PRODUCTION AND POPULATION-DYNAMICS OF TUBIFEX-TUBIFEX IN THE PROFUNDAL ZONE OF A FRESH-WATER RESERVOIR IN NORTH ITALY

During a study of a pumped storage system from May 1979-June 1980 the profundal macrobenthos of the upper reservoir (Lago di S. Maria Valves tino) was sampled at a fixed station and the population of the tubific id Tubifex tubifex studied in detail. Eggs, embryos and the individual s living in an extra-cocoon stage were counted and individually weighe d from monthly samples, according to the methods described in Bonomi & Di Cola (1980). Numerical recruitment during the study period was est imated as 257000 ind m(-2) yr(-1); of which 110000 died either as eggs or as embryos, i.e. inside the cocoons, and a further 128000 died bef ore they attained sexual maturation. The data seem to confirm the typi cal demographic strategy of if tubifex i.e. high fecundity and high mo rtality in the early life stages. The total annual production of the s pecies was estimated at 91.7 g (w.w.) m(-2). The low P/B ratio (2.0 yr (-1)) is considered to be mainly due to high population densities.