Decline of recruitment success in cockles and other bivalves in the Wadden Sea: possible role of climate change, predation on postlarvae and fisheries

In the last similar to 15 yr, frequent recruitment failures in the main bivalve species in the western Wadden Sea led to insufficient food supply for some specialised bird species, sharpening conflicts between nature conservationists and fishermen. To study possible causes of the recent recruitment failure in bivalves, we compare long-term data sets (1973 to 2002) of annual abundance of spat of 3 of the most important species of bivalves (cockle Cerastoderma edule, gaper clam Mya arenaria, and Baltic tellin Macoma balthica) on Balgzand, a tidal-flat area in the westernmost part of the Wadden Sea. In the 3 species, recruitment success declined significantly over the period of observation, particularly at offshore sampling sites which were characterized by low intertidal levels and sandy sediments. In these areas, we found high biomass values of a predator of bivalve postlarvae, the shrimp Crangon crangon. In each of the 3 bivalve species, annual recruitment (estimated as numerical density of spat in August) in these areas was negatively related to shrimp biomass at the time of settlement of postlarvae (May/June). Shrimp biomass has increased over the last similar to 30 yr. High near-shore flats showed invariably low shrimp biomass values and appear to serve as a refuge for postlarval bivalves in years of high predation pressure. Only in this coastal part of Balgzand was no decline in bivalve recruitment found; in fact, cockle recruitment even increased. Alternative explanations for the observed changes in recruitment of bivalves are discussed, including changes in sediment composition and bottom-disturbing fishing for cockles, mussels and lugworms. It is concluded that the recruitment trends on Balgzand (and other parts of the Wadden Sea) are governed primarily by natural processes, in particular increases in predation pressure on early benthic stages, which in turn appears to be largely governed by the warming climate.