Diel vertical migration of sea scallop larvae Placopecten magellanicus in a shallow embayment

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Abstract
Vertical distribution of sea scallop larvae was studied while anchored for 50 h at a shallow location (< 25 m) off Grand Manan Island [Nova Scotia, Canada] in the outer Bay of Fundy. Changes in the depth-averaged concentration of larvae (no. m-3) were unrelated to changes in the centre of mass (ZCM) of larvae, and appear to reflect the movement of a patch (or patches) of larvae back and forst past the anchor station. A small-amplitude diel vertical migration, comparable to observations under controlled conditions, was indicated by changes in the ZCM. During the day the ZCM ranged from 5.2 to 11.5 m, while at night the ZCM was shallower and less varible (4.1 to 6.8 m). The concentration of all size groups was greatly reduced within 1 to 8 m of the bottom, and evidence for vertical stratification by size was lacking. Sea scallop larvae appear to be unable to form aggregations below a critical level of water column stratification.