The biology of<i>Mya arenaria</i>(Bivalvia) in the eutrophic inner Oslofjord

Abstract

Abstract Both newly settled and adult M. arenaria had an aggregated distribution. The maximum densities found (115/m2) were not higher than in Scandinavian localities not subjected to organic enrichment. Over winter there was heavy mortality of newly settled individuals and adults. Two spawning periods were found and it is suggested that high summer water-temperatures (over 20° C) inhibited spawning in mid-summer. The length/dry weight regression had values that fitted the standard allometric relationship and the condition factor showed a decrease in summer associated with spawning. A Gompertz equation described accurately the growth rate of the population; growth rate was similar to that of other populations in Scandinavia. Substituting day-degrees for time in the Gompertz equation a model with seasonally varying growth was obtained. This model was used to obtain production figures. Production was calculated as 42.52 g dry weight/m2 per year. This is over twice that of a population of similar age structure studied in the Roskilde Fjord, Denmark. The P/B¯ ratio for the inner Oslofjord population was 1.59 which is almost twice that obtained from Robertson's equation of P/B¯ for the appropriate lifespan. It is possible, therefore, that the inner Oslofjord population has higher production and a higher P/B¯ ratio than populations from non-eutrophic areas.