Settlement Response in Blue Mussels (Mytilus edulis L.) in a Hatchery Setting

A critical aspect of blue mussel (Mytilus edulis L.) commercial aquaculture is the industry’s dependence on a highly variable supply of wild seed. The objective of the present study was to investigate hatchery production of blue mussel seed with respect to larval settlement responses to different types of rope collectors. Mussel larvae were cultured at the Downeast Institute (DEI; Beals, ME, USA) using techniques similar to those employed in the culture of soft-shell clam seed (Mya arenaria). Wild blue mussel broodstock collected from the rocky intertidal adjacent to DEI were conditioned using a mixed microalgal diet and spawned via thermal-shock. The resulting larvae were reared on a mixed microalgal diet at 18°C for approximately two weeks in 400 L culture tanks. The study consisted of two trials (July and September) which entailed the exposure of competent, pediveliger larvae (approximately five thousand per experimental tank) to rope collectors (polyethylene and polypropylene) exhibiting diverse structural features such as long loops, short loops, long filaments, short filaments and smooth (Trial I - four rope types; Trial II - five rope types). In Trial I, rope segments (3 cm long) were placed at two different levels in four culture tanks (top and bottom of water column). In Trial II, segments were placed at one level in six culture tanks (middle of water column). In both trials, each type of rope was replicated five times. After a period of five days, rope collectors were removed from experimental tanks and the number of settled larvae on each segment was counted. Rope collectors with the highest structural complexity (long loops) elicited the strongest settlement response in mussel larvae, while rope collectors with the lowest complexity (smooth) elicited the weakest response. Position within tank (top vs bottom; Trial I) had no discernible effect on settlement numbers. These findings suggest that hatchery-reared mussel seed could be a reliable alternative to wild seed and that ropes with complex features should be used as larval collectors as they appear to enhance the number of settled individuals which, in turn, could potentially reduce production costs.