Large-scale synchronization of annual recruitment success and stock size in Wadden Sea populations of the mussel Mytilus edulis L.

Abstract

Simultaneous abundance or shortage of mussels over vast areas may seriously affect fishery as well as shellfish-eating birds. We studied synchrony in annual recruit numbers and stock sizes (biomass) of mussels on the base of long-term observations in various parts of the Dutch (and German) Wadden Sea, including regular monitoring on Balgzand (a 50-km2 tidal flat area) and published or unpublished records for other parts of the Wadden Sea. Annual records for 37 years of mussel seed abundance in the eastern and western half of the Dutch Wadden Sea proved to be mutually well correlated and were also significantly correlated with annually assessed numerical densities of mussel recruits on Balgzand. The scarce long-term series available on mussel biomass pointed to significantly positive correlations between stock sizes on Balgzand and those in the northern German Wadden Sea, at about 300 km distance. The incidence of severe winters, which occurrence is synchronized over areas in the order of thousands of km, is identified as the dominant causative factor behind Wadden Sea-wide recruitment synchrony. Severe winters are known to reduce abundance of predators on tiny bivalve spat, and this process may overrule local processes causing abundance variation in bivalves. As such extreme winters are infrequent (usually only one or two per decade), sensible studies on the phenomenon of synchronization in abundance of Wadden Sea bivalves should be based on data series of sufficient length, covering decades.