The separate and combined effects of epibenthic predation and presence of macro-infauna on the recruitment success of bivalves in shallow soft-bottom areas on the Swedish west coast

Abstract

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.