Razorback sucker studies on Lake Mead, Nevada: 2000-2001 annual report

Holden, Paul; Abate, Paul; Welker, Tim

INTRODUCTION The razorback sucker (Xyrauchen texanus [Abbott]) is an endemic fish species of the Colorado River Basin. It was historically widespread and common throughout the larger rivers of the Colorado River Basin (Minckley et a!. 1991 ). The distribution and abundance of the razorback sucker are greatly reduced from historic levels, and it is one of four large-river fishes endemic to the basin that is presently considered endangered by the U.S. Department of the Interior (USFWS 1991 ). One of the major factors causing the decline of razorback sucker and other large-river fishes has been the construction of main stem dams and the resultant cool tail waters and reservoir habitats that replaced a warm, riverine environment (Holden and Stalnaker 1975, Joseph eta!. 1977, Wick et a!. 1982, Minckley et a!. 1991 ). Competition and predation from non-native fishes that have successfully established in the Colorado River and its reservoirs have also contributed to their decline. Razorback sucker persisted in several of the reservoirs that were constructed in the lower Colorado River Basin; however, these populations were comprised primarily of adult fishes that apparently recruited during the first few years of reservoir formation. The population oflong-lived adults then would disappear 40 to 50 years following reservoir creation and the initial recruitment period (Minckley 1983). The largest reservoir population, estimated at 75,000 in the 1980s, occurred in Lake Mohave, Arizona and Nevada, but it had declined to about 9,000 by 1999 (P. Marsh, Arizona State University, personal communication). Adult razorback sucker are most evident in Lake Mohave from January through April when they congregate in shallow shoreline areas to spawn, and larvae can be very numerous soon after hatching. Predation by bass, carp, catfish, sunfish, and other non-native species appears to be the major reason for the lack of razorback sucker recruitment (Minckley et a!. 1991 ). Lake Mead appeared to follow the trend of other lower Colorado River Basin reservoirs, with recruitment to these populations occurring only near the time of reservoir creation. Lake Mead was formed in 1935 when Hoover Dam was closed, and razorback sucker were relatively common lakewide throughout the 1950s and 1960s. Their numbers became noticeably reduced in the 1970s, approximately 40 years after closure of the dam (Minckley 1973, McCall 1980, Minckley eta!. 1991, Holden 1994, Sjoberg 1995). From 1980 through 1989, neither the Nevada Division of Wildlife (NDOW) nor the Arizona Game and Fish Department collected razorback sucker from Lake Mead (Sjoberg 1995). This may be in part because of changes in their lake sampling programs, but it was a considerable difference from the 30-plus razorback sucker collected during sportfish surveys in the 1970s. These results are not surprising and fit well within the pattern of razorback sucker dying off 40 years or so following reservoir development, as was seen in other lower Colorado River Basin reservoirs.

Razorback sucker studies on Lake Mead, Nevada: 2000-2001 annual report

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