The spatial variations in the densities of adult (> 1 year old) mussels Mytilus edulis L. in the Exe estuary, South-west England, are investigated in relation to six potentially significant environmental gradients; distance up-estuary and up-shore, substrate softness, mussel bed topography, bed area and proportion covered by algae. The most important correlate of mussel density was the up-shore gradient, as measured by exposure time; mussels were densest just below mid-tide level. The level of recruitment of spat mussels (0 yr) to each bed depended on the densities of adults already present; more spat were recruited where adults were denser. Their subsequent mortality was strongly density dependent, with the numbers surviving the winter also being related to the density of adults. Hence the population as a whole was self-sustaining and densities on the individual beds were related to the up-shore gradient of exposure time. The question of how adult densities became established in the first place is therefore discussed. In former times, fishermen laid many mussel beds over the estuary and it is concluded that, once abandoned, only those placed at or below the mid-tide level survived. |
