Recruitment of spat to horse mussel populations is usually erratic
(Brown, 1984). Clues to this may be obtained from studies of the age distribution of the
populations. However, the animals do need to be aged as modality in size distributions is
not a good guide to the ages. An initial study of age distribution in each of the
designated beds is recommended. In contrast to Mytilus, simple counting of annual
growth rings after gentle cleaning with bleach solution seems to be a reliable method in
most cases. In cases where difficulty is encountered the shell sectioning - acetate peel
technique, as often used for Mytilus, can successfully be used (Anwar et al.,
1990). After this, ways need to be considered for monitoring recruitment on an annual
basis and possibly on a seasonal basis. Methods need to be found to do this that are not
too destructive. In beds of Mytilus, spat recruitment has been recorded by putting
out spat collectors. Plastic pan scourers have been used as a standardised item for this.
As an initial trial, it would be possible to attach pan scourers to moored weights to see
if Modiolus spat would settle on them.