THE LIFE-CYCLE, POPULATION-DYNAMICS, GROWTH AND SECONDARY PRODUCTION OF THE SNAIL VIVIPARUS-CONTECTUS (MILLET) (GASTROPODA, PROSOBRANCHIA) IN THE MARSHES OF THE RIVER STRYMONAS, SERRES, MACEDONIA, NORTHERN GREECE

The life cycle, population dynamics, growth and secondary production of the prosobranch freshwater snail Viviparus contectus were studied in the marshes of Strymonas River upstream of its entry into the artificial Lake Kerkini, Serres, Macedonia, northern Greece. Demographic analysis of the population of V. contectus revealed that two or more generations existed in the field throughout the year. The sex ratio was 1:1. Reproduction took place in the beginning of spring or at the end of autumn, depending on the water level and water temperature in the study area. Growth of newly born individuals mainly took place during spring and early summer. Von Berialanffy's method suggested that V. contectus may live up to five years to reach its maximum possible size (51 mm). Annual secondary production, calculated by Hynes' size frequency method, revealed a mean annual density of three individuals/m(2), a mean standing crop ((B) over bar) of 4.19 g dry body weight/(m(2).year), and an annual production (P) of 13.45 +/- 12.9 g dry body weigh/(m(2).year). Annual turnover ratio (P/(B) over bar) was 3.21 and turnover time was 113.7 days.