Determining conditions that promote razorback sucker recruitment in Lake Mead: a summary of the 2000-2002 pilot study

Golden, Michael; Holden, Paul

This report presents findings from the third year of study to determine if differences in cover, nutrients, and/or food can account for razorback sucker recruitment in certain areas of Lake Mead... EXECUTIVE SUMMARY Las Vegas Bay and Echo Bay in Lake Mead have small, recruiting 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, razorback sucker reproduction has been documented in 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 in 2000 and 2001. The Lake Mohave locations were chosen because they are known to have razorback sucker reproduction but no recruitment. During the first 2 years 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 in 2000. However, dropping lake levels in 200 I substantially reduced cover at Echo Bay and Las Vegas Bay. We also compared turbidity, another possible source of cover for young razorback sucker, between locations in 2001. Again Echo Bay and Las Vegas Bay had significantly higher levels of turbidity. We hypothesized that increased vegetative cover and/or turbidity may provide larval and juvenile razorback suckers with protection from predation by nonnative fishes. We felt that the long-term lake level fluctuations in Lake Mead are probably responsible for the increased cover and suggested continued studies to find links between environmental conditions and razorback sucker recruitment. In 2002 we continued to collect 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. The 2002 information generally reinforced what we had already surmised from the first 2 years of study: data from this 3-year pilot study indicate that while higher nutrient levels and increased food availability may help to increase razorback sucker recruitment at Las Vegas Bay, they are certainly not responsible for the recruitment seen at Echo Bay. Instead, a reduction in depredation of larval and juvenile razorback sucker, resulting from increased cover, seems to be the more probable mechanism allowing recruitment in these two locations. We recommend examining the water level records and any water quality measurements relating to turbidity for Lake Mead to attempt to determine what years were probably good cover years for razorback sucker larvae and juveniles. Comparing these data with our existing aging data, and continuing to collect and age adult razorback suckers from Las Vegas Bay and Echo Bay, should enable us to determine if larger numbers of razorback suckers have recruited in years where more cover was present. Compiling this evidence should solidify the link between cover and recruitment.

Determining conditions that promote razorback sucker recruitment in Lake Mead: a summary of the 2000-2002 pilot study

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