A study was conducted in the Îles de la Madeleine (Gulf of St. Lawrence, Canada) to evaluate if it is possible to predict the optimal collector deployment period to reduce the abundance of undesirable species and maintain a high collection of scallop spat. The timing of spat collection was characterized by weekly deployment of spat collectors for one-week periods. These collectors were placed at 2, 5, and 8 m above the bottom to represent the settlement range on commercial line. Other test collectors were deployed weekly between the third and eighth weeks after the onset of spawning at 2 m above the bottom and then were all retrieved in December, June, and October to evaluate cumulative collection for these six different deployment periods. The first peak of Mytilus edulis and Hiatella arctica occurred in September before the Placopecten magellanicus peak. Weekly spat monitoring could be useful to identify when those peaks occur. As a result, growers can delay the collector deployment until after these peaks. The weekly spat collection monitoring appears effective to identify peak settlement of scallop and undesirable species and could be used as a husbandry practice to help growers determine the optimal time to deploy their collectors. Collectors deployed 5–6 wk after spawning was initiated (September 29 and October 6) provided the highest abundance of scallops one year later, at the usual commercial retrieval period. The scallop abundance observed in collectors deployed on October 6, 2003 and retrieved in October 2004 was of 52% of the initial numbers found in December 2003 whereas for mussels, losses were of about 69% over the same period. The abundance of hiatella and anomia increased between December and the following June and decreased afterwards.
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