Previous studies have shown a high year-to-year variability in recruitment success of bivalves. Especially after mild winters recruitment was generally low, but the lower egg production after mild winters in Macoma balthica could only explain 7% of its recruitment variance. In the present study, I tested the hypothesis that the combined effect of a high predation pressure together with a high abundance of adult macrofauna contributes to an explanation of the low recruitment success of bivalves after mild winters. In field experiments in shallow soft-bottom bays at the Swedish west coast, adult benthic fauna (mainly consisting of lugworms Arenicola marina and cockles Cerastoderma edule) was removed and predators (mainly shrimps Crangon crangon and crabs Carcinus maenas) were excluded in some plots/cages, whereas in other plots/cages high densities of adult lugworms or cockles and predators were present. Both the absence of adult macrofauna and the absence of predators increased recruitment success, but the effect of the combined absence of adult macrofauna and predators enhanced recruitment success even more. The combined presence of high macrofauna densities and high predation pressure reduced the recruitment success for Mya arenaria by about 80% compared to the situation in which adult macrofauna and predators were absent. For C. edule, spat densities the reduction was nearly 90% and for Tellinacea spat even about 95%.
Thus failure in recruitment success after a mild winter can to a large extent be explained by the presence of high densities of adult macrofauna combined with high predation pressure.
During a now 30-year-running monitoring program of the zoobenthos in the westernmost part of the Dutch Wadden Sea, an enormous year-to-year varia- bility was observed in annual recruitment of particularly the bivalves (Beukema et al., 2001). A strong correlation was found between winter temperature and the recruitment success of bivalves (Beukema, 1992). A succession of recruitment failures was observed in a number of bivalve species after series of mild winters and this had a strong impact on the ecosystem(Beukema and Cadée, 1996). In the 4 years 1997– 2000 the winters were extremely mild, not only in Holland but also in Sweden, perhaps as a result of global warming (Alexandersson, 2000). It is therefore important to know what causes the recruitment failure of bivalves after mild winters and what are the expected consequences of declines in the bivalve stocks.