Growth and mortality rates of the zebra mussel, Dreissena polymorpha, in the Lower Mississippi River

We investigated seasonal growth and mortality of a population of zebra mussels, Dreissena polymorpha, near the extreme southern limit of their current North American distribution. Dreissena in the Lower Mississippi River experience seasonal regimes of temperature and suspended sediment that are distinctly different from those found in northern lentic environments. Shell growth and mortality were determined biweekly using marked animals in caged environments. Tissue condition was also estimated biweekly using mussels from caged and wild environments. Our results demonstrate that, although annual growth is comparable with that reported for more temperate environments, the seasonal pattern of increase is different. Shell growth and tissue condition were highest during the early spring. Tissue condition declined and shell growth;th ceased during the summer, when temperatures remained at 29-30 degrees C for 3 months. This midsummer depression was accompanied by increased mortality, particularly for mussels greater than 15 mm. Condition improved for all sizes in fall and winter. These patterns of growth and mortality appear to be primarily driven by fluctuations in seasonal temperature and spawning condition with some influence of suspended sediment. Our observations indicate that periodic summer mortality events will occur, and the extent of this mortality will depend primarily on temperature, population size structure, and spring tissue condition.