Condition and variability in Mytilus edulis (L.) from different habitats in Ireland

Abstract

Twenty-one samples of Mytilus edulis L., taken from three habitats; commercial rafts, commercial subtidal beds, and non-commercial intertidal regions, were examined. Three condition indices: dry meat weight/internal volume, shell weight/internal volume, and dry meat weight/shell weight; and two morphological ratios: internal volume versus length and shell weight versus length, were calculated for each sample. The relative weight of the shells increased and the meat weight decreased, as we examined samples from rafts, subtidal, and intertidal regions, in that order. The size-related increase of shell weight was relatively greater than the increase of volume in intertidal mussels, in contrast to raft mussels. These findings agree with the commercial practice of relocating mussels to subtidal beds to improve their meat yield. There were no significant clines in the data associated with geographical location, although there were very marked local variations in all factors measured. Variability was as great in raft mussels as in wild intertidal ones. Meat weight was consistently more variable than shell weight, and these two variabilities were not correlated. Because of the strong local variation reflected in the significantly different values for the ratio meat weight/shell weight, in virtually all populations, we concluded that comparisons of stocks or growth must be carried out in the same location to have any validity.