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.

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