Long-term variability in bivalve recruitment, mortality, and growth and their contribution to fluctuations in food stocks of shellfish-eating birds

Bivalve biomass in coastal marine areas tends to fluctuate heavily from year to year, but the relative impact of underlying processes (such as recruitment, mortality, and growth) on this variability is poorly known. The present study deals with 3 bivalve species that live on tidal flats and are readily exploitable as food for shellfish-eating birds, which have suffered in recent years from short food supply in the Wadden Sea. Long-term (similar to 40 yr) monitoring of bivalve populations on tidal flats in the western Wadden Sea has revealed high year-to-year variability in numerical densities, whereas variability in growth rates and resulting mean individual weights-at-age was relatively low. High biomass values were found in particular when strong cohorts had arisen from successful recruitment in preceding years, whereas failing recruitment for some years in succession led to low biomass of bivalve stocks. Occasionally, temporarily enhanced mortality rates resulted in temporarily strongly reduced biomass values. Only in Macoma balthica were biomass values (at similar foregoing recruitment and mortality levels) lower in years with below-average individual weights than after years with above-average ones. In all 3 species, processes that govern numbers rather than individual weights were most decisive for biomass variability. Recruitment and mortality rates in only 3 species together explained a significant proportion of the variability of the total (multi-species) bird-accessible bivalve biomass. The low bird-food supply in recent years in the Wadden Sea appears to be due to a shift in the major structuring factors since the mid-1990s: as a consequence of the absence of cold winters in the area after 1997, annual recruitment in some major bivalve species has consistently failed and was not sufficiently compensated by better survival. A recent period of consistently enhanced mortality rates in M. balthica even aggravated the situation.