Eastern oyster aquaculture relies on effective larviculture resulting in high-quality seed. Nursery space is frequently a limiting factor in production, and successful spawns may generate more eyed larvae than can be set immediately. Hatcheries seeking to maximize output from productive spawns may refrigerate eyed larvae or increase setting densities. However, the effects of these strategies on growth and multi-sets (multiple oysters joined together) are not well documented.
To evaluate the effect of setting densities, eyed larvae were placed into silos containing microcultch at 300-, 400-, 500-, and 600-thousand larvae per silo in two separate trials. Set rates, shell height at 1- and 3-weeks post-set, and the frequency of multi-sets were assessed for each density. Setting density did not impact initial set rates or growth progression to retention on a 710 µm screen after one week on flow-through seawater. In one trial, the 600-thousand density treatment produced more multi-sets than the lower density treatments (Table 1). In both trials, a density of 600-thousand larvae exhibited reduced shell height at 3 weeks post-set.
To investigate the effects of refrigeration, eyed larvae were set on microcultch in silos immediately (fresh) or after refrigeration at 4°C for 4 hours or 1-4 days. Set rates were recorded after a 4-day setting period. Recovery at various sizes was tracked in the nursery. Set rates were significantly higher when larvae were refrigerated for 2 days or less but were lower after 4 days of refrigeration. There was no significant difference among treatments in the number of multi-sets. After 1 week on flow-through seawater, retention on a 710 µm screen was lower in larvae refrigerated for 2 and 4 days, however after standard nursery grading practices, such as density reductions and culling of slow growers, no differences in recovery were detected at 2 mm. No differences in shell height or percent recovery were observed among treatments after six weeks in the field.
Collectively, these results suggest that setting densities of at least 500-thousand eyed larvae per silo can be employed without adverse effects on performance. Furthermore, while refrigeration may temporarily stunt seed growth, these effects are mitigated during nursery grading, resulting in no long-term differences in seed performance.