Effects of planting orientation and size on survival of juvenile Pacific geoducks <i>Panopea generosa</i>

ABSTRACT Objective Seed mortality in geoduck aquaculture is a major bottleneck for production and is often attributed to predation. In the cultivation of the Pacific geoduck Panopea generosa, predator exclusion devices significantly reduce seed mortality, but farmers still report low postplanting survival, and the factors that influence this mortality are largely unknown. We conducted a laboratory study to assess the effect of siphon orientation and clam size (wet weight and shell length) on the postplanting survival of hatchery-reared juvenile Pacific geoducks. Methods Juvenile geoducks (N = 540) were individually marked, and shell length (mm) and wet weight (g) were measured and used to calculate a condition factor (g/cm). The experimental system consisted of thirty 20-L containers in a flow-through seawater system. Individuals were randomly assigned to a container based on size, and each container was randomly assigned a treatment factor representing the siphon angle with respect to the sediment surface (siphon oriented upright, horizontal, or inverted) at the time of planting. Geoducks were planted at 15 cm depth in each container, and survival was assessed after 14 d. Results We found that juvenile geoduck survivorship was dependent on planting orientation and that this relationship was highly size-dependent. Mean survivorship (±SD) was highest for geoducks planted upright (68.5 ± 7.6%), followed by geoducks planted horizontally (50.7 ± 8.7%) and those that were inverted (34.1 ± 11.7%). Across all planting orientations, juvenile survivorship was positively correlated with shell length, condition, and wet weight, with the latter two metrics best predicting survival. Orientation did not affect the survivorship of geoducks that had a condition of 1.6 g/cm or a wet weight of at least 2.5 g. Conclusions Our results show that planting Pacific geoducks upright increases juvenile survivorship, possibly leading to increased outplanting production. If controlling planting orientation is impossible, we recommend selecting juvenile geoducks of the highest possible condition to maximize survival. To our knowledge, this is the first analysis demonstrating that planting orientation affects mortality in juvenile Pacific geoducks.