Settlement, density, survival and shell growth of zebra mussels, dreissena polymorpha, in a recently invaded low latitude, warm water texas reservoir

Abstract

Zebra mussels, Dreissena polymorpha, introduced to US freshwaters in 1986, have expanded into at least 19 Texas reservoir lakes since 2009. In Texas, they occupy the lowest latitudes in their North American and European range where summer surface water temperatures can exceed their upper thermal limit of 30 °C. Little is known of its population dynamics in such warm-water bodies, knowledge of which will be important for development/implementation of effective mussel management strategies. Zebra mussels were first discovered in Belton Lake, central Texas, in 2013 and developed a dense population by 2014. Belton Lake mussel settlement dynamics were studied from March 2016 to May 2017 on steel chains vertically suspended from a floating marina dock. Three chains were deployed at bimonthly intervals with mussel densities recorded monthly thereafter at each meter of depth until experiment termination. Shell growth rates and densities were also determined for emersed mussels at various sites in the reservoir after a 52-day period of high water. Spring cohort settlement occurred from June to September 2016 and fall cohort settlement from November 2016 to May 2017. Peak spring and fall cohort settlement densities were 1,025 mussels m-2 on July 2016 and 175 m-2 on February 2017, respectfully. Bottom to surface hypoxia from August (51.58%–71.14% DO) to October 2016 (49.04%–52.13% DO) extirpated the spring cohort by September 2016 and delayed fall cohort settlement until December 2016. Mean mussel settlement density increased with distance downstream from 0 mussels m-2 in the reservoir’s shallow, inlet end to 21,160 mussels m-2 at its deeper wider outlet end suggesting that mussel densities tend to be greatest in deeper, wider, low-flow, down-stream ends of impounded water bodies. Mean mussel shell growth rates across six sites were 127.9 μm day-1, one of the fastest recorded for mussels in Europe and North America. Two annual reproductive periods, rapid growth rates and early maturity are likely to lead to zebra mussel population expansion to problematic sizes within 1–2 years after introduction into warm southwestern water bodies, leaving little time for development/implementation of effective mussel macrofouling control/mitigation strategies. Thus, development of effective macrofouling management/mitigation plans should be undertaken prior to establishment of mussels in a water body.