Evaluation of separation of seed mussels from macroalgae using chlorination

Growing seed mussels (spat) to larger sizes in single-seed nursery culture systems prior to seeding onto coastal mussel farms is an effective approach for greatly reducing costly spat losses during this stage of mussel aquaculture. However, the production of single-seed mussel spat in nursery culture systems relies on first separating spat from the settlement substrata to which they are attached, whether spat are settled in a hatchery or gathered from the wild. This study sought to determine whether chlorination could be an effective approach for separating wild Greenshell mussel (Perna canaliculus) spat from beachcast macroalgal material (Kaitaia spat material) harvested from a beach in northern New Zealand. The results demonstrate that chlorination (i.e. in solutions of up to 0.625% sodium hypochlorite) is a highly effective approach for separating spat, resulting in the detachment of up to 94.6% in one treatment without increasing mortality relative to controls. The effectiveness of chlorination was modulated by spat density, with increasing spat densities, lowering rates of detachment. While chlorination is effective at separating small spat (i.e. successfully separating 72.4% of spat <750 μm in shell length) overall spat densities, further investigation is required to determine how to effectively separate larger spat (i.e. with chlorination only successfully separating 21.5% of spat >750 μm). The high survival (relative to the control treatments) also suggests that chlorine concentrations and exposure durations could be increased from those used in this study. These results demonstrate that chlorination can be used to separate spat from substrate prior to their transfer to nursery systems for ongrowing, paving the way for such approaches for reducing spat losses to be integrated into the aquaculture production cycle.