Abundance and stomach content analysis of threadfin shad in Lake Mead, Nevada: Do invasive quagga mussels affect this prey species?

Lake Mead, Nevada is the largest reservoir by volume in the United States, as well as a popular sport fishing destination. In January 2007, the invasive quagga mussel Dreissena rostriformis bugensis (Andrusov, 1897) was discovered in the reservoir and concerns began to arise about potential alterations to the aquatic environment. The Lake Mead sport fishery is reliant on the sustainability of prey species like the threadfin shad [Dorosoma petenense (Gunther, 1867)]. This study examined 20-year trends in historic abundance of the threadfin shad, before, and shortly after, the discovery of quagga mussels in the system. Larval shad trawl data collected in Las Vegas Bay and Overton Arm portions of Lake Mead were analyzed in the present study. Two-way analysis of variances showed that the abundance of this prey fish has not changed following quagga mussel invasion (P > 0.05). Post-quagga mussel discovery collections of adult threadfin shad from Las Vegas Bay ranged from 113 to 212 mm total length (TL) (mean = 184 mm; n = 170). Shad from Overton Arm ranged from 131 to 197 mm TL (mean = 150 mm; n = 27). Stomach contents were analyzed. The proportion of in stomach contents differed significantly from spring to winter in Las Vegas Bay (P = 0.008); whereas the proportion of copepods did not show statistically relevant differences regarding seasonality (P = 0.562). Initial trends in lower trophic level dynamics in response to quagga mussel invasion has yet to reveal significant effects in Las Vegas Bay or Overton Arm. Long-term monitoring on threadfin shad is needed to evaluate potential impacts from invasive quagga mussels in Lake Mead.