The early stages of mussel farming are often extremely inefficient with a large proportion of seed (spat) typically lost early in the production cycle. An effective approach for decreasing such losses is to grow spat to larger sizes in nursery systems prior to seeding. However, for such an approach to be commercially viable, spat must first be separated from their settlement substrates into a single seed format, i.e., singulated. This experimental study sought to determine whether the use of chemical treatments (i.e., chlorine and protease enzymes) at various concentrations and exposure times, or altering the holding conditions of the spat (i.e, salinity, air exposure, starvation and oxygen deprivation), could be an effective approach for singulating juvenile Greenshell ™ mussel ( Perna canaliculus ) spat from spat-collector ropes. The proportion of mussels singulated from spat-collector ropes after treatment were measured then reintroduced into seawater. After 24h of rearing, the survival and recovery rate of spat was also measured.
Chemical treatments involving proteases (PAP, PXT, and BAP) and chlorine demonstrated varying results. Proteases, along with chlorine, were capable of singualting up to 65% (PAP) and 61% of larger spat, while for smaller spat, singualtion rates reached 85% (PXT) and 79%, respectively. However, these treatments were associated with elevated spat mortality, particularly with increased concentrations and exposure times, resulting in recovery rates as low as 34% for large spat and 26% for small spat. On the other hand, alterations in environmental conditions showed singualtion rates ranging from 28.5% to 80.2%, with the highest singulation rate observed in tanks without aeration and without food for a period of 24h. Despite these variations in singulation rates, survival remained consistently high (>80% survival) across all treatments, leading to the highest mean recovery of 65% for spat of all sizes.
These findings shed light on ongoing efforts to enhance the efficiency and sustainability of mussel farming practices. They underscore the potential utility of chemical treatments or modifications to spat holding conditions for singulating spat from settlement substrates before transferral to nursery systems for further growth. Notably, the study reveals a tradeoff between survival and singulation efficacy, with chemical treatments showing higher singulation rates but increased spat mortality compared to alterations in environmental conditions. Moreover, considerations regarding the timing and practicality of these methods warrant further exploration in optimizing mussel farming operations.