Variability in annual recruitment success as a determinant of long-term and large-scale variation in annual production of intertidal Wadden Sea mussels (Mytilus edulis)

J. J. Beukema,R. Dekker

doi:10.1007/s10152-006-0054-3

J. J. Beukema, R. Dekker

Open Access

https://doi.org/10.1007/s10152-006-0054-3

Copy DOI

Abstract

AbstractTo understand the background of the strong variation and recent decline of stocks and production of mussels (Mytilus edulis) on tidal flats of the Wadden Sea, we analysed long-term (twice-annual for 26 years) and multi-station (15 sites) estimates of numbers, mean individual weights, biomass, and annual production on Balgzand, a 50-km2tidal-flat area in the westernmost part of the Wadden Sea (The Netherlands). Somatic production was estimated from summed growth increments of soft tissues per half-year period and expressed in ash-free dry mass (AFDM). In adults, positive values in spring/summer regularly alternated with negative values in autumn/winter, when up to ∼25% (mean: 14%) of individual weight gains in the preceding season were lost. No weight losses were observed during the first winter of the life of mussels. The 26-year mean of net somatic tissue production P amounted to 5.5 g AFDM m−2 a−1at a mean biomass B of 3.2 g AFDM m−2; the ratioP/Bvaried strongly with age composition of the mussel population and ranged between 0.5 and 3.0 a−1(mean: 1.7). Within the restricted areas of mussel beds, mean biomass and annual production values were two orders of magnitude higher. In the Wadden Sea, mussel beds cover a typical 1% of extensive tidal flat areas. Numerical densities of recruits showed straight-line relationships with subsequent life-time year-class production. Once recruits had reached an age of ∼10 months, their numbers predicted subsequent production within narrow limits. Production per recruit averaged 0.21 g AFDM for 10-mo recruits and was not related to recruit density. Local variation in annual production varied strongly, with maximal values between mid-tide and low-tide level, where recruitment was also maximal. Production per recruit was higher at low than at high intertidal levels. Frequently failing recruitment is indicated as the main cause of declining mussel stocks in the Wadden Sea. As in other bivalve species, a declining frequency of the occurrence of cold winters appears to govern declining recruitment success and consequently declining production.

Highlights

Mussels (Mytilus edulis L.) play an important role in the ecosystem of coastal waters and estuaries such as the Wadden Sea (Dankers and Zuidema 1995)
Details on sampling sites and methods can be found in earlier publications (e.g., Beukema and Cadée 1997)
Mean ash-free dry mass (AFDM) per individual only increased in autumn/ winter if growth was suYciently fast

Summary

Introduction

Mussels (Mytilus edulis L.) play an important role in the ecosystem of coastal waters and estuaries such as the Wadden Sea (Dankers and Zuidema 1995). Helgol Mar Res (2007) 61:71–86 and Dankers 1988; Asmus and Asmus 1991; Dankers 1993), and on other intertidal animals by creation of a speciWc mussel-bed faunal assemblage (Dittmann 1990; Saier 2002). They are an important food source for such specialised bird species as oystercatchers (Hulscher 1996) and eider ducks (Swennen 1976). Mussel Wshery dredges for seed and half-grown mussels to stock culture plots. In some years, this Wshery severely aVected natural mussel beds (Smit et al 1998; Dankers et al 2004). In the western half of the Dutch Wadden Sea, mussel stocks on tidal Xats remained low for the last 15 years (Steenbergen et al 2005) and nowadays mussel beds are few there (see map 1 in Goudswaard et al (2006))

Objectives

Methods

Results

Conclusion