The production of relaid blue mussels ( Mytilus edulis L.) in a Danish fjord

Mussels (Mytilus edulis L.) smaller than the commercial size caught in Limfjorden, as in other areas, are typically discarded. However, during the period 1990 to 1993 these small mussels were returned, after sorting to mussel beds for later harvest; a process defined as ''relay''. This paper presents data from two commercial culture beds and from two smaller experimental beds established to study growth and mortality of these small mussel discards. The data were analysed by a yield- per-recruit model to calculate yields from such relays. This model was also used to predict the optimal time of harvest. The parameters utilized in the model were: (1) initial mortality due to harvesting, unshipping and sorting; (2) growth and mortality between relay and harvest; and, (3) the drained wet weight of a mussel of a given shell length. The initial mortality was estimated from observations of mussels damaged by harvesting, unshipping, sorting and exposure to desiccation on land in aquaria survival experiments. Growth and mortality rates after relay were estimated from diver investigations of the experimental beds. The relationship between the drained wet weight and the shell length was established based on samples from commercial landings in Limfjorden. The calculated harvest was compared to the observed yield from two commercial culture beds. The productivity in kg m(-2) in a bed of relaid mussels was found to be 2-7 times higher than in natural mussel beds in Limfjorden. (C) 1997 International Council for the Exploration of the Sea.