Sweetening the Deal: Testing Pre‐Seeding Exposure to Dissolved Sugar as a Conditioning Method for Mussel Spat

ABSTRACT The poor retention of mussel spat during the early stages of longline aquaculture is a global issue, leading to inefficient use of juveniles and reduced production yields. In New Zealand, the Greenshell mussel ( Perna canaliculus ) industry relies heavily on wild‐sourced spat, often seeded onto grow ropes in poor nutritional condition, contributing to substantial losses of mussels in the early‐stage of production. This study tested whether short‐term immersion in sucrose‐enriched seawater immediately prior to seeding could improve spat condition and enhance retention and growth, as previous laboratory studies have shown mussel spat can rapidly take up dissolved sugars. Wild spat harvested from Ninety Mile Beach that were in poor nutritional condition were transported to a farm site in the Firth of Thames and immersed for 4 h in one of three treatments: (1) No immersion (control), (2) filtered seawater or (3) filtered seawater with dissolved sucrose (1 mg mL −1 ). Spat were then seeded directly onto grow ropes on a coastal longline farm. After 85 days, spat densities declined from ∼359,000 m −1 to just 543 m −1 across all treatments, a >99% loss. Immersion of spat in dissolved sugar for 4 h prior to seeding out did not significantly improve the subsequent retention, growth or nutritional condition of the spat. These findings demonstrate that a single 4‐h immersion in sugar‐enriched seawater was insufficient to improve outcomes on farms. More effective approaches, such as extended nursery periods and better control over seeding densities are needed to improve production efficiency and address the broader challenge of high early‐stage losses in global mussel aquaculture.