INCREASED ABUNDANCE AND GROWTH OF THE SUSPENSION-FEEDING BIVALVE CORBULA-GIBBA IN A SHALLOW PART OF THE EUTROPHIC LIMFJORD, DENMARK

The abundance, growth and production of Corbula gibba have been studied by intensive sampling over a 2-year period in a shallow part of the eutrophied Limfjord (Denmark). Size-frequency distributions and growth marks on the shells were used to follow successive cohorts over time. The abundance of Corbula gibba was high, the highest density found in 1-mm sieve samples (almost-equal-to 53 000) being one order of magnitude higher than previously recorded. The analysis of samples sieved through 250-mu-m sieves revealed that settling took place in August. The mortality of newly settled individuals was high, followed by low and constant mortality during the following winter. The growth of C. gibba was fast compared with records from the Irish Sea and the Sound and the specimens reached a mean size of 6 to 7 mm one year after settlement. It is suggested that the between-year-variation in growth rate was caused by variable frequencies of wind-induced resuspension of settled organic matter. The production of C. gibba varied between stations but was generally high (mean: 26.8 g AFDW.y-1.m-2) compared with previous records from subtidal areas. The P/B ratio is among the highest recorded (mean: 4.2 y-1). Growth seems to be higher at present than during the first part of this century, probably due to the increased eutrophication of the Limfjord. It is suggested that growth rate may be a good complement to the use of total abundance and biomass in monitoring, as the latter parameters are more variable due to stochastic events such as sampling variance and inter- seasonal variations in settlement success caused by physical factors.