Variability in annual recruitment success as a determinant of long-term and large-scale variation in annual production of intertidal Wadden Sea mussels (<Emphasis Type="Italic">Mytilus edulis</Emphasis>) |
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Authors: | J J Beukema R Dekker |
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Institution: | (1) Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790AB Den Burg, Texel, The Netherlands |
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Abstract: | To 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-km2 tidal-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−1 at a mean biomass B of 3.2 g AFDM m−2; the ratio P/B varied 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. |
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Keywords: | Long-term data Annual recruitment Secondary production Tidal flats Wadden Sea |
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