Michael Booth, Carolynn S. Culver

Since its discovery in Lake Mead in 2007, the invasive quagga mussel (Dreissena rostriformis bugensis) has rapidly spread throughout the lower Colorado River drainage. However, the invasion in California remained confined to the Colorado and water systems throughout southern California that receive water from the Colorado River until December 2013, when quagga mussels were first observed in Lake Piru, a reservoir in the Santa Clara River watershed of Ventura County, California. This paper describes population dynamics in Lake Piru and spread within the Santa Clara watershed over the first seven years since invasion. There was an initial boom period in the first year after colonization, with the highest densities and settlement rates between 2014 and 2017, a period of low and relatively stable lake water levels. Lake Piru’s temperature, oxygen, and water chemistry conditions were consistently within preferred bounds for quagga mussels. The mussel population initially expanded on hard substrates and eventually onto soft sediments as mussels attached to debris and each other. The peak settlement rate (June 2015, 1.9 x 105 mussels m-2 month-1) in Lake Piru was within the range observed in Lake Mead (1 x 104 – 1 x 106 mussels m-2 month-1) during the initial invasion, but typically settlement was 10-100 times lower than rates observed in Lake Mead. Despite these lower settling rates, within 2 years of detection (late 2015), mussels were regularly found on soft sediments. However, a fill event in 2017 dramatically increased the lake level and lead to substantial sediment deposition, which smothered the mussels on soft sediments and reduced the overall mussel population relative to the lake volume. Veligers (and settlement rates) were extremely low during the initial fill period, but rebounded within 6 months, despite relatively low levels of reproductive mussels observed in the lake. Large mussels were rare in 2018-2019, likely due to increased mortality during 2017, and the size distribution remained biased towards small size classes into 2021. Environmental conditions (particularly fill status and water temperature), rather than adult density, appear to be the primary driver of veliger abundance in this system, while recruitment was primarily explained by veliger abundance. While lake level fluctuations have potential to control the mussel population in the lake to some extent, lowering lake levels requires extended periods of elevated base flow as well as out-of-season water releases. These elevated water releases increase the flux of veligers into the creek and lead to downstream recruitment, particularly when veliger abundance is high. While seems relatively unlikely that quaggas will establish and reproduce in the Santa Clara River itself due to the unstable bed, highly fluctuating flows, and lack of lentic habitat needed for reproduction, periodic colonization within the river occurs, and is likely to lead to infestations in water systems that directly take in raw river water.