Refining Spat‐Collection Practices to Optimise Single‐Seed Spat Production for New Zealand's Greenshell ( Perna canaliculus ) Mussel Industry

ABSTRACT Mussel aquaculture depends on a consistent spat supply, often sourced from spat‐collection ropes suspended in coastal waters. However, settlement on these ropes can be highly variable due to environmental factors. Therefore, improving spat supply from this source relies on optimising aspects of the process, particularly the timing of deployment of spat‐collection ropes and the duration that they remain in the water (soak duration). This study aimed to determine whether short, frequent deployments of spat‐collection ropes could be used to increase the collection of Greenshell mussel ( Perna canaliculus ) spat for aquaculture. Spat‐collection ropes were deployed in Whangapē Harbour, New Zealand, across four weekly deployments (D1–D4). From each deployment, spat‐collection ropes were retrieved after a soak duration of 7, 14, 21 or 28 days. The highest settlement (> 6000 spat m −1 ) was recorded on ropes deployed during D1 and retrieved after a 7‐day soak duration. In contrast, D2–D4 showed significantly lower settlement than D1, irrespective of soak duration. Notably, retrieving spat‐collection ropes after four consecutive 7‐day soak durations collected 30% more spat than ropes retrieved after two 14‐day soak durations, and 53% more than a single 28‐day soak duration. A second experiment assessed whether spat collected over shorter durations were amenable to singulation, the process of detaching spat into unattached individuals preferred by mussel nurseries. Nine chemical or stressor singulation treatments were tested to promote detachment of the spat from the collection rope. Chlorine exposure (0.625% NaOCl in seawater) produced the highest singulation rate (91.5%) without reducing spat survival (∼85% vs. 89% in the control). These findings demonstrate that combining short, weekly deployments of spat‐collection ropes with the use of chlorine treatments to rapidly remove settled spat enables the ropes to be redeployed quickly, allowing for repeated collection cycles and increasing the mussel spat supply for aquaculture production.