Immersion Can Trigger Detachment Of Juvenile Mussels

The nursery stages of mussel aquaculture are highly inefficient with most juvenile mussels being lost within the first few months after seeding, although the precise timing of losses is largely unknown. One possibility is that many juveniles are lost at seeding because they are either impaired or killed by the process of transporting juveniles from collection sites to coastal nursery farms, or because they detach and move away from aquaculture substrata at, or soon after deployment, via secondary settlement behaviour. Three laboratory experiments were used to assess detachment of juvenile mussels from aquaculture substrates following periods of emersion for two batches of wild juveniles undergoing actual transport and one batch of hatchery-reared juveniles during simulated transport periods. The percentage of juveniles retained on their substrates (i.e., not detaching) after a twominute immersion in seawater and the percentage of dead juveniles were quantified, as was the size of the mussels that were retained on, or detached from, their substrates. Detachment occurred in all experiments (16.9?53.8%) and was greater when the average size of the mussels was small (0.75 ? 0.01 mm). Across all the experiments, larger juvenile mussels were retained in greater numbers with mussels less than 1.75 mm in shell length having a greater propensity to detach from their substrates upon immersion. Very few mussels were dead at the end of the experiments and large proportions of mussels reattached in the aquaria, when given the opportunity. Together, the results of this study indicate that detachment of juvenile mussels is size-specific and driven by secondary settlement behaviour occurring upon or soon after (< 2 mins) immersion. New nursery techniques are required to grow small juvenile mussels to a size beyond which their migratory secondary settlement behaviour is reduced, so that they are less likely to detach upon or soon after immersion while seeding out to farm sites.