Quagga mussel (Dreissena rostriformis bugensis) population structure during the early invasion of Lakes Mead and Mohave January-March 2007

Shell length-frequencies were determined for samples of Dreissena rostriformis bugensis collected <2 m depth from Las Vegas Boat Harbor (LVBH), Callville Bay Marina (two samples, CBM-1 and CBM-2), and Lake Mead Marina (LMM) in the Boulder Basin of Lake Mead and Katherine Landing Marina (KLM) in the lower end of Lake Mohave within two months of initial discovery of D. rostriformis bugensis in Lake Mead at LVBH on 06/01/2007. Two annual shell length (SL) cohorts were present at all four sites consisting of individuals presumed to have settled during spring-fall of 2005 and 2006, respectively. Members of the most recently settled 2006 cohort numerically dominated all samples; relative ratios of individuals in the LVBH, CBM-2, LMM, and KLM 2006 versus 2005 cohorts being 174:1, 95:1, 172:1 and 65:1, respectively. Mean shell lengths of the LVBH, CBM-1, CBM-2, KLM and LMM 2006 cohorts were 6.86 (s.e. = ±0.126), 9.59 (±0.080), 9.31 (±0.194), 10.49 (±0.287) and 12.15 mm (±0.105), respectively. The mean shell lengths of the 2005 cohorts in the LVBH, CBM-2, KLM and LMM samples were 17.5 mm (n = 1), 18.9 mm (n = 4), 21.2 mm (n = 1) and 20.1 mm (n = 4), respectively. The occurrence of the 2005 cohort in all four sampled sites suggested that quagga mussel introduction to the Boulder Basin occurred prior to 2005. This supposition is based on the very low probability of individuals from a localized initial introduction appearing in the samples. It is hypothesized that a reproducing quagga mussel population must have been established somewhere in the Boulder Basin by 2003 or 2004 in order to generate enough veliger larvae to settle as relatively wide-spread members of the 2005 cohort and be transported downstream to establish a 2005 cohort at KLM in lower Lake Mohave. Essentially equivalent population structures at all three sampled sites makes speculation on the original site of quagga mussel introduction to the Boulder Basin difficult.