Recruitment, abundance and production ofMya arenariaandCardium edulein marine shallow waters, Western Sweden

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Abstract
Abundance, recruitment, biomass, growth, and production of the bivalves Mya arenaria L. and Cardium edule L. were studied in two predominantly sandy areas and one muddy area in 1977 to 1982. Random samples were taken in the shallow waters between 0 and 1 m depth with the aim to collect all sizes of the bivalves. The areas investigated are characterized by summer temperatures up to 30 °C on the bottom, ice in winter, salinities normally between 15 and 25 ‰, and a tidal amplitude of 0.1 m. Mya and Cardium spats colonized the bottoms in vast numbers in June. Mya reached maximum means of between 13 000–458 000 ind. · m−2 in the sandy areas and 2000–4000 ind. · m−2 in the soft bottom area. Corresponding mean numbers of Cardium were in these areas 5000–59 000 and 600–1400 ind. · m−2, respectively. High density (local maximum 950 000 ind. · m−2) of Mya in 1979 followed the coldest winter recorded during this study and gave a production at 50–100 cm depth during the first four months exceeding 300 g ash-free dry weight (ADW) · m−2. Cardium reached a high biomass and production in 1979, which probably was a consequence of the exceptional recruitment of Mya that year. The following year this strong 1979 year class of Mya, however, most probably depressed the abundance, biomass and production of 0-group Mya and Cardium. Thus, a strong year class seems to have a great interspecific and intraspecific impact on these bivalves. One of the main predators, Crangon crangon, consumed in one year 36 and 68 % of the annual production of Mya and Cardium, respectively. Effects of predation can, however, be reduced by high recruitment and by rapid growth beyond the critical size for being captured by the main predators; thus survival of the bivalves is density and size dependent. About 75 % of the production of the 0-group bivalves occurred in general within 2 months. Production/mean biomass (P/B) of the 0-group were found in the range of 2.0 to 13.5 for Mya and 2.2 to 21.0 for Cardium. Comparisons with other areas showed the abundance and production recorded in this study to be generally higher.

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