9586 - Laserfiche WebLink

Home Browse Search

PROCEEDINGS OF THE DESERT FISHES COUNCIL, 1994 SYMPOSIUM concentrated in the Overton Arm and Gregg's Basin/Grand Wash portions of the lake. An additional eight records are from the Temple Bar area immediately adjacent to the lower Gregg's Basin. These areas, along with Las Vegas Wash, represent inflow areas and tend to be more productive than the relatively sterile central Virgin and Boulder basins. Sampling efforts represent a broad base of sites, times and methods throughout most areas of the reservoir. Thirty of 44 documented NDOW contacts occurred in the November to May period when razorbacks are most likely to be found in shallow water littoral zones. All of the NDOW records outside of that winter/spring time frame occurred near or above Temple Bar. No records of collections or contact with razorback sucker could be found in available agency field records between 1980 and 1990. Extensive gill and trammel netting was conducted on Overton Arm during the 1986-1988 period by the University of Nevada, Las Vegas, in conjunction with the cooperative nutrient enhancement project, including sites where historic collections of adult razorbacks had occurred. No records of razorback sucker captures were found in field data files from those efforts. The lack of observations or contacts incidental to other routine sport fish sampling activities indicates a probable reduction in occurrence when compared to the more frequent observations through the early 1970's. However, changes in agency sampling techniques in the 1980's, such as use of mid-water and meter-net trawling and decreased use of conventional net surveys, may have significantly decreased the opportunity to contact or observe adult razorback suckers for much of this period. Current dedicated Lake Mead razorback survey efforts began in April 1990 when NDOW personnel observed an apparent spawning aggregation of approximately 15 adult suckers in the Blackbird Point area of Las Vegas Wash. Three adult razorbacks were collected using electrofishing equipment and apparent spawning behavior was observed. Specific sampling efforts were not conducted in 1991, although a single adult razorback sucker was captured at Bark Bay on Overton Arm in March during spring gill net surveys. Intensive trammel net surveys targeted specifically at razorback suckers in known areas of occurrence were initiated in spring 1992. Efforts were concentrated at Blackbird Point, near Las Vegas Wash, and Echo Bay on Overton Arm, based on 1990 collections and reports of recent observations. A total of 25 adult razorback suckers were captured at the two locations, 12 at Blackbird Point and 13 at Echo Bay. Efforts for 1993 were expanded to include historic collection locations and other sites thought to have characteristics of preferred razorback spawning habitat, primarily the presence of well-washed coarse gravel or cobble substrate and exposure to limited wave action to reduce siltation. A total of 92 trammel VOLUME XXVI - published 1995 net nights of survey were conducted at 21 sites in all four primary basins. The majority of sets were placed in the January through April period. A total of 21 adult razorback suckers were captured during 1993, all at the Blackbird Point (11) and Echo Bay (10) sites. All captures occurred in trammel net sets during the months of February and April. In 1994 a total of 113 trammel net nights of sampling effort were conducted at 40 sites lakewide. A total of five razorback suckers were captured, all at the Blackbird Point site. Adult razorback suckers captured in Lake Mead since 1990, with only two exceptions, have been marked to facilitate identification in the event of recapture. Initially five fish were tagged using Floy brand T-lock anchor tags. Beginning in March 1992 all adult fish were tagged using PIT tags provided by the Bureau of Reclamation. Through 1994 a total of 42 adult fish were marked by NDOW personnel using PIT tags. A total of six recaptures of PIT tagged fish were documented in 1993, from fish tagged in 1992 or 1993, three at Blackbird Point and three at the Echo Bay site. Observations of the historic population of razorback suckers in Lake Mead appear to closely follow predicted trends for captive populations in lower basin mainstem reservoirs. Jonez and Sumner (1954) reported razorbacks as abundant and widespread throughout the lake with particular seasonal occurrence in inflow and potential spawning areas. In the early 1950's the impoundment was 15 to 20 years old and a logical assumption is that entrapment of resident adult fish from dam closure or a high level of survival and recruitment of juvenile fish from spawning at the time of impoundment would result in retention and observation of relatively large numbers of older-aged adult fish at the time of their studies. Jonez and Sumner specifically mentioned observations of fish in the 290 mm to 620 mm or larger size range, and that at no time were fish smaller than 290 mm observed in Lake Mead. Although their length estimates appear to be based only on visual observations, and age estimation of adult razorbacks based on length measurements is highly subjective, it is probable that most of the adult fish observed by Jonez and Sumner dated to a period at or near the time of reservoir filling. Their lack of observations of smaller adult fish supports the presumption that significant recruitment to the population was not occurring during this period. Both anecdotal and documented observations throughout the historic period indicate that adult fish were broadly distributed throughout the reservoir, including upper lake areas to Paiute Point and throughout the Overton Arm. A decline in anecdotal observations of adult fish after the mid-1960's correlates with the predicted longevity of the species and indicates a probable loss of the original cohort of adult fish to old age in the 1979s and 1980's. 25