile sucker with protection from predation by normative fishes. We suggested that the long-term lake level fluctuations in Lake Mead may be responsible for the increased cover and suggested continued studies to find links between environmental conditions and razorback sucker recruitment. In 2001 we collected information on water quality and nutrients, zooplankton density, and cover at Las Vegas Bay, Echo Bay, and Trail Rapids Bay in Lake Mead and the Arizona Bay and Tequila Cove areas of Lake Mohave. As in 2000, Las Vegas Bay had higher nutrient levels than all other locations. Overall, most sites had higher ammonium and phosphate levels in 2001. The 2000 results showed no real trends in zooplankton density, but in 2001 we found that Las Vegas Bay and Tequila Cove had a higher zooplankton density than the other locations. Zooplankton density was substantially higher at Las Vegas Bay, Tequila Cove, and Arizona Bay in 2001 versus 2000. Cover was substantially reduced at Echo Bay and Las Vegas Bay in 2001. In May 2001 no significant differences were seen in percent cover at any of the locations. Lake level lowered over 24 feet from March 2000 to May 2001, and left much of the submerged vegetation that provided cover in 2000 dry on shore. However, turbidity, which also provides cover, was significantly higher at Las Vegas Bay and Echo Bay than the other study locations. We recommend continuing and potentially expanding the study in future years. Cataloguing the conditions present in different years, under different lake elevations, and correlating them with the presence of a strong razorback sucker year class should identify what suite of factors is important in allowing razorback sucker recruitment in Lake Mead. Information on factors necessary for recruitment would assist in managing for the recovery of the species in the Lower Colorado River system