Interactive Effects of Stocking Density and Algal Feed Ration on Growth, Survival, and Ingestion Rate of Larval Geoduck Clams

The combined effects of initial stocking density (2, 5, 10 individuals [inds]/mL) and feed ration (5×103, 20×103, 40×103, 100×103 cells·ind−1·d−1) on growth, survival, and ingestion rate of larval geoduck clams Panopea generosa fed Isochrysis sp. (Tahitian strain, TISO clone) were assessed. All three measurements were significantly affected by stocking density, ration, and their interaction. Growth rate ranged from 2.15 to 3.85 μm/d in the various treatments. It increased with stocking density at 5×103 cells·ind−1·d−1, was significantly higher at 5 than at 2 inds/mL at 20×103 cells·ind−1·d−1, and was significantly higher at 2 and 5 than at 10 inds/mL at 40×103 and 100×103 cells·ind−1·d−1. Growth rate increased with increasing ration up to 40×103 cells·ind−1·d−1 at 2 and 5 inds/mL, but decreased with increasing ration at 10 inds/mL. Percent survival over the experiment duration (23 d) ranged from 7% to 56% in the various treatments and generally decreased with increasing stocking density. There was no significant effect of ration on percent survival at 2 and 5 inds/mL, but survival decreased with increasing ration at 10 inds/mL. Ingestion rate ranged from 2×103 to 29×103 cells·ind−1·d−1 in the various treatments. It was not significantly affected by stocking density at 5×103 and 20×103 cells·ind−1·d−1, but larvae held at 2 and 5 inds/mL had significantly higher ingestion rates than those stocked at 10 inds/mL at 40×103 and 100×103 cells·ind−1·d−1. Ingestion rate increased with ration at 2 and 5 inds/mL but was not significantly affected by ration at 10 inds/mL. Due to the significant interaction of the treatment factors, stocking density‐specific ration levels must be selected in order to maximize growth and survival.